Nature of the interaction between central and peripheral

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Nature of the interaction between central and peripheral chemoreceptor drives in human subjects Claudette M. St. Croix, D.A. Cunningham, and D.H. Paterson

Abstract: The purpose of the current study was to investigate the nature of the interaction between the central and peripheral chemoreflex loops in humans, using the different speeds of response of the central and peripheral chemoreceptors to enable a temporal separation of their chemical stimulation. Subjects were exposed to an end-tidal PCO2 of 8–10 torr (1 torr = 1 mmHg = 133.3 Pa) above resting PCO2, with end-tidal PO2 = 100 torr, for 8 min. Thirty seconds after the hypercapnic stimulus was withdrawn, a 5-min hypoxic stimulus (end-tidal PO2 = 50 torr) was introduced. The 30-s interval was believed to be sufficient time for the peripheral chemoreceptors to adapt to the new level of carbon dioxide. Over the subsequent 5 min of hypoxia, however, the central chemoreceptors were exposed to diminishing hypercapnia. The response to the hypoxic step was compared with the effect of the same hypoxic step without the preceding period of hypercapnia. In 4 of the 5 subjects studied, the ventilatory response to hypoxia was unaffected by relative hypercapnia at the central chemoreceptor, suggesting that the central and peripheral chemoreflexes were independent of each other. Key words: respiratory control, chemoreceptors, hypercapnia, hypoxia. Résumé : La présente étude a eu pour but d’examiner la nature de l’interaction entre les boucles de chémoréflexes périphériques et centraux chez les humains, en utilisant le fait que les différentes vitesses de réponse des chémorécepteurs périphériphes et centraux permettent de dissocier leurs stimulations chimiques respectives. Les sujets ont été exposés, pendant 8 min, à une PCO2 de fin d’expiration de 8–10 torr (1 torr = 1 mmHg = 133.3 Pa) plus élevée que la PCO2 de repos ainsi qu’à une PO2 de fin d’expiration de 100 torr. Un stimulus hypoxique d’une durée de 5 min (PO2 de fin d’expiration de 50 torr) a été appliqué 30 s après le retrait du stimulus hypercapnique. L’intervalle de 30 s a été considéré comme étant suffisant pour permettre aux chémorécepteurs périphériques de s’adapter au nouveau taux de gaz carbonique. Toutefois, durant les 5 min de stimulation hypoxique, les chémorécepteurs centraux ont été exposés à une hypercapnie décroissante. La réponse à l’échelon hypoxique a été comparée à celle du même échelon hypoxique sans la période d’hypercapnie. Chez 4 des 5 sujets examinés, la réponse ventilatoire à l’hypoxie n’a pas été affectée par l’hypercapnie perçue au niveau du chémorécepteur central, ce qui suggère que les chémoréflexes périphériques et centraux étaient indépendants l’un de l’autre. Mots clés : contrôle de la respiration, chémorécepteurs, hypercapnie, hypoxie. [Traduit par la Rédaction]

Introduction

The slope of the ventilation to alveolar PCO2 (V⋅ e–PaCO2) relation is increased when alveolar PO2 (PaO2) is below euoxic values (Nielsen and Smith 1952). When PaO2 rises above euoxic values, the slope is reduced. This suggests that hypoxic and hypercapnic feedback stimuli interact multiplicatively in their effects on ventilation (Cunningham et al. 1986). In anes-

Received November 20, 1995. C.M. St. Croix and D.A. Cunningham.1 Centre for Activity and Ageing (affiliated with the Faculty of Kinesiology and Faculty of Medicine at The University of Western Ontario and The Lawson Research Institute at the St. Joseph’s Health Centre) and the Department of Physiology, The University of Western Ontario, London, ON N6A 3K7, Canada. D.H. Paterson. Centre for Activity and Ageing, Faculty of Kinesiology, Thames Hall, The University of Western Ontario, London, ON N6A 3K7, Canada. 1

Author for correspondence.

Can. J. Physiol. Pharmacol. 74: 640–646 (1996).

thetized cats, the literature supports the peripheral chemoreceptor as the exclusive site of multiplicative interaction between CO2 and hypoxia (Hornbein et al. 1961; Van Beek et al. 1983) and indicates that the interaction between peripheral and central inputs is strictly additive (Cunningham et al. 1986; Van Beek et al. 1983). It is commonly accepted that the central and peripheral chemoreflexes contribute independently to the total ventilatory drive in human subjects (Berkenbosch et al. 1992). However, the possibility remains that interaction between peripheral chemoreceptor afferent signals and signals from the putative chemosensitive areas of the ventrolateral medulla takes place within the central nervous system. The main problem in determining the nature of the peripheral–central chemoreflex interaction in human subjects is the difficulty in changing the peripheral CO2–H+ drive without affecting the central CO2–H+ drive. Robbins (1988) used the differing speeds of response of the central and peripheral chemoreceptors to enable a temporal separation of their chemical stimulation. The dynamic end-tidal forcing technique was used to give a period of time when the central chemoreceptors were exposed to residual hypercapnia while the peripheral © 1996 NRC Canada

