Clinical Research
Role of Respiratory Motor Output in Within-Breath
Modulation of Muscle Sympathetic Nerve
Activity in Humans
Claudette M. St. Croix, Makoto Satoh, Barbara J. Morgan, James B. Skatrud, Jerome A. Dempsey
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Abstract—We measured muscle sympathetic nerve activity (MSNA, peroneal microneurography) in 5 healthy humans
under conditions of matched tidal volume, breathing frequency, and end-tidal CO2, but varying respiratory motor output
as follows: (1) passive positive pressure mechanical ventilation, (2) voluntary hyperventilation, (3) assisted mechanical
ventilation that required the subject to generate –2.5 cm H2O to trigger each positive pressure breath, and (4) added
inspiratory resistance. Spectral analyses showed marked respiratory periodicities in MSNA; however, the amplitude of
the peak power was not changed with changing inspiratory effort. Time domain analyses showed that maximum MSNA
always occurred at end expiration (25% to 30% of total activity) and minimum activity at end inspiration (2% to 3%
of total activity), and the amplitude of the variation was not different among conditions despite marked changes in
respiratory motor output. Furthermore, qualitative changes in intrathoracic pressure were without influence on the
respiratory modulation of MSNA. In all conditions, within-breath changes in MSNA were inversely related to small
changes in diastolic pressure (1 to 3 mm Hg), suggesting that respiratory rhythmicity in MSNA was secondary to
loading/unloading of carotid sinus baroreceptors. Furthermore, at any given diastolic pressure, within-breath MSNA
varied inversely with lung volume, demonstrating an additional influence of lung inflation feedback on sympathetic
discharge. Our data provide evidence against a significant effect of respiratory motor output on the within-breath
modulation of MSNA and suggest that feedback from baroreceptors and pulmonary stretch receptors are the dominant
determinants of the respiratory modulation of MSNA in the intact human. (Circ Res. 1999;85:457-469.)
Key Words: autonomic nervous system n cardiorespiratory interaction n positive pressure ventilation
T
he independent influence of the respiratory rhythm generator on the timing of sympathetic outflow has been
studied using reduced preparations in which peripheral reflex
mechanisms were eliminated by vagotomy and/or sinoaortic
denervation. In these deafferented animal preparations, sympathetic neurons fire mainly during inspiration (ie, in synchrony with phrenic discharge), with their minimum activity
occurring during expiration.1,2 The close temporal relationship between phrenic discharge and sympathetic activity
recorded in anesthetized, paralyzed, and vagotomized animals has led to the hypothesis that the 2 neural outputs either
arise from the same brain stem neurons or are driven by a
common oscillator.1,3 However, in the intact human, sympathetic outflow to skeletal muscle (muscle sympathetic nerve
activity; MSNA) declines during inspiration, reaching its
nadir at end inspiration/early expiration, and then rises,
reaching its peak at end expiration.4,5 The influence of central
respiratory motor output on the within-breath modulation of
MSNA has not been studied in humans.
The purpose of the present study was to examine the
relative contribution of central respiratory drive in generating
the respiratory modulation of MSNA in intact human subjects. We varied respiratory motor output using passive
positive pressure mechanical hyperventilation at high tidal
volume (VT) to eliminate respiratory motor output, and
voluntary elevation of VT and inspiratory flow rate, with and
without added inspiratory resistance, to increase respiratory
motor output. We also determined the effects of intrathoracic
pressure on the modulation of MSNA and cardiac frequency
and controlled for the effects of lung inflation and chemoreceptor reflexes.
Materials and Methods
General Procedures
Six men ages 27 to 59 years served as subjects after providing
written, informed consent. All subjects were normotensive and free
from cardiopulmonary disease. All experimental procedures and
protocols were approved by the University of Wisconsin Center for
Health Sciences and the Middleton Memorial Veterans Hospital
Human Research Review Committees. Airflow rates, VT, mask
pressure (Pm), and end-tidal partial pressure of CO2 (PETCO2) were
measured using equipment and techniques described previously.6 A
Received March 23, 1999; accepted July 6, 1999.
From the Departments of Preventive Medicine (C.M.S., M.S., J.A.D.), Surgery (B.J.M.), and Medicine (J.B.S.), University of Wisconsin and the
Middleton Memorial Veterans Hospital, Madison, Wis.
Correspondence to Jerome A. Dempsey, PhD, Department of Preventive Medicine, University of Wisconsin-Madison, 504 N Walnut St, Madison, WI
53705. E-mail [email protected]
© 1999 American Heart Association, Inc.
