Clinical Science and Molecular Medicine (1977) 52, 61-14.
Acid-base and respiratory changes after prolonged
exposure to 1%carbon dioxide
B. J. W. P I N G R E E
Tenth Submarine Squadron, Royal Navy, and
Department of Bioengineering, University of Strathclyde, Scotland
(Received 13 June 1975; accepted 19 August 1976)
Key words : acid-base equilibrium, blood gas
analysis, hypercapnia.
summary
1. The acid-base and respiratory status of
fifteen healthy male subjects living in an atmosphere of 1% COz in air was studied over the
course of 44 days, during an operational patrol
in a nuclear submarine.
2. Observations were made during a control
period before exposureto COr,at 4 day intervals
during the patrol, and finally during the period
after its completion. Samples of arterial blood
from each subject were analysed for Pa,cor, pH
and Poz immediately after collection. Concurrently mixed expired Pcozand Poz,together
with minute volume and respiratory rate, were
measured.
3. A mild uncompensated respiratory acidosis
in which arterial pH was depressed by 0.02 pH
unit was demonstrated throughout the period
of exposure to COz.This was associated with an
acute rise in Pa,co, of 0.14 kPa, accompanied
by an increase in minute volume from 11.5 to
15 I/min. By mid-patrol minute ventilation had
returned to control value and a further elevation
in Pa,cor of 0.31 kPa was detected together with
a rise in plasma bicarbonate of almost 1 mmol/l.
4. On return to air after completion of the
patrol, the acid-base changes appeared to be
quickly reversed. Minute volume decreased
slightly initially, but it too subsequently
returned to the control value. A fall in Pa,or
of about 2.5 kPa was recorded at this time,
together with a reductionin f o r d vital capacity
of 8%.
Introduction
In recent years interest has grown in the physiological problems associated with the operation
of undersea and space vehicles. As a consequence of this it has been necessary to define a
maximum acceptable limit for the atmospheric
COz concentration in such vehicles to permit
safe prolonged exposure. This has stimulated a
number of studies into chronic hypercapnia in
man at COr concentrations of up to 4% over
periods of from 3 to 56 days (Chapin, Otis &
Rahn, 1955; Schaefer, Hastings, Carey &
Nichols, 1963; Glatte, Motsay & Welch, 1967;
Clark, Sinclair & Welch, 1971). Of these studies
the one most comparable in scope with this
present work is that of Schaefer et al. (1963), in
which twenty-one subjects were exposed to
1.5% CO, for 42 days.
Schaefer et al. (1963) demonstrated the wellestablished acute sequelae of exposure to a
raised ambient COz concentration. Thus for his
subjects there was an initial rise in mean PA,CO~
of 0.25 kPa together with a fall in venous blood
pH of 0.06 unit and an elevation in minute
ventilation of approximately 1 1 min-I m-z.
After some 23 days, adaptation (defined by a
compensation of the respiratory acidosis) had
occurred. On the basis of this and other work,
Schaefer (1961) postulated a triple-tolerance
concept to account for changes he had shown
after chronic exposure to various COz concentrations. For COz concentrations of 3% down
Correspondence: Dr B. J. W.Pingree. G. D. Searle
and Co.Ltd, High Wycombe, Bucks., U.K.
67
68
B. J , W. Pingree
to 0.8% this concept envisages slow adaptive
processes in electrolyte exchange and acid-base
regulation which might induce pathophysiological states on greatly prolonged exposure.
For COz concentrations below 05-04%
Schaefer (1961) suggests that no significant
adaptive changes occur, although he states that
this last concentration is not yet fully established.
For prolonged submarine operations it is
desirable that this last concentration described
by Schaefer is closely studied even if it should
require work similar to Schaefer’s at COz
concentrations only slightly less than 1 5 % .
Thus it is important to know whether there is
indeed a COz threshold below which adaptive
changes no longer occur and whether the
threshold can be accurately quantified. In
current nuclear submarine practice COz can be
conveniently kept to between 0.5 and 1% but
far greater demands are made on air-purification machinery if COz concentrations of less
than 05% are required. Only detailed knowledge of the physiological response to prolonged
exposure to low concentrations of COz will
indicate whether the effort should be made to
reduce operating COz concentrations.