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St. Croix et al. Fig. 1. Schematic diagram describing the time-related changes in PetCO2 and PetO2 forcing functions in each of the three experimental protocols employed.

chemoreceptors were exposed to eucapnia. Hypoxic sensitivity was measured against this background and when both environments were eucapnic. In 2 of the 3 subjects studied, the ventilatory response to hypoxia was augmented when central PCO2 was high. These results were taken to provide some evidence for a degree of multiplicative interaction between the central and peripheral chemoreceptors in humans. However, there may not have been adequate time between the administration of the protocols to eliminate the possibility of the hypercapnic or hypoxic stimulus in one protocol potentiating or diminishing the ventilatory response in a succeeding protocol. The purpose of this study was to investigate the possibility of interaction between the central and peripheral ventilatory chemoreflex loops in humans. We modelled the experiments after those of Robbins (1988) with modifications to the administration of the experimental protocols. The results of the study by Robbins (1988) suggest that the peripheral response to hypoxia is augmented by central hypercapnia. These results were not conclusive, however, because of potential imperfections in methodology, and the fact that a positive result was detected in only two subjects. We hypothesized that, given adequate time for recovery following the hypercapnic– hypoxic perturbations, the results would not support interaction between the central and peripheral chemoreflex loops.

Methods Respiratory apparatus and gas analysis Subjects were seated and breathed through a mouthpiece with the nose occluded. Inspired and expired ventilations were measured using a low resistance bidirectional turbine (VMM 110, Summit Technology, Oakville, Ont.) and volume transducer (VMM–2A, Summit Technology) calibrated with a syringe of known volume (3.01 L). Respiratory flows and timing information were measured using a pneumotachograph (model 3800, Hans Rudolph Inc., Kansas City, Mo.) and differential pressure transducer (MP45–871, Validyne, Northridge, Calif.). Inspired and expired gases were sampled continuously (20 mL⋅min–1) at the mouth and analyzed by a mass spectrometer (MGA 2000, AIRSPEC, Biggin Hill, U.K.) calibrated with precision-analyzed gas mixtures. Analogue signals were sampled and digitized every 20 ms by computer. Gas concentration signals were aligned with the inspired and expired volumes after correcting for the time delay appropriate for the instrument. Two microcomputers were used. The data acquisition computer collected the experimental variables every 20 ms and stored them for later analysis. Accurate control of end-tidal gases was achieved using a computer-controlled fast gas-mixing system similar to that described in more detail by Howson et al. (1987) and Robbins et al. (1982). The control computer compared the measured end-tidal gas tensions with the target end-tidal tensions (entered into the control computer before the experiment according to the protocol). The variables used for feedback control were end-tidal PCO2 (PetCO2) and end-tidal PO2 (PetO2). The inspired PCO2 and PO2 required were converted by an algorithm into appropriate values for flows of CO2, O2, and N2. The sensing process for PetCO2 and PetO2 was repeated at the end of each breath and the control computer adjusted the gas mixture to force the end-tidal PCO2 and PO2 towards the desired values. Subjects and protocol Five males ranging in age from 22 to 35 years (mean age = 28 years) acted as subjects for the experiments. All subjects were nonsmokers with no history of cardiovascular or respiratory disease. The study requirements were fully explained to all participants, with each subject giving informed consent prior to volunteering to participate in the study. The research was approved by The University of Western Ontario Committee on Human Research. The experimental protocols were modelled after Robbins (1988), who used the differing speeds of response of the central and peripheral chemoreceptors to enable a temporal separation of their chemical stimulation. The three different protocols that were required are illustrated schematically in Fig. 1. In protocol A, the subject was exposed to an end-tidal PCO2 8–10 torr (1 torr = 1 mmHg = 133.3 Pa) above resting, with PetO2 = 100 torr, for 8 min. Thirty seconds after the hypercapnic stimulus was withdrawn, a 5-min hypoxic stimulus (PetO2 = 50 torr) was introduced. The 30-s interval should be sufficient for the peripheral chemoreceptor to adapt to the new level of PCO2. However, the central chemoreceptor environment changes more slowly, and over the subsequent 5 min of hypoxia, the central chemoreceptors were exposed to diminishing hypercapnia. The other two protocols were controls. Protocol B was similar to protocol A, but without the hypoxic step. In protocol C a 5-min step down in PetO2 from 100 to 50 torr was administered at the resting level of PetCO2, and without the preceding period of hypercapnia. Strict adherence to the protocol outlined by Robbins (1988), wherein protocols A and B were administered in one breathing period, resulted in the ventilatory response to the second hypercapnic stimulus being 9.2 L⋅min–1 (31%) higher than the ventilatory response to the first hypercapnic stimulus. When two type C protocols were administered in the same breathing period, the ventilatory response to the second hypoxic stimulus was 8.3 L⋅min–1 (30%) lower than the response to the first hypoxic stimulus. Therefore, on each visit, three periods of breathing on the apparatus were planned, © 1996 NRC Canada