Circulation Research is available at http://www.circresaha.org
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TABLE 1. Experimental Protocols Used and the Effects of These Interventions on the Proposed Central and Peripheral Influences on
Respiratory Modulation of Muscle Sympathetic Nerve Activity
Protocol
Central Inspiratory
Motor Output
Tidal Volume
Intrathoracic Pressure
(Peak Inspiration, cm H2O)
PETCO2,
mm Hg
Systemic Arterial Pressure
(Inspiration)
Spontaneous eupnea
Low
0.6 L
22
37
Increasing
Voluntary hyperventilation
High
23 eupnea
23
27
Increasing
Passive PPV
0
23 eupnea
121
25
Increasing
Assisted PPV
High
23 eupnea
118
25
Increasing
Very high
23 eupnea
218
26
Increasing
Voluntary hyperventilation1inspiratory resistor
single-lead ECG was continuously recorded. Diaphragmatic electromyogram (EMGdi) was obtained from surface electrodes placed over
the sixth and seventh intercostal spaces in the anterior axillary line.
The raw EMGdi signal was amplified and band-pass filtered from 30
to 1000 Hz (model P511, Grass Instruments). Arterial pressure was
measured beat by beat using the finger photoplethysmography
technique (Finapres model 2300, Ohmeda) and with an automated
sphygmomanometer (Dinamap, Critikon) at 1-minute intervals.
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Recordings of Sympathetic Nerve Activity
Multiunit recordings of postganglionic MSNA were obtained from
the peroneal nerve of the right leg as described previously.7,8 The
neural signals were passed to a differential preamplifier, an amplifier, a band-pass filter (700 to 2000 Hz), and an integrator (time
constant5100 ms; total gain5100 000). When acceptable MSNA
recordings (spontaneous pulse synchronous activity with signal-tonoise ratio .3:1) were obtained, the subject was instructed to
maintain the leg in a relaxed position for the duration of the study.
Segments of the neural recording that showed evidence of mechanoreceptor or a-motoneuron activity were excluded from the analysis.
MSNA data from one subject were discarded because of inadequate
neural recordings.
Experimental Protocols
Subjects were studied supine in the postabsorptive state. All protocols were performed during the application of a nonhypotensive level
(applied pressure, ,20 mm Hg) of lower body negative pressure
(LBNP) to augment basal MSNA,9 thereby facilitating the study of
respiratory modulation. In a single subject, who was studied both
with and without lower body suction, LBNP increased MSNA
frequency compared with normal resting conditions (35.461.5
versus 26.462.6 bursts per minute). However, in agreement with
previous reports,5 we saw that the respiratory modulation of MSNA
under all experimental conditions was unaffected by LBNP.
The experimental protocols used are summarized in Table 1.
These different protocols were designed to study the effects of
increasing or decreasing central respiratory motor output on the
within-breath modulation of MSNA and to control for the effects of
changing lung volume, intrathoracic pressure, and systemic arterial
pressure.
Voluntary Hyperventilation
The subject voluntarily maintained VT at twice the eupneic level,
using visual feedback from an oscilloscope. The subject maintained
a constant respiratory frequency (f) and a duty cycle [inspiratory time
(Ti)/total time (TTOT)] of 50% using auditory feedback with distinct
inspiratory and expiratory tones. VT, f, and Ti/TTOT were held at the
same level for each of the subsequent experimental protocols.
Passive Positive Pressure Mechanical Ventilation
(Passive PPV)
PPV was applied until the respiratory muscles were inhibited and
was then continued for a minimum of 5 minutes during which blood
pressure, ECG, and nerve activity were continuously recorded.
Respiratory muscle inhibition was determined from specific criteria,
as previously outlined,10 including the absence of EMGdi, stabilization of the Pm waveform, constancy of peak positive end-inspiratory
Pm, and a significant (.10-second) prolongation of expiratory time
on cessation of mechanical ventilation (Figure 1). Subjects visited
the laboratory on a separate occasion for a familiarization session so
that they were able to relax during the passive mechanical ventilator
trials on the test day.
Assisted Positive Pressure Mechanical Ventilation
(Assisted PPV)
Each subject had to generate a threshold Pm of –2.5 cm H2O to trigger
each positive pressure mechanical breath. The subjects were asked to
maintain the set f by following computer-generated audio signals.
The aim of this protocol was to produce positive intrathoracic
pressure changes during inspiration and at the same time require a
high amount of respiratory muscle activation, as determined by the
EMGdi.
Inspiratory Resistor
In 3 subjects, the voluntary hyperventilation trials were repeated with
the addition of an inspiratory resistive load.
Before each series of voluntary hyperventilation, passive PPV, and
assisted PPV protocols, baseline measurements of ventilatory and
cardiovascular variable data were made during 5 minutes of spontaneous breathing. These measurements of MSNA made during
spontaneous breathing (22.764.7 and 22.663.1 bursts per minute,
respectively; P.0.05) showed that baseline nerve traffic did not
change over time. The voluntary hyperventilation trial served as a
“control” protocol, during which all potential mechanical and neural
influences on the respiratory modulation of MSNA were present.