In support of the limits proposed by Schaefer
for his lowest COz concentration, Peck (1971)
was unable to detect any evidence of respiratory
acidosis during a submarine patrol in which
atmospheric COz had a mean value of 0.85%.
This work, however, was based on measurements
in arterialized capillary and venous blood, and
was confined to blood-gas and pH analysis,
with some plasma electrolyte study, there being
no measurement of respiratory minute volume.
The present investigation was done to
establish whether acute or adaptive acid-base
and respiratory processes are stimulated during
prolonged exposure to concentrations of COz of
approximately 1 %, as existed during an operational submarine patrol. In this work advantage
was taken of recent developments in blood gasanalysis equipment to measure arterial blood
samples immediately they had been taken, in
apparatus installed aboard the submarine. By
the use of arterial blood it was envisaged that a
possible source of variability in the data of some
earlier workers, who had used venous or
capillary samples, would be eliminated. This consideration was important in view of the small
changes, if any, it was expected might occur.
Methods
Subjects
Fifteen healthy male volunteers acted as
subjects in this study. They worked in various
departments within the submarine and they had
each undertaken from nil to five previous
nuclear submarine patrols. No subject had been
on a patrol during the 3 months before the start
of this study. The subjects’ ages ranged from 21
to 37 years. There was no history of respiratory
disease in any volunteer and none was under
treatment for any medical condition at the time
of the start of the experiment. Such medication
as was prescribed to subjects during the study
consisted of no more than the very occasional
paracetamol for headache. Diet was unrestricted
and was of a varied and generally balanced
nature but dietary analysis was not carried out
in this work. Full subject data are presented in
Table 1.
Consent was obtained from each subject to
undertake the procedures contemplated after
explanation of their nature, purpose and risks.
The protocol for the study was approved by the
Flag Officer Submarines, Royal Navy.
Atmosphere
During the patrol the PcoZvaried between 0.8
and 1.2kPa but was generally close to 0.93 kPa.
The ambient pressure in the vessel was maintained in the region of 106 kPa, giving a nominal
C o t concentration of 1%. Oxygen concentration was maintained between 19 and 22% and
carbon monoxide was less than 20 p.p.m. At no
time during the 44days patrol was the submarine
ventilated with outside air.
Experimental procedure
Specimens of radial arterial blood and mixed
expired air were collected for analysis from each
subject during a control period before the start
of the patrol. This procedure was repeated at 4
days intervals as the patrol progressed until the
vessel was opened to the air when it surfaced
after 44 days at the end of the patrol. There
were then three further sampling occasions, at
12 days, 36 days and 3 months after completion
of the patrol. The post-patrol measurements
were restricted by the unavailability of some of
the subjects during this period.
Age (years)
Height (m)
Weight (kg)
Surface area (Dubois, mz)
Smoking habit (cigaretteslday)
Forced vital capacity bcfore
patrol (1 BTPS)
Forced vital capacity after
patrol (l
BTPS)
No. of previous patrols
3.9
3.3
2
5.4
46
1
5.3
4.9
5
24
1.78
76.5
1.94
0
21
1.88
80.8
2.07
25
24
1.78
66.2
1.84
0
3
2
1
4.4
2
3-9
2
4.8
2
5.1
1
3.7
2
4.5
2
5.2
5.1
48
3.1
4.5
4.2
5.5
4 02
34
1.80
68.2
1.87
20
20
1.75
66.8
1.81
20
23
1.70
64.4
1.76
10
25
1.70
66.8
1.77
25
29
1.85
69.8
1.94
22
1.73
79.5
1.94
20
24
1.73
66.5
1.79
20
10
9
8
7
6
5
4
Subject no.
TABLE
1. Data for the subjects of the study
4.1
27
1.70
79.0
1.91
0
11
5.1
37
1.78
64.4
1.82
20
12
5.3
22
1.88
89.5
216
0
13
3.9
32
1.70
63.5
1.73
15
14
4.8
402
31
1.80
65.0
1.84
2
9
3
3
E
$
5
h
3
70
B. J. W. Pingree
Samples were taken from seated subjects
after they had first rested and then accustomed
themselves to the collection apparatus by a
period of breathing on open circuit. Their mixed
expired air was collected over 4 min and the
respiratory rate during this collection period
recorded. As soon as the expired air had been
collected a sample of radial arterial blood was
taken and immediately analysed for Pco2, pH
and Po1 in a Radiometer BMS3 blood gasanalysis apparatus. Blood sampling was performed under local anaesthesia to reduce the
discomfort occasioned by arterial puncture. This
precaution was desirable for the subjects who
were to undergo a series of such punctures, and
also to reduce any hyperventilation which might
otherwise be provoked.
