Comparative
of breathing
aspects of the dynamics
in newborn mammals
JACOPO
P. MORTOLA
Department
of Physiology,
McGill
University,
Montreal,
MORTOLA,
JACOPO P. Comparative
aspects of the dynamics
of breathing
in newborn mammals.
J. Appl. Physiol.: Respirat.
Environ.
Exercise Physiol.
54(5): 1229-1235, 1983.-Static
and
dynamic properties
of the respiratory
system have been studied
in anesthetized,
tracheostomized
newborns of six species, ranging in size from rats to piglets. Respiratory
system compliance
(Crs), total resistance of respiratory
system (Rrs), and expiratory time constant
(7) have been measured
in the paralyzed
passively ventilated
animals. Crs is found to be proportional
to
body weight (BW0m8’) and Rrs to BW-0*75; T is independent
of
body size, the shortest value being in kittens and guinea pigs
and a value of about 0.14 s in the other species. Including
the
upper airway resistance,
7 becomes approximately
0.22 s. This
value is similar to the expiratory
time of the fastest breathing
species; therefore
in the smallest
species the high breathing
rate can be regarded
as a mechanism
to raise end-expiratory
level. On a few occasions, dynamic
lung compliance
and pulmonary
resistance,
measured
in spontaneously
breathing
kittens, puppies, and piglets were, respectively,
smaller and larger
than Crs and Rrs, suggesting that the hysteresis of the pressurevolume curve may be substantial.
Rrs was almost linear within
the volume
and flow range investigated,
with the Rohrer’s
constant
K2 always being less than 2.5% of KI. The Reynolds
number
increases with body size (a BWoe51) more than is predictable
from the changes in tracheal diameter,
since the tracheal flow velocity is not an interspecific
constant.
resistance;
compliance;
respiratory
time
number; flow-volume
curve; allometry
constant;
Reynolds
METABOLIC
RATE of newborn
animals implies
a relatively high minute ventilation.
This can be achieved
with a high tidal volume (VT) or an elevated breathing
frequency, or both. Each pattern has its energetic implications. One deep breath requires an elastic work substantially larger than the sum of the elastic work of two
breaths of half depth (18). The frictional
work increases
proportionally
with the breathing rate. In adult mammals
and humans it has been shown that the commonly chosen
pattern of breathing minimizes the respiratory
work (2,
18) and/or the average force of the respiratory muscles
(12)
From a functional
point of view, the importance
of a
small dead space is also evident, particularly
in small
animals like newborns with a high breathing
rate, The
exponential
increase in resistance with the decrease in
airway diameter is a constraint to the reduction in dead
space. Together with the smaller frictional
work, a relatively small airway resistance for a given compliance
implies a short time constant (T), which can be particuTHE HIGH
0161-7567/83/0000-0000$01.50
Copyright
0 1983 the American
Physiological
Quebec H3G 1 Y6, Canada
larly advantageous
inasmuch as it provides a more ready
volumetric
response for any given pressure applied. On
the other hand, a relatively
high T-to-expiratory
time
ratio may help in raising the end-expiratory
level, as
suggested in infants (17). If either case was an important
aspect of the newborn ventilatory
dynamics, one would
expect to find a body size dependency of 7 and possibly
an interspecies constancy of the product of 7 and breathing rate, as suggested for adult animals (6, 20).
In this work, the dynamic properties of the respiratory
system of newborns of several species have been analyzed. One aim was to provide
some data to further
understand
the respiratory
mechanics of newborn mammals, which have been as yet studied only in their static
the comparative
apaspects (1, 3, 8, 9). Additionally,
proach over the body size range from rats to infants
enables us to derive some conclusions on the structurefunction
relationship
of the respiratory
system. This
information,
even though it should be cautiously generalized, can offer some basis for further studies on respiratory mechanics in the neonatal period.