The sequence of the trials was as follows: (1) passive PPV,
(2) voluntary hyperventilation, (3) assisted PPV, (4) repeat passive
PPV, (5) repeat voluntary hyperventilation, and (6) inspiratory
resistor. This sequence ensured that each experimental protocol was
bracketed by a voluntary hyperventilation trial with which any
within-breath and per-minute changes in MSNA were compared.
This design ensured that potential baseline shifts and/or timedependent changes in MSNA did not affect our results.
Data Analysis
Respiratory and cardiovascular parameters were computed as described previously.11 Bursts of MSNA were identified by visual
inspection of the integrated neurogram by one investigator (C.M.S.).
The amplitude of each burst of MSNA and the beat-to-beat levels of
arterial blood pressure were determined by computer. MSNA was
expressed both as burst frequency and total activity (calculated as the
product of burst frequency and relative burst amplitude and presented in arbitrary units).
Time Domain Analyses
The blood pressure signal was advanced by 0.2 seconds to correct for
the propagation time from the central circulation to the finger on the
basis of previous studies (B. Morgan, A. Xie, unpublished data,
1998) of the delay between the peak of the R-wave from the ECG
signal and the corresponding peak in the blood pressure waveform.
Bursts of MSNA were also advanced in time to account for nerve
conduction delays using the subject’s height and an estimate of
conduction velocity in the peroneal nerve of 1.11 m/sec.12 The exact
locations and amplitudes of the bursts of MSNA and the arterial
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Figure 1. Raw data from a representative subject during eupneic breathing and under conditions of matched VT (2 times eupnea),
breathing frequency (f), and PETCO2, but varying respiratory motor output as evidenced by marked changes in EMGdi, as follows: voluntary hyperventilation, passive positive pressure ventilation, assisted positive pressure ventilation, and added inspiratory resistance (augmented hyperventilation). Note that arterial pressure and MSNA have not yet been corrected for conduction delays.
pressure waves within the breath cycle were determined for each
breath in each protocol both as a function of time and breath volume.
Both burst frequency and total MSNA (frequency3relative amplitude) in each 12.5% time interval from the onset of inspiration to end
expiration were then normalized (as a percentage of the number of
bursts and total MSNA for each breath and averaged over the entire
condition). Arterial pressure was expressed as the change from the
last quartile of expiration (end expiration). To statistically test for
differences in the within-breath patterns of modulation of MSNA
among the 4 conditions (spontaneous breathing, voluntary hyperventilation, passive PPV, and assisted PPV), principal component
analysis13 on the covariance matrix of the time series data were used
to identify independent patterns that contributed to the composite
pattern represented by the raw data points. A principal component
was considered statistically significant if it explained .5% of the
total observed variance. This technique computes both the common
shape and the individual relative importance of each significant
pattern in the MSNA in each subject, under each condition.
Total minute values for MSNA, heart rate, and blood pressure
were calculated during all protocols and compared using 1-way
ANOVA with repeated measures. Post hoc analyses were performed
if differences were detected. Significance was set at P,0.05.
Frequency Domain Analyses
Power spectral densities were obtained from analyses of 16-second
epochs of arterial pressure, respiration (airflow), and sympathetic
nerve activity. The power spectra for the R-R interval data were
obtained using previously described methods.11
Results
Differences in Respiratory Motor Output
Figure 1 and Table 2 show the different conditions across
which respiratory motor output was varied. Note that with
voluntary hyperventilation, the 2-fold increase in VT above
spontaneous eupnea was accompanied by increased EMGdi,
more negative swings in Pm, and reduced PETCO2. This is
contrasted with passive mechanical hyperventilation with
similar VT, duty cycle, and PETCO2, but with positive Pm and
no discernable EMGdi during mechanical ventilation and with
a postventilator apnea. The assisted PPV condition matched
the VT and positive Pm during the passive mechanical ventilation condition, only at high levels of respiratory motor
output, as evidenced by the EMGdi. Finally, the increased
inspiratory resistance trial matched the VT and breathing
pattern of voluntary hyperventilation, only at exaggerated
levels of respiratory motor output (see EMGdi) and negative
Pm. Average heart rate, systolic pressure, and diastolic pressure over the entire condition were not different among any of
the conditions studied (Table 2). However, there was a small
but significant (P,0.05) increase in mean MSNA (burst
frequency3burst amplitude) for the 5 subjects during passive
PPV (range, 19 to 38 arbitrary units/min) when compared
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TABLE 2. Average (n55) Per-Minute Measurements of Respiratory and Cardiovascular Variables for Each
Experimental Condition
Spontaneous
Voluntary
Hyperventilation
Passive PPV
Assisted PPV
Inspiratory
Resistor (n53)
VT, L
0.6260.11*
1.2060.11
1.1560.13
1.2260.12
1.0260.06
f, breaths per minute
12.862.6
13.361.3
13.361.3
13.161.3
14.861.04
PETCO2, mm Hg
37.164.7*
26.863.2
24.562.3
24.662.8
26.064.5
Ti/TTOT
0.4160.05†
0.4860.02
0.4360.03†
0.4560.04
Variable
0.5060.01
21.860.5
22.660.3
120.660.9
118.063.7
Heart rate, bpm
56.068.5
56.869.5
58.369.7
56.968.5
MSNA, burst frequency, bursts per minute
24.964.3
23.165.1
26.366.4
23.264.8
23.1611.6
MSNA, total activity§
24.264.4
22.963.3
29.166.4*
22.663.2
22.869.2
Systolic BP, mm Hg
114.6612.6
110.4610.8
110.5610.7
107.6611.6
108.2620.3
Diastolic BP, mm Hg
68.269.7
64.1611.0
66.1612.6
64.3612.2
59.1616.3
Pm, cm H2O‡
218.261.8
57.7612.2
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BP indicates blood pressure.