The measuring cuvette in the BMS3 equipment was surrounded by a water bath maintained at a constant 37°C. When the measurements on blood had been completed the apparatus was used to determine the Pcoz and Po1 of
a small sample of the mixed expired air, the
volume of the remainder then being measured
in a dry gas meter. The composition of the
atmosphere in the submarine at the time of
sampling was determined by a Beckman gas
chromatograph installed as part of the vessel’s
routine atmospheric monitoring equipment.
This same apparatus was also used to obtain a
continuous record of ambient CO, throughout
the patrol.
The Pcoz and Poz electrodes were calibrated
immediately before blood sampling from each
subject. Calibration was done with bottled gas
mixtures analysed by Haldane’s method (duplicate analyses within 0.03%). The pH electrode
was calibrated with Radiometer precision buffer
solutions of pH 7.383 +0.005 and 6.841 k0.005,
at 37°C. Plasma [HCOs-] was derived from
P C Oand
~ pH data using the Siggaard-Andersen
(1963) alignment nomogram. The electrode
system was assessed by tonometry in the
Department of Medicine, University of Edinburgh. For the C o t electrode, mean blood-gas
difference at tonometer gas Pco2of 6.34 kPa was
3.99 x
kPa, with a coefficient of variation
on duplicate measurements of the same blood
sample of 1.4% (five sets of duplicate determinations). For the Oz electrode, mean blood-gas
difference at tonometer gas Pozof 15.79 kPa was
9.45 x 10- kPa, with a coefficient of variation
on duplicate measurements of the same blood
sample of 2.4% (nine sets of determinations).
For further details of this work see Pingree
(1973).
Statistical methods
A two-factor analysis of variance was made
from the data, using the interaction of subjects
by times to test the significance between times.
The times considered were control, the initial
and final halves of the exposure period and the
post-patrol period after return to air. For these
periods the mean value of the variables for each
subject was taken to be the best approximation
to the true value. This approach was necessary
because of the absence of data for some subjects
on occasions when radial stab was unsuccessful.
Additionally Schaefer’s triple-tolerance hypothesis would suggest adaptive changes to be
occurring after some 3-4 weeks’ exposure, or
roughly half way through the patrol. Thus
bearing in mind the necessity for meaning of
subject data mentioned, it was decided to
divide the patrol into initial and final halves in
the analysis of variance.
More complete data were obtained for the
control and exposure periods than for the postexposure period when a number of subjects was
unavailable. Accordingly the conclusions made
for this period are generally less precise. The
results for subject no. 11, who did not participate
after the fourth sampling occasion, were disregarded in the analysis. Details of the analysis
of variance appear in Pingree (1973).
Results
Acid-base measurements
The data recorded for arterial Pco2, pH and
[HC03-] are summarized in Fig. 1.
Pa,co2 in the initial half of the exposure
period was significantly elevated by 0.13 kPa
( P < 0.05),relative to a control value of 5.09 kPa.
A further significant rise to 0.46 kPa above
control was demonstrated in the latter half of
the period of exposure ( P < 0.001). On return to
air blood samples taken at the first sampling
occasion showed that Pa,co2 remained elevated
0.13 kPa above control (P< 0.01), but this
elevation was not sustained, there being no
significant difference between control and the
postexposure period taken overall (P < 0.05).
Response to mild chronic hypercapnia
-Air
71
I
T
l.
T
0 4
8 12 16 20 24 28 32 36 40
Time (days)
FIG.1. Time-course of acid-base and respiratory variables for the fifteen subjects studied. Mean
values are plotted. with f 1 SD limits indicated by horizontal bars.
Arterial pH was depressed during exposure to
(P< 0.01).
There was no significant difference (P<0.05)
between arterial pH during the initial and final
periods of the patrol. pH returned to the control
value immediately after the patrol (P< 0.05) and
remained at this throughout the post-exposure
period.