METHODS
Experi.ments were performed on six species of newborn
animals (rats, rabbits, guinea pigs, cats, dogs, and pigs)
within the first week of life. Ages and body weights are
presented in Table 1. Animals were anesthetized
with
pentobarbital
sodium (30 mg/kg body wt ip) and placed
in the supine position, and a tracheal cannula was inserted just below the larynx. One arm of the tracheal
cannula was connected to a Statham pressure transducer
for tracheal pressure measurements.
The characteristics
of the instrumental
connections
and the tracheal Reynolds number (Re) were such that the error determined
by the Bernoulli
effect in the measurement
of the lateral
tracheal pressure was probably less than 10% even in the
largest species (5). The cannula was connected to a
pneumotachograph
for measurements
of respiratory flow
and, by integration,
VT was measured. The pneumotachograph consisted of a small cannula (ID 0.7-2.6 mm,
length 22-25 mm, depending
on the species) with two
sidearms connected to a differential
pressure transducer
(Hewlett-Packard
model 270). The pneumotachograph
was tested by connecting it to a ventilator,
which provided oscillatory flows into a sealed box; the pressure in
the box and the electronically
integrated
signal of the
differential
pressure of the pneumotachograph
were simultaneously
recorded and displayed on an X-Y storage
Society
1229
J. P. MORTOLA
1230
oscilloscope. No loops were observed up to the maximal
frequency of the ventilator
(107 cycles/min).
The linearity of the flow signal was also checked up to flows of 150
kg-l.
In kittens, puppies, and piglets, a saline-filled
catheter
was placed in the lower third of the esophagus and
connected
to ,a Hewlett-Packard
pressure transducer
(model HP-1280C)
for measurements
of esophageal pressure. The ID of the catheter was 1.4 mm and length 40
cm; its frequency response was adequate up to 107 cycles/
min. In the smaller species, the frequency response of a
ml
l
s-l
l
1. Compliance
TABLE
No. of
Animals
Rats
5-6
Rabbits
9
Guinea pigs
5
and resistance
5-7
in newborns
Age
BW
Crs
3.6
to.2
3.9
0.014
to.002
0.102
kO.037
0.116
kO.013
0.171
to.044
0.498
-+0.109
1.047
to.065
kO.015
0.185
t0.068
0.144
to.014
0.363
kO.164
0.698
to.254
1.952
to.076
k1.3
Kittens
values
4.3
kO.9
3.9
-+1.8
Puppies
6
3.8
t2.9
Piglets
3
2.0
kO.5
catheter narrow enough for a proper esophageal placement was not adequate. Therefore
in newborn rabbits,
guinea pigs, and rats, the esophageal pressure was not
measured.
After a period of spontaneous
ventilation
(Fig. 1)) a
paralyzing agent (succinylcholine
chloride 10 mg/kg iv,
or pancuronium
bromide in excess of 1 mg/kg iv) was
then injected. Additional
doses of the anesthetic and
paralyzing drugs were administered
as needed. Mechanical ventilation
was initiated at a frequency of 20 breaths/
min and an end-inspiratory
pressure of 7 cmHa0 (Fig. 1).
0.059
Rrs
7 rs
2.663
to.892
0.747
to.373
0.399
to.098
0.254
to.130
0.257
to.174
0.068
to.013
0.148
to.012
0.123
kO.045
0.057
to.012
0.073
kO.019
0.154
to.046
0.133
to.030
CLdyn
0.929
k0.542
0.697
to.343
2.607
S.799
TPR
0.148
to.052
0.292
to.089
0.052
to.020
K
1 rs
2.481
kO.988
0.598
to.354
0.385
kO.072
0.254
to.130
0.250
kO.180
0.063
to.013
K2
rs
0.0378
to.0357
0.0141
kO.0183
0.0014
to.0032
0.0003
t0.0006
0.0003
to.0004
0.0001
to.0001
Values are means t SD. No. of animals, min-max; age (days); BW, body weight (kg); Crs, compliance of the respiratory
system (ml/cmHnO);
dynamic lung compliance (ml/cmH20);
Rrs, total resistance of the respiratory
system (cmH2O.m1-’
s); TPR, total pulmonary resistance
system; rrs, expiratory time constant of the respiratory
system (s).