*Significantly different from other conditions.
†Significantly different from voluntary hyperventilation.
‡Peak inspiratory mask pressure. During assisted PPV, subjects generated 23.561.0 cm H2O to trigger each positive pressure,
ventilator breath.
§Total activity refers to the product of burst frequency and peak amplitude and is expressed in arbitrary units per minute.
with spontaneous breathing (range, 19 to 32 arbitrary units/min),
voluntary hyperventilation (range, 19 to 33 arbitrary units/min),
or assisted PPV (range, 18 to 33 arbitrary units/min). Nevertheless, there was no significant difference (P.0.05) in burst
frequency among any of the 4 conditions (Table 2).
Within-Breath Modulation of Arterial Pressure and
MSNA: Frequency Domain Analyses
During spontaneous eupneic breathing, power spectral analyses of MSNA and diastolic pressure identified 2 distinct
peaks in power at frequencies corresponding to the respiration and heart period in all subjects. Representative spectra
from 1 subject are presented in Figure 2. In all 5 subjects, the
amplitude of the peak power in MSNA at the respiratory
frequency was increased during voluntary hyperventilation
(VT51.2 L) when compared with eupneic breathing (VT50.6
L). However, there were no further changes in the amplitude
of the peak power of MSNA at either the respiratory or
cardiac frequency, as respiratory motor output was varied
from 0 (passive PPV) to very high (added inspiratory resistance), and Pm was changed from negative to positive, at the
same VT and frequency.
Within-Breath Modulation of Arterial Pressure and
MSNA: Time Domain Analyses
Data for MSNA are presented only in terms of total activity
(burst frequency3relative burst amplitude), because the pattern and amplitude of the within-breath variations in burst
frequency and total activity were the same under all conditions. During spontaneous breathing at low VT, when respiratory motor output was low, a within-breath variation of
MSNA was evident in all subjects (Figure 3A), with peak
activity occurring at end expiration (mean, 22.767.5% of
total within-breath activity) and minimum activity (mean,
4.263.8% of total activity) at end inspiration. The amplitude
of this respiratory modulation of MSNA was accentuated
during voluntary hyperventilation, when both VT and respiratory motor output were increased and Pm was more negative
(Figure 3B). In all 5 subjects, MSNA fell progressively from
onset to late inspiration, with the nadir at end inspiration
(mean, 2.261.6% of total activity), and then rose sharply to
a peak at end expiration (mean, 25.566.8% of total activity).
There was some variability among subjects, with 2 of the 5
showing peak activity slightly earlier in expiration.
Respiratory motor output was eliminated during passive PPV
(Figure 3C), but the within-breath pattern of MSNA was
qualitatively similar to that observed during active ventilation at
the same VT. However, during passive PPV, the maximum
activity (31.4610.2% of total activity) occurred at a slightly
earlier quartile in expiration, rather than end expiration. The
variability in MSNA among subjects was also more pronounced
during the ventilator trials, with one subject showing a pattern
that was markedly different from the remaining 4 during both
passive and assisted mechanical ventilation. Assisted mechanical ventilation with positive pressure during inspiration and high
respiratory motor output showed a similar within-breath modulation of MSNA, as did passive mechanical ventilation at equal
VT and positive Pm, but no respiratory motor output.