Plasma bicarbonate fell 0.7 mmol/l during the
first half of the patrol, from a control value of
23-5mmol/l (P<0.05),
but during the latter half
of the patrol rose to 24.3 mmol/l (P< 0.01). The
first sampling occasion on return to air on
completion of COz exposure gave a plasma
bicarbonate concentration which was not significantly different from the control value and
this persisted for the whole of the past-exposure
period.
Respiratory measurements
COz by approximately 0-02 pH unit
The data on minute ventilation, respiratory
rate and arterial Poz are summarized in Fig. 1.
Minute volume rose acutely on exposure to
the elevated ambient COs. The magnitude of
the mean acute rise was 3.5 l/min, from 11.5
l/min (P< 0001). After this maximum had been
attained, minute volume subsequently diminished until by the latter half of the exposure
period it had attained a value which was 2 5
l/min (P< 0.05) less than control. Analysis of the
less-complete data for the post-exposure period
suggested a further reduction in minute volume
for this time.
Respiratory rate was elevated during the first
half of the patrol by a mean value of 1.5
breathslmin, relative to a control of 13.5
72
B. J. W. Pingree
breathslmin (P < 0.01). There was no significant
difference between mean respiratory rate for the
latter half of the patrol and control.
Arterial Pozremained unchanged relative to a
control of 12.7 kPa during the initial half of the
period of exposure to the submarine atmosphere,
but rose to 13.7 kPa ( P <0.01) during the latter
half. During the post-exposure period mean
Pa,02 was of the order of 11.3 kPa.
Table 1 shows that for most subjects a
reduction in forced vital capacity was recorded
after the patrol. Mean values were 4.68 I BTPS
(control), and 4.32 1 BTPS (after the patrol)
( P < 0.05).
Discussion
It is apparent that the acute change after
exposure to 1 % COz was a rise in ventilation
opposing an increase in Pa,coz. Thus despite an
ambient Pco, of almost 1 kPa, the rise in
Pa,coz and hence PA,CO,was no more than 0.14
kPa, accompanied by an increase in expired
minute ventilation of 3.5 I/min from the control
value of 1 1 5 I/min. If the relation between
suggested by
minute ventilation and PA,CO~
Lloyd & Cunningham (1963) is considered, i.e.
V , = S(PA,CO~
- B), and assuming Pa,co2 =
PA,co~,
then the acute change from control to
the first sampling occasion gives values of s and
B of 25 1 min-' kPa- and 4.2 kPa (Pa,o, = 13
kPa). These values are typical of those to be
expected.
If compensation of arterial pH is used as the
index of adaptation to chronic hypercapnia,
then there is no evidence that adaptation has
occurred during the exposure studied. Certainly
full compensation of the type specified by
Schaefer could not be demonstrated. Despite
this, Comparison of the two exposure periods
does show that chronically there are adaptive
features. Thus minute ventilation is reduced, and
Pa,coz and plasma [HCOa-] rise. It is accordingly worth examining criteria for adaptation
which have been used by other workers to see
whether any of these might be more applicable
in this instance.
Chapin et 41.(1955), who investigated purely
respiratory variables, proposed that adaptation
could be detected if a reduction in sensitivity of
the respiratory centre occurred after exposure
to a raised CO, environment. Using this index
they found that, after exposure to 3% CO,,
adaptation was achieved within 13 h. After this
respiratory adaptation there was a further
Since no acidsustained elevation in PA,co~.
base data were recorded in this work it is not
possible to compare directly the adaptation
demonstrated by Chapin et al. with that
described by Schaefer. However, Clark et a[.
(1971) in a far-reaching study of exposure to 3 %
CO, for 30 days measured both respiratory and
acid-base parameters. Their conclusion was
that reduction in ventilatory response occurs
more rapidly than compensation of arterial pH.
In addition this compensation was never complete, so that unlike Schaefer (1961) they
referred to maximal rather than full compensation as the adaptive criterion.