(cmH20 r-r&‘. s); K1 rs and K2 rs, Rohrer’s constants of the respiratory
CL
dyn,
l
V
20
i
25
ml
ml
se;’
PTR
2
cm
P
3
cm H20
PL
V
v
10
15
ml
I
t-i201
ml
seii’
PTR
5
cm
H20
P
3
cm
t-i20
PL
I
I
I
I
I
I
FIG. 1. Newborn
piglet, 0.98 kg, 1.5
days. Experimental
records (top to hottom) of tidal volume (V), airflow
(V),
tracheal pressure (Ptr) , and esophageal
pressure (Ppl). Inspiration
is upward. A,
animal is spontaneously
breathing, and
dynamic lung compliance
and pulmonary resistance can be measured. B, animal is paralyzed and ventilated. C, airways are occluded at end inspiration
to
measure compliance of respiratory
system. Slope of flow-volume
curve after
reopening
of airways
represents
time
constant
of respiratory
system.
Time
mark, 1 s. High-amplitude
oscillations of
esophageal pressure wave in C are artifacts.
RESPIRATORY
DYNAMICS
1231
IN NEWBORNS
For measurements
of the expiratory
flow-volume
curve,
the lungs were periodically
inflated to a known amount,
and the following
expiration
was recorded at a higher
paper speed (125 mm/s) (Fig. 1).
Body temperature
was measured with a rectal probe
in kittens, puppies, and piglets and maintained
around
37°C through
the whole experiment
by adjusting the
distance of a heating lamp. In all the species, the signal
obtained with three subcutaneous
electrodes was fed into
a loudspeaker for continuous monitoring
of cardiac activity. Respiratory
airflow, VT, tracheal pressure, and esophageal pressure were simultaneously
recorded on a multichannel pen recorder (Gould model 250).
Dynamic
lung compliance
and total pulmonary
resistance. These measurements
were obtained in kittens,
puppies, and piglets. At least six breaths were analyzed
in each animal. The pleural pressure wave and the inspiratory flow wave of the spontaneous breaths were digitized. The data were fed into a Hewlett-Packard
microcomputer (HP-85) that integrated
the flow signal every
0.02 s to obtain the inspired volume and constructed the
pressure-volume
(P/V, y-axis) vs. flow-volume
(V/V, xaxis) plots. According to the equation of motion of the
lung, P=V/C+RV
and therefore P/V= l/C+R
V/V; the
interest of this representation
is that the slope of the
function
represents
the total pulmonary
resistance
(TPR), and the intercept is the reciprocal of the dynamic
lung compliance
( CL~& (6, 7, 15). Even though the
relationship
was very close to a linear function
(4, 11),
indicating that the flow resistance component related to
turbulence was negligible, polynomial
regression analysis
was used to derive CLAIM, and the first and second Rohrer’s constants, K1 and K2, according to the equation
P/V=~/CL~~~+K&V-+K&~/V.
For each animal, the
mean inspiratory
flow (Vinsp)
was then computed,
and TPR was calculated as K1+K&nsp.
In each animal
the average coefficient
of variation
of these parameters
was less than 10%.
Compliance,
resistance, and time constant of the respiratory
system. These measurements
were obtained in
all the animals after paralysis and under artificial ventilation. Respiratory
system compliance
(Crs) was measured by occluding the airways at end inspiration
and
measuring the corresponding
tracheal positive pressure.