Three subjects completed the resistor trials. VT, f, Ti/TTOT,
and PETCO2 were the same as during the voluntary hyperventilation trials, but inspiratory effort was greatly augmented, as
shown by the 7-fold decrease in the nadir for inspiratory Pm
(Table 2) and the much larger EMGdi (Figure 1). The
respiratory modulation of MSNA was qualitatively the same
during both voluntary hyperventilation (Figure 4A) and
voluntary breathing, with increased inspiratory resistance
(Figure 4B). In all 3 subjects, MSNA decreased from onset to
late inspiration and increased during expiration, reaching a
peak at end expiration/early inspiration.
Changes in diastolic pressure during inspiration and expiration for spontaneous breathing, voluntary hyperventilation,
passive PPV, and assisted mechanical ventilation are shown
in Figure 3. The within-breath pattern of change in systolic,
mean arterial, and pulse pressures (not shown) were the same
as those shown for diastolic pressure under all conditions. On
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Figure 2. Power spectral densities for a representative subject under conditions of varying respiratory motor output. Arrows indicate
respiratory (Resp) and cardiac (Card) frequencies.
average, during inspiration, arterial pressure increased 1 to
2 mm Hg (relative to end expiration) and decreased during
expiration under all 4 conditions, with considerable variability among subjects. The within-breath pattern of change in
diastolic pressure shown during voluntary hyperventilation
(Figure 4A) was more pronounced during resistor breathing
(Figure 4B), with diastolic pressure increasing by .4 mm Hg
during inspiration in 2 of the 3 subjects. The third subject
showed a continual increase in diastolic pressure from early
inspiration to late expiration.
Principal Component Analysis
Principal component analysis of both MSNA and diastolic
pressure for each of the 4 conditions revealed 2 distinct
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Figure 3. Individual and mean (n55) data for the within-breath changes in MSNA, R-R interval, and diastolic pressure (DP) during
spontaneous breathing (A), voluntary hyperventilation (B), passive PPV (C), and assisted PPV (D).
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Figure 4. Individual and mean (n53) data for the within-breath changes in MSNA, R-R interval, and diastolic pressure (DP) in 3 subjects during voluntary hyperventilation (A) and added inspiratory resistance (B).
patterns, as follows: (1) a basic underlying pattern that was
similar to the mean responses pictured in Figure 3, and (2) an
asymmetry component reflecting the variability among subjects. When combined, these 2 components explained .90%
of the total variance in each variable, for each condition.
Marked differences in respiratory motor output among the 4
conditions (spontaneous eupnea, voluntary hyperventilation,
passive PPV, and assisted PPV) had no effect on either the
shape or the relative importance of the individual significant
patterns. For MSNA, the basic pattern accounted for 70% to
88% (asymmetry, 10% to 29%) of the total variance. The
asymmetry component was relatively more important in
describing the average arterial pressure pattern, reflecting the
greater variability among subjects in this variable, explaining
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Figure 5. Effect of changing lung volume on the respiratory modulation of MSNA during spontaneous breathing, voluntary hyperventilation, passive PPV, and assisted PPV. Data are presented as high lung volume points (last 50% of inspiration plus first 50% of expiration) or low lung volume points (last 50% of expiration plus first 50% of inspiration). NS indicates nonsignificant slope (P.0.05).
28% to 39% of the total variance, with the basic pattern
explaining the remaining 47% to 61%. Neither of the 2
principal components describing the within-breath pattern of
change in either MSNA, nor in diastolic pressure, were
significantly different among the 4 conditions (P.0.05).
Cross-correlation of the principal components describing the
within-breath patterns of variation in MSNA and diastolic
pressure revealed a significant inverse relationship between
diastolic pressure and MSNA under all conditions with
correlation coefficients of – 0.75 (spontaneous), – 0.89 (voluntary hyperventilation), – 0.87 (passive PPV), and – 0.72
(assisted PPV).
Effects of Arterial Pressure Versus Lung Volume on
Within-Breath Modulation of MSNA
To “control for” within-breath changes in diastolic pressure
and to examine the effects of changing lung volume on the
respiratory modulation of MSNA, we compared the MSNA at
any given change in diastolic pressure that occurred at higher
lung volumes (the latter 50% of inspiration plus the first 50%
of expiration) versus the (remaining) lower lung volumes
(Figure 5). The mean level of MSNA at any given change in
diastolic pressure was significantly higher at low versus
higher lung volumes (P,0.05). This effect of lung volume on
MSNA held for all conditions of active and passive ventilation. In addition, the MSNA at any given lung volume
showed a significant negative correlation with the change in
diastolic pressure during both voluntary hyperventilation and
passive PPV. The slope of this relationship was significantly
higher (P,0.05) at the lower lung volumes.