Inspection of the results for minute ventilation
shows an acute increase. This is unsustained,
however, so that by the second sampling occasion, on day 8, ventilation has returned to a
value close to control. This acute response
represents a level of ventilation that was significantly greater than that recorded at any other
sampling occasion ( P < 0.001). Although ventilatory response studies were not done in this
work it is assumed that the respiratory adaptation to exposure to 1 % CO, that has apparently
occurred by day 8 is of the type described by
Chapin et al. (1955). The comparatively wide
SD limits for minute ventilation on the initial
four exposure sampling occasions would appear
to be a manifestation of subject variability in
the time-course of ventilatory adaptation.
However, it must also be borne in mind that the
subjects were untrained at the start of the study
and this fact may also contribute to the initial
scatter recorded for this variable.
In a study of mineral metabolism during
exposure to 0.7% COz in a submarine for 7
weeks Gray, Morris & Brooks (1973) demonstrated a period of 10 days during the middle of
the exposure period when acid excretion shows a
marked elevation compared with the rest of the
patrol. Since the net result of renal excretion of
H+ is the addition of HC03- to the body, this
period of acid excretion would suggest that
Compensation of the prevailing respiratory
acidosis is occurring at this time. Such compensation would appear to be of the same nature as
that which was demonstrated for the latter half
of the present study when plasma [HC03-] was
noticeably increased, in association with the
raised Pa,coz. This increase in [HCO1-] is
Response to mild chronic hypercapnia
presumably brought about by a renal response
to elevated Pa,coz (Relman, Etsten & Schwartz,
1953; Sullivan, Dorman, MacLeod & Parks,
1955).
Inspection of Fig. 1 shows that there was a
mean fall in Pa,02 of about 2.5 kPa from 14 kPa
to 11.5 kPa (PeO.01) on return to air after
exposure to CO,. This drop is approximately 1
kPa greater than can be accounted for by a
reduction in oxygen concentration from 22%
(in the latter part of the patrol) to 20.9% of air.
While it is emphasized that the post-patrol data
are less complete than the preceding data, since
many subjects were unavailable at this time,
there was a fall in minute ventilation on return
to air of 2 llmin (P<O.O5), which may have
contributed to the post-patrol PA,o, depression.
A reduction in Pa,oz might also be accounted
for by any increase in the alveolar to arterial
oxygen tension gradient. Calculations of this
variable from the alveolar gas equation to
estimate PA@,revealed no significant difference
(P< 0.05) in the (PA,o,-Pa,o,) gradient in the
population during the periods studied, despite
mean value changes of the order of 1 kPa, to
suggest an increase during the post-exposure
period. An increase in the (PA,o,-Pa,02)
gradient on return to air after exposure to CO,
was reported by Schaefer et al. (1963), although
in their work this had become apparent during
the latter half of the exposure period.
It is noteworthy that the reduction in mean
forced vital capacity recorded at the end of the
patrol was 0.36 I BTPS, from a mean control
value of 4.68 1 BTPS. For most crew members a
submarine patrol represents a period during
which their customary physical activity is
reduced. It is therefore considered most likely
that this forced vital capacity change reflects a
temporary reduction from inactivity, in the
strength of skeletal musculature.
Although the adaptive process demonstrated
in this work was not the compensation of
respiratory acidosis specified by Schaefer,
adaptation under the conditions studied gives
some support to Schaefer’s triple-tolerance
concept. Thus for exposure to CO, concentrations of 1% adaptation did indeed occur, and
time to adaptation corresponded approximately
to that predicted. It is evident from the foregoing that the postulated C 0 2concentration at
which adaptive changes do not occur, must be
below 1 % at least.
73
This study was carried out in the course of an
operational Polaris submarine patrol, so that
some aspects of the work were constrained by
external factors that could not be altered in the
interests of preferred scientific procedure. For
this reason, a comparable laboratory study
would be desirable in which ventilatory response
to CO,, and even cerebrospinal fluid acid-base
data, are obtained, with more thorough coverage
of the control and post-exposure periods.
Acknowledgments
I thank the Medical Director-General (Royal
Navy) for permission to publish this paper, the
Flag Officer Submarines for allowing the work
to proceed, and Miss H. Ferres of the RAF
Institute of Aviation Medicine for the analysis of
variance. I am also indebted to Dr D. C. Flenley,
University of Edinburgh, for advice and help
with this study, to Dr T. Gilchrist, University of
Strathclyde, for continuous support, and of
course to the enthusiastic co-operation of the
subjects who participated.
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