Since all the animals were ventilated
at about 7 cmH20,
Crs was therefore computed on the linear portion of the
pressure-volume
diagram (8) with a similar inflating pressure. The passive time constant of the respiratory
system
was calculated
through
the analysis of-the expiration
after the release of airway occlusion above the end-expiratory level (4, 11, 25) (Fig. 1). For this purpose, expiratory loops of flow (x-axis) and volume ( y-axis), measured every 0.02 s, were constructed
as previously
described (15). Flow data were obtained by continuous
digitizing of the expiratory
flow signal. This input was
fed into a Hewlett-Packard
microcomputer
(HP-85) that
provided
the integrated
signal every 0.02 s. Using this
method, any possible time lag between flow and volume
was avoided. Then the expiratory
flow (x-axis) and driving pressure (y-axis) loops were constructed,
as at any
given volume the driving pressure of the passive expiration is equal to V/Crs. The late portion of the pressurel
flow curve so constructed
was very close to a linear
function. Nevertheless,
in some cases a slightly better fit
could be obtained with a second-degree polynomial equation. The first and second Rohrer’s constants (K1 and K2)
for the whole respiratory
system have been computed,
and the sum K1 + K~*Vexp
(where Vexp is the
mean expiratory
flow) will be referred
to as the total
resistance of the respiratory
system (Rrs). The resistance
of the pneumotachograph
was in each case less than 7%
of Rrs and is included in the Rrs value. From Crs and
Rrs the passive expiratory
time constant (rrs) of the
respiratory
system was obtained (Tag = Rrs Crs).
AZZometry. The equations relating respiratory variables
to body weight (allometric
equations)
were calculated
from the raw data of the present study (from rat to
piglets) and previous data in infants (15). The exponents
and intercepts relating the variables are derived from the
least-squares regression analysis of the logarithm
of the
basic data. When more than one value for a parameter
was available
for a single species, the mean value was
used for the regression analysis, even though all data
points were plotted. The critical values of the correlation
coefficients
(r) and differences
of the slopes (b, the
exponent of the log-transformed
equation y = axb) were
tested for a (P < 0.05) level of significance
for a twotailed t test.
With the allometric
cancellation
(21) one allometric
equation is divided by another allometric equation. The
remaining exponent is termed residual mass index and in
general, if it is less than 0.08, not statistically
different
from zero, and the quotient
may be considered
size
independent
(21).
l
RESULTS
Mean values and standard deviations of age, body size,
Crs, Rrs, K1, and K2 are presented for each species in
Table 1.
In the body size range from rats to infants, the slope of
the log-log function between Crs and BW is 0.804, significantly less than unity (Fig. 2 and Table 2), indicating a
small trend to a relatively
stiffer respiratory
system in
the larger species.
The expiratory
flow-volume
loops for a representative
newborn of each species are shown in Fig. 3. The last
portion of expiration was very close to a linear function
in all the animals (the correlation coefficient never below
0.90), indicating
that the lower respiratory
tract of the
newborn,
within the experimental
range of flow and
volume, practically
behaves as a linear resistor. The
second Rohrer’s constant K2, derived from the polynomial analysis of the P/V relationship
as described in
METHODS,
was in fact very small (Table l), ranging in the
different species from 0.1% (kittens and puppies) to 2.4%
(newborn rabbits) of the EC1value.
The resistance of the respiratory
system (Rrs = K1 +
K2Vexp)
decreases in the different species in proportion
to BW-Oe7” (Fig. 4 and Table 2). This slope is significantly
different from unity, indicating that, at least for the size
range considered, Rrs tends to decrease progressively
less
with the increase in species size. Obviously a similar but
inverse exponent correlates the maximal expiratory flow
J. P. MORTOLA
1232
Hz0
mbcm
-1
0. 80
SLOPE
( r
0.98)
NTS
ii
RABBITS
0
0
/
0.001
RATS
I
I
I
1
0.01
0.1
1
10
f
BW
FIG. 2. Compliance
Data for infants
TABLE
fnY =
of respiratory
from Ref. 15.
2. Equations
system
describing
(Crs, mlcmH20-‘)-body
several
respiratory
weight
variables
(BW, kg) relationship
as functions
kg
(in log-log scale) in newborns
of 7 species.
of body weight
aAxb) in newborns
Species No.