Posthyperventilation Apnea
The apneas that followed passive PPV showed a dissociation
between central respiratory motor output and MSNA (Figure
6). In the 3 instances in which the duration of the apnea was
.30 seconds, we observed the following: (a) in the initial
seconds of the postventilator apnea (when PETCO2 was still
low and arterial pressure stable), MSNA burst frequency and
amplitude remained unchanged relative to voluntary hyperventilation or the mechanical ventilation period; (b) as the
apnea proceeded, and CO2 was rising, MSNA burst frequency
and amplitude increased before any appearance of significant
respiratory motor output, as noted by the unchanging Pm or
VT; and (c) at apnea termination, MSNA remained high
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Figure 6. Raw data from a representative subject during voluntary hyperventilation, passive mechanical ventilation, and the postventilator apneic period. The apneic period shows that in the initial seconds of the apnea when respiratory motor output was absent (a),
MSNA remained unchanged relative to voluntary hyperventilation or the preceding period of mechanical ventilation; late in the apneic
period (b), MSNA was increased, but respiratory motor output was undetectable; and at apnea termination (c), both MSNA and respiratory motor output were increased. Note that arterial pressure and MSNA have not yet been corrected for conduction delays.
coincident with the appearance of significant respiratory
motor output.
Respiratory Sinus Arrhythmia (RSA): Frequency
Domain Analyses
During spontaneous eupneic breathing, power spectral analyses of the R-R interval data in each subject revealed a small
peak in power at the respiratory frequency (see representative
subject in Figure 2). The amplitude of the peak in power was
markedly increased during voluntary hyperventilation at high
VT. In all subjects, assisted PPV at the same VT markedly
reduced the amplitude of the power at the respiratory frequency compared with voluntary hyperventilation. The amplitude of RSA was further reduced with passive mechanical
ventilation.
RSA: Time Domain Analyses
All subjects showed a small RSA during spontaneous breathing that was characterized by a decrease in R-R interval
throughout inspiration and an increase during expiration
(Figure 3A). The RSA was significantly accentuated with
increased VT during voluntary hyperventilation (Figure 3B).
When respiratory motor output was eliminated during passive
PPV at the same elevated VT, RSA was eliminated in 4 of the
5 subjects and markedly reduced in the fifth subject (Figure
3C). The RSA was also considerably reduced during assisted
mechanical ventilation when respiratory motor output was
high, but positive Pm was equal to that during passive
mechanical ventilation (Figure 3D). During the resistor trials,
when inspiratory effort was greatly increased and Pm markedly decreased, the RSA amplitude was enhanced (Figure 4).
Discussion
Summary of Findings
We examined the breathing-induced variations in MSNA
under conditions in which feedback influences, such as
within-breath changes in systemic arterial pressure, lung
volume, and intrathoracic pressure, either remained constant
or were controlled, and the magnitude of respiratory motor
output was varied from undetectable to severalfold greater
than normal. We found that maximum MSNA always occurred at end expiration and minimum MSNA occurred at
end inspiration, and the amplitude of this modulation was not
different among widely varying amounts of respiratory motor
output. Similarly, variations in intrathoracic pressure, per se,
were also without systematic, independent effects on MSNA;
this was demonstrated by comparing negative pressure (voluntary) with positive pressure ventilation at comparable
levels of respiratory motor output, intravascular pressure, and
lung volume. We propose that central respiratory motor
output provides a relatively minor, if any, contribution to the
respiratory modulation of MSNA in intact humans. Rather,
our findings support the postulate that peripheral reflexes
elicited by changes in systemic arterial pressure and lung
volume are the dominant influences on the within-breath
modulation of MSNA.
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Influence of Central Respiratory Motor Output
on MSNA
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Our data provide evidence against a significant independent
effect of central respiratory motor output on the within-breath
modulation of MSNA in the intact human. These findings
were consistent across several types of change in respiratory
motor output, including (1) voluntary increases in respiratory
motor output, (2) hypocapnia-induced elimination of respiratory motor output, and (3) chemoreceptor stimulation during
the latter stages of posthyperventilation apnea.
These findings differ from the strong central respiratory
component inferred from studies in anesthetized animals
deprived of vagal and baroreceptor feedback.1,2 Why did our
findings not reveal this strong contribution from central
respiratory motor output? We hypothesize that feedback
mechanisms from baroreceptors and pulmonary stretch receptors are the dominant determinants of the respiratory modulation of MSNA in the intact state. This idea is confirmed in
part by studies in intact, anesthetized cats, which showed a
pattern of inspiratory inhibition and expiratory activation of
MSNA during eupnea that was similar to the pattern observed
in intact humans.2 However, when respiratory motor output
was increased via CO2 inhalation in this animal model, the
MSNA modulation showed inspiratory excitation and expiratory inhibition.2 This pattern implies that the role of central
respiratory motor output is important even in the intact cat
and remains quite different from that in the human, in whom
inspiratory inhibition and expiratory excitation of MSNA
occurs even when respiratory motor output is increased via
high CO2 or high voluntary drive to breathe (Figure 8, Seals
et al5).