(body size
range,
rats-infants)
ACRS = f(ABW)
ARRS = f(ABW)
Avmax* = f(ABW)
AV” = f(ABW)
ARe* = f(ABW)
7
7
7
7
7
Intercept of
Log-Log Linear
Regression
0.133
1.994
1.933
3.007
3.355
a
1.36
98.5
86
1,016
2,266
Correlation
Coef r
b
0.804-f-
0.980
0.973
-0.75q
0.7891
0.2305
0.510t
0.977
0.887
0.964
Level of Signif of
r’
SDofb
co.01
(0.01
CO.01
0.0666
0.0728
co.01
co.01
0.0490
0.0571
0.0594
BW, body weight (kg); CRS, compliance of the respiratory
system (m.lcmH~O-‘);
RRS, resistance of the respiratory
system (cmHnO*l-’
es);
vmax, maximal expiratory flow (ml. s-l); V, linear flow velocity (cm. s-l); Re, Reynolds number. ILevel of significance of r using a two-tailed
test.
* Computed in the trachea at a driving pressure of 7 cmHaO. t Significantly
different from unity (P < 0.05). $ Significantly
different from unity
(P < 0.02), not significantly
different from 0.804 (P > 0.05.). 5 Significantly
different from zero (P < 0.05).
(vmax), achieved with a given driving pressure, to body
weight (vmax a! BWos7’, Table 2). Since the cross-sectional area (A) of the trachea of the newborn species
from rats to piglets is proportional
to BW”*56 (13), it
follows that the tracheal linear velocity (v) = vmax/A
is
proportional
to B W(“*7g-o.56)= BW”*23 (Table 2). Individual
computations
at a driving pressure of 7 cmH20 indicate
that the linear flow velocity is 324 cm. s-l in newborn
rats and as high as 933 cm. s-l in piglets. At the same
driving pressure, from the v and previously
published
data on tracheal diameter
(D) (13), it is possible to
calculate in each species the Reynolds number, Re =
(vDd)/y,
where d is the density of the air (0.001226 g/
ml) and of is its viscosity (0.00018 poise); Re is proportional to BWos51 (Table 2).
The respiratory
time constant (Tag = Crs Rrs) for .the
size range from newborn rats to infants can be derived to
be proportional
to BW(0*80-0*75) = BW”*05; this residual
mass index, being below 0.08, has probably no physiological significance (21). Close examination
of the individual
values of 7rs in the six species studied (Table 1) reveals
that kittens and guinea pigs have a short 7rs, while both
smaller and heavier species have very similar values, in
average 0.14 s.
The values of CL+~ and TPR for kittens, puppies, and
piglets are presented in Table 1. The range in body size
and the number of values are too small for a meaningful
allometric analysis. It can be noted that in the puppy the
mean value of CL
is slightly less than Crs; by considering each animal individually,
it appears that this was
the case in three puppies and two piglets. The mean
value of TPR in the puppy was larger than Rrs, and, on
an individual
basis, this was the case in two puppies and
one kitten. The product of CLdyn and TPR, which repredyn
RESPIRATORY
DYNAMICS
N. 6.
1233
IN NEWBORNS
RAT
N.B.
2
4
6
8
10
N.B.
RABBIT
2
4
6
8
10
GUINEA
2.4
PIG
4.8
7.2
9.6
12
W
N. B.
2
N.B.
KITTEN
7.2-
-J
0
0
>
5.84.3-,
6’
7.2
14.4
/’ #’
,’
#’
,’
,’
/ ,’
02
PUPPY
N. 6.
lo-
,002
0
'
H,O
cm
. I
0
2
6-
21.6
-1
,’
8-
28.8
36
10
20
30
ml-
FIG.
3. Expiratory
flow (x-axis, ml. s-‘)-volume
(y-axis, ml) relationships after reopening of the airways previously occluded at various lung
volumes. One animal of each species is presented. Slope of late portion
RS
40
0 /’
FLOW,
R
PIGLET
40
set
50
40
80
120
160
200
-1
of curve is almost linear and represents time constant
system, 3 values are indicated by dashed Lines.
of respiratory
set
SLOPE
-0.75
( r
0
97)
1000
cl
A
cl
RABBITS
3
0
GUINEA
A
\l
0
l
PIGS
KITTENS
100
PUPPIES
PIGLETS
INFANTS
10
0.001
FIG. 4. Respiratory
system
in infants from Ref. 15.
0.1
0.01
resistance
(Rrs, cmI-LOmll
+)-body
weight
sents the inspiratory
time constant of the lung (TL), is in
each species greater than TV, the TL/T~~ ratio being 188%
in kittens, 132% in puppies, and 102% in piglets.
DISCUSSION
In a previous
is proportional
study we observed that Crs of newborns
to BW”eg2 (9). In the present study the
1
(BW, kg) relationship
10
(in log-log scale) in newborns
of 7 species. Data
exponent relating Crs to BW is only 0.80, and being
significantly
different
from unity indicates that in the
size range from rats to infants, the respiratory
system has
a small trend to become progressively
stiffer. The difference in the exponents could be due to the different
species sampled, to differences
in methodology,
and to
the slightly more restricted
range in body size of the
1234
J. P. MORTOLA
present study. If the data pertinent
to newborn goats
were included w, as previously
done (9)) the exponent
would increase to 0.89. It is important
to stress, as it has
been pointed out (22), that with the allometric approach
both the “narrow”
and “expansive”
views can be misleading. The former can lead to erroneous conclusions if
simplistic extrapolations
are done; the latter may overlook subtle trends characteristic
of certain ranges. In this
work comparison is made on the 200-fold body size range
from newborn
rats to infants; no data are presently
available to indicate that extrapolation
of a trend out of
this range is necessarily correct.
The respiratory
system time constant does not seem
to have a body size dependency.
Newborn rats have 7rs
values, similar to puppies and piglets, of about 0.14 s.
This value can be substantially
higher if laryngeal and
nasal resistance were included. In the puppy, upper airway resistance (Rua) is about 0.075 cmHz0 l rl1. s. kg
(14), or about 37% of Rrs + Rua. Therefore
7rs of the
whole system, including
the upper airways, would be
about 0.23 s. It is interesting that in the kitten, which has
a relatively
smaller Rrs, the upper airway resistance is
relatively larger (14), such that the 7rs of the whole system
is not smaller than in the puppy. If the (Rrs + Rua) kg
values found in kittens and puppies were also present in
the other species, it appears that the total expiratory
7rs
can be about 60% higher than the value measured in our
tracheostomized
species, approximately
of the order of
0.22-0.23 s, close to the 0.21 s that was measured in l- to
&day-old
infants (15). With an inspiratory
time-to-total
breath duration ratio (TI/TT)
of 0.5, as it is often measured in infants
(15), only at frequencies
above 130
breaths/min
would the expiratory
time be too short for
an adequate emptying of the lungs. Vinegar et al. (24)
described mathematically
the relationship
between VT,
respiratory
frequency
(f) and the difference between the
end-expiratory
level, and the passive resting volume of
the system (FRC-Vn). A simpler form of the equation for
unobstructed
expiration lasting one-half of the cycle in a
system with linear compliance and resistance, is (FRCV) = VT/( ,3o/fT-l ) (modified from Ref. 24). In an unanesnthetized puppy, with a VT of approximately
5 ml and
a rate of 60 breaths/min,
FRC-VR would be 0.57 ml.
Since VR is about 11 ml (8), the increase in end-expiratory
level would correspond to only 5% VR. In other words, on
the basis of our 7rs measurements,
the possibility that the
end-expiratory
level is maintained
above the passive
resting position of the respiratory
system as the unique
result of the high breathing rate (17) would seem realistic
only for the smallest newborn species, which have very
high breathing
frequencies
(13). In infants this mechanism to raise the end-expiratory
level may apply during
the marked tachypnea of the first hours of life (10) or in
combination
with other mechanisms aimed to maintain
an elevated lung volume, such as breaking of expiration
with inspiratory
muscle activity or glottis closure (10, 15,
16) .