Our findings showing a dissociation between respiratory
motor output and MSNA during and after hypocapniainduced apnea imply quite different chemoreceptor thresholds for activation of the respiratory and sympathetic outputs
(see Figure 6). This concept is consistent with 2 sets of
findings in anesthetized cats. Huang et al14 showed that
transient carotid chemoreceptor stimulation (via sodium cyanide) during hypocapnia-induced apnea stimulated cervical
sympathetic outflow in the absence of phrenic nerve activity.
Trzebski and Kubin15 demonstrated that the PaCO2 threshold
for sympathetic activation during posthyperventilation apnea
was 36 mm Hg, whereas the threshold for resumption of
phrenic activity was 44 mm Hg.
Our conclusions concerning the absence of an effect of
respiratory motor output on MSNA are predicated on the
assumption that passive PPV eliminated central respiratory
motor output. Truly passive mechanical ventilation markedly
reduces phasic output from medullary respiratory neurons.16,17 However, it may not inhibit all medullary respiratory neuronal activity; in fact, tonic expiratory nerve activity16 and oscillatory glossopharyngeal nerve activity18 have
been shown to persist during phrenic apneas secondary to
mechanical hyperventilation. The significant prolongation of
expiratory time after each passive trial indicated that any
residual component of central respiratory drive that remained
during the mechanical ventilation was not sufficient to initiate
a breath once the ventilator was turned off. So, phasic
respiratory motor output, if not completely eliminated, must
Within-Breath Changes in Sympathetic Activity
467
have been markedly inhibited. Furthermore, despite this
greatly reduced central respiratory motor output during passive PPV, the amplitude of the within-breath variation in
MSNA was observed to be equal to that shown during active
voluntary hyperventilation against a resistive load, when a
very large amount of inspiratory effort was required to
generate the same VT.
Influence of Arterial Pressure Changes
In agreement with most previous studies in humans4,5 and
intact cats,2 we showed that MSNA changed reciprocally with
changes in arterial pressure. This correlation between the
within-breath pattern of change in arterial pressure and the
respiratory modulation of MSNA was unaffected by PPV in
a background of either very high (assisted) or 0 (passive)
respiratory motor output, or by dramatically increasing inspiratory effort by voluntary efforts during resistor breathing.
Whereas the within-breath changes in diastolic pressure
averaged ,3 mm Hg under all conditions, small changes in
baroreceptor input have been shown to trigger large changes
in MSNA both in humans and intact animals.19,20 A strong
influence of breathing-related arterial baroreceptor activity on
sympathetic outflow was also demonstrated in anesthetized
cats. Respiratory modulation of MSNA (peroneal nerve)
during hyperventilation-induced phrenic silence was unaffected by vagotomy but was eliminated by bilateral carotid
occlusion.2
Our findings suggest that during normal negative-pressure
breathing, sympathetic inhibition during inspiration is caused,
at least in part, by activation of carotid and aortic baroreceptors resulting from a rise in intravascular pressure and decline
in intrathoracic pressure.21 Interestingly, during PPV, the
aortic arch baroreceptors were most likely deactivated during
inspiration at the same time that the carotid sinus baroreceptors were activated; ie, the 2 sets of receptors provided
conflicting information to the central nervous system. In this
situation, we also observed sympathetic inhibition during
inspiration similar to that observed during normal negative
pressure ventilation. Does this mean that carotid sinus baroreceptors predominate over aortic arch baroreceptors in the
respiratory modulation of MSNA? Other investigators have
advanced the hypothesis that neither set of receptors is
predominant under all conditions.22 Instead, when inputs
from aortic arch and carotid sinus baroreceptors conflict, the
net sympathetic response is determined by the input from
the set of receptors that is activated. In the present study, the
sympathoinhibition observed during inspiration with positive
pressure ventilation was most likely the result of activation of
carotid sinus baroreceptors caused by a rise in intravascular
pressure, suggesting that the carotid reflex was dominant in
that instance.
Influence of Lung Stretch
Activation of pulmonary stretch receptors also has a direct
inhibitory action on sympathetic activity in anesthetized cats
and rats.23,24 However, data from patients without intact
pulmonary innervation below the carina (as a result of
orthotopic heart-lung transplantation) suggest that intact lung
inflation reflexes are not obligatory for the respiratory oscil-
468
Circulation Research
September 3, 1999
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lation in MSNA during eupneic breathing, but that vagally
mediated lung inflation feedback is the primary mechanism
through which the within-breath variation in muscle sympathetic discharge is augmented at high VT in the human.5 We
showed that the level of MSNA, at any given change in
diastolic pressure, was higher at low versus higher lung
volume phases of the breath cycle, which is suggestive of an
independent effect of lung inflation on sympathetic outflow.
However, the greatest influence of afferent input from pulmonary stretch receptors may be in modulating sympathetic
responsiveness to baroreceptor influences. In fact, our data
showed that the slope of the relationship between changes in
diastolic pressure and MSNA was greater at lower lung
volumes (see Figure 5). These correlative findings are consistent with reports showing that the sensitivity of the
sympathetic nervous system to baroreceptor influence fluctuates during breathing, with the greatest responsiveness occurring at low lung volumes when spontaneous MSNA is
highest.4,25
Influence of Intrathoracic Pressure
The use of positive intrathoracic pressure to produce passive
ventilation introduced additional confounding influences on
venous return, cardiac filling, and aortic transmural pressure.
However, we did not find any effect of positive pressure, per
se, on the respiratory modulation of MSNA as shown by the
similarities in the patterns of MSNA modulation observed
during voluntary hyperventilation (negative pressure) and
assisted mechanical ventilation (positive pressure) at similar
high levels of central respiratory motor output and VT.
Macefield and Wallin26 also showed no significant effect of
PPV on respiratory modulation of MSNA in humans; however, they also reported that respiration-associated changes in
arterial pressure were in the opposite direction during positive
versus negative pressure ventilation.26
RSA
Strong evidence in support of a dominant role for central
respiratory motor output in causing RSA was provided by the
persistence of heart rate modulation, occurring synchronously
with phrenic nerve activity, reported in the anesthetized dog
during constant flow mechanical ventilation, which eliminated phasic inputs related to respiration.27 Similarly, our
findings also point to an effect of central respiratory motor
output as indicated by the reduction in RSA amplitude during
passive versus active mechanical ventilation (see Figure 3C
versus 3D) and also during voluntary increases in VT at high
versus very high levels of respiratory motor output (see
Figure 4A versus 4B). However, we also observed that RSA
was greatly attenuated during assisted PPV, even when
respiratory motor output was very high (see Figure 3B versus
3D). These findings are consistent with the previously reported effects of PPV in greatly attenuating RSA compared
with the persistent RSA observed during negative pressure
mechanical ventilation.28 Such findings point strongly to
atrial stretch and cardiac vagal afferent activity as important
modifiers of RSA. Secondly, intact neural feedback from
pulmonary stretch receptors also seems to be obligatory to
RSA as shown by the absence of neurally mediated RSA in
lung transplant patients (with intact hearts)– even in the
presence of large superimposed respiration synchronous
swings in intrathoracic pressure and also in central respiratory
motor output.11 In turn, as with the respiratory modulation of
MSNA (see previous section), feedback from lung inflation
may be important to RSA because of its ability to limit
accessibility of medullary vagal neurons to sensory input
from systemic baroreceptors.25 So, to date, two apparently
obligatory feedback mechanisms for RSA have been identified in the human, namely those related to intrathoracic
pressure and those related to lung inflation. These mechanisms are clearly not redundant, because neither one alone
will produce RSA; rather, they may be mutually dependent.
In summary, it is clear that cardiovascular and pulmonary
feedback mechanisms have different relative influences on
the within-breath changes in MSNA as compared with the
respiratory modulation of heart rate. First, passive PPV
abolished RSA but had no effect on the respiratory modulation of MSNA. Second, the within-breath variation in heart
rate was critically dependent on input from pulmonary stretch
receptors,11 whereas lung denervation did not alter breathingrelated oscillations in MSNA during eupneic breathing.5
Finally, our present data using passive versus active mechanical ventilation also showed that central respiratory motor
output, per se, had a negligible independent role in the
respiratory modulation of MSNA in the intact human but did
contribute to the amplitude of RSA.
Acknowledgments
This work was supported by the National Heart, Lung, and Blood
Institute (Grants R01-15469 and R29-57401), the Veterans Affairs
Medical Research Service, and (in part) by a research fellowship
from the American Heart Association of Wisconsin and the Hazel
Mae Mayer Foundation (to C.M.S.). We thank D. Puleo, D.F.
Pegelow, and A. Jacques for technical support, and Dr B. Goodman
for his assistance with the statistical analyses.
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Role of Respiratory Motor Output in Within-Breath Modulation of Muscle Sympathetic
Nerve Activity in Humans
Claudette M. St. Croix, Makoto Satoh, Barbara J. Morgan, James B. Skatrud and Jerome A.
Dempsey
Circ Res. 1999;85:457-469
doi: 10.1161/01.RES.85.5.457
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