From a structural
point of view, a high chest wall
compliance is an unavoidable
charac teristic of newborn
mammals, which must undergo a substantial
squeezing
through the pelvic canal at birth. From a functional point
of view, however, the consequent long time constant may
l
decrease the efficiency
of the ventilatory
pump. The
distortion
of the chest wall during inspiration
due to an
inadequate
support of the contracting
diaphragm,
the
instability of the system to external or internal loads, and
the high rate of pressure development
to inflate the lungs
would all be functional
problems in an animal breathing
at a high rate and with a long T~~.Several factors could
shorten the inspiratory
7rs to acceptable functional
values. The laryngeal resistance is known to be less during
inspiration.
The compliance of the lung is also probably
smaller during inspiration
than during expiration.
In a
few of our animals CL dyn was even smaller than Crs and
TPR was higher than Rrs, possibly suggesting a substantial hysteresis of the lung. Finally, it is likely that in
dynamic conditions
the pressure losses to expand the
chest are higher than estimated from the passive pressure-volume
curve of the respiratory
system, effectively
reducing the compliance. In fact, part of the muscle force
potentially
available
for inspiration
does not generate
pressure because it is lost during the shortening
of the
inspiratory
muscles (force-length
relationship)
and their
contraction
with a finite velocity (force-velocity
relationship). In infants these latter factors can effectively reduce
the inspiratory
time constant by about 30% (15).
In all of the newborns the late portion of the flowvolume curve was very close to a linear function. Indeed,
K2 was always a very small fraction of K1. This is hardly
surprising,
since the small airway dimensions
favor a
small Re. The fact that in spontaneously
breathing
infants K2 was found to be less than 2% of K1 (15) suggests
that the upper airways do not markedly
change the
airflow characteristics.
The difference in the exponent between the allometric
equations of expiratory flow and tracheal cross-sectional
area implies that, in newborns, the linear flow velocity is
not an in terspeci .es constant, distinc t from that suggested
in adults (23). A similar conclusion was previously made
on the basis of tracheal measurements
in newborns of
different
species (13). A corollary
of this is that the
Reynolds
number
is not simply determined
by the
changes in tracheal diameter, as found in adults (5)) but
increases with the newborn size progressively
more than
predictable
from the changes in tracheal dimensions.
Because values of alveolar minute ventilation
during
unanesthetized
eupneic breathing are not available, only
approximative
estimations
of the respiratory
work can
be made. If the alveolar volume is assumed to be two
times the dead space volume, as seems to be the case in
infants (19), it is possible to compute that the elastic
work of a puppy with a 7rs of 0.22 s (including the upper
airways) at the eupneic frequency (54 breaths/min,
Ref.
13) is about 50% of the total. The minimum
work frequency, according to the formula proposed by Otis et al.
(18), would be 43 breaths/min,
while the value corresponding to the minimum force amplitude,
according to
the concept introduced
by Mead (12), would be 59
breaths/min.
Neither
of these values would therefore
differ markedly from the observed rate.
In summary, over the size range from newborn rats to
piglets, the expiratory 7 of the respiratory
system is found
to be relatively constant, since compliance and resistance
change in proportion.
Only in the smallest species the
RESPIRATORY
DYNAMICS
IN NEWBORNS
high breathing
frequency
could be regarded as mechanism per se sufficient
to substantially
raise the endexpiratory
level.
I thank Anna Noworaj for the kind and skillful technical assistance.
I also thank Sandra James and Christine
Pamplin for typing the
manuscript.
1235
This study was supported by the Canadian Medical Research Council and the Hospital for Sick Children Foundation, Toronto.
Address reprint requests to J. P. Montola, Dept. of Physiology,
McGill University,
3655 Drummond
St., Montreal, Quebec H39 lY6,
Canada.
Received
24 May 1982; accepted in final form 23 November
1982.
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