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Fig. 2. The experimental results of protocols A (d), B (,), and C (.) for each of the five subjects. Top, PetO2 (torr); middle, PetCO2 (torr); ⋅ bottom, Ve (L⋅min–1). Dotted lines mark the start and end of the hypoxic step.

corresponding to one of the three protocols. Each breathing period was separated by at least 30 min. Each of the five subjects contributed six sets of data to each of the three protocols. Data analysis The data were analyzed in a manner similar to that of Robbins (1988). For each protocol, a mean of the respiratory variables for the 2-min steady-state period prior to the first step was calculated along with the means for each 30-s period following the step. The results were then combined to yield an average response for each subject to each step type. Six individual responses contributed to each average response. The effect of hypoxia on ventilation was examined by subtracting the ventilatory response to protocol B from the ventilatory response to protocol A. The effect of hypoxia in protocol C was measured by subtracting each 30-s data point from the 2-min control point. The results of these calculations gave the ventilatory response to hypoxia under two sets of conditions. The response to the hypoxic step in protocol A was then compared with the effect of the same hypoxic step without the preceding period of hypercapnia in protocol C, using a one-sided two-sample t test. The null hypothesis was that the ventilatory response in step type A was the same as (or smaller than) the ventilatory response in step type C. If the hypoxic

response was affected by relative hypercapnia at the central chemoreceptor, then the ventilation in protocol A should initially have been greater than in protocol C, only becoming the same as central eucapnia was restored. A two-component exponential model (Bellville et al. 1979) was used to estimate the temporal parameters of the ventilatory response to the step decrease in CO2 in protocol B. For each individual, the ⋅ breath-by-breath data for Ve, PetCO2, and PetO2 from each test were interpolated over 1-s intervals, and all tests for a given protocol were ensemble-averaged to increase the signal-to-noise ratio. The total ventilatory response was made up of the sum of contributions of the ⋅ ⋅ peripheral (Vp(t)) and central (Vc(t)) chemoreflex loops and a drift term (Drift(t)): ⋅ ⋅ ⋅ ⋅ Ve(t) = Vb + Vc(t) + Vp(t) + Drift(t) where ⋅ Vc(t) = Gc(1 − e−(t − T )/τ ) c

c

and ⋅ Vp(t) = Gp(1 − e−(t − T )/τ ) p

p

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St. Croix et al. Fig. 3. The differences between the hypoxic response 30 s after a step decrease in PetCO2 and the hypoxic response during steady-state eucapnic breathing. Error bars show the 95% confidence intervals. Broken lines mark the start and end of the hypoxic period. *p < 0.05.

⋅ ⋅ Vb is the baseline ventilation and Ve(t) is the time-dependent variation ⋅ in Ve. The parameters Gc, τc, and Tc are the gain, time constant of the response, and time delay of the central chemoreflex loop, respectively. The parameters Gp, τp, and Tp are the gain, time constant of the response, and time delay of the peripheral chemoreflex loop, respectively. To obtain optimal parameter estimation, a computerized optimization routine was applied. A grid search was applied to obtain optimal time delays. The minimum time delays were chosen to be 1 s, and τp was constrained to be at least 0.3 s, based on previous studies (Bellville et al. 1979; Dahan et al. 1990).

Results The results for each subject are shown in Fig. 2. The quality of the end-tidal profiles was the same for the hypercapnic steps in protocols A and B and for the hypoxic steps in protocols A and C. Four of the five subjects showed no significant differences between the ventilatory response to hypoxia in protocol A and in protocol C (Fig. 3). In subject 1569, however, 4 of the 10 points were significantly greater than zero. Typical ventilatory responses to the hypercapnic control protocol (B) with the best-fit model and residuals plot are shown for a representative subject in Fig. 4. The estimated parameters are listed in Table 1. The time constants for the fast and slow components of the ventilatory response to a step down in PetCO2 averaged 10.2 ± 2.9 and 147.0 ± 28.7 s, respectively (Table 1).

Discussion In 4 of the 5 subjects studied, the drives from the central and peripheral chemoreceptors were independent. The differences between the results of the current study and Robbins (1988) might be explained in part by the modifications to the administration of the protocols. The literature supports the findings that a 5-min hypoxic exposure potentiates a subsequent hypercapnic test (Davidson and Cameron 1985) and depresses hypoxic sensitivity (Easton et al. 1988) for up to 1 h. Administering two protocols in the same breathing session, as was done in the study by Robbins (1988), had the effect of lowering the average response to hypoxia in protocol C and increasing the average ventilatory response to hypercapnia in protocol A or B, thus yielding a larger difference between A and C. The effects of administering the protocols in this manner are in the direction required to explain the results of the Robbins (1988) study. In the present study, each protocol was separated by a minimum of 30 min of breathing room air (off the apparatus), and there was no evidence to suggest that the ventilatory response to hypoxia was increased by central hypercapnia. While subject 1569 showed some evidence for multiplicative interaction between the chemoreflexes, the ventilation remained higher throughout the hypoxic period in protocol A, rather than decreasing as central eucapnia was restored, as Robbins (1988) observed in his two subjects. It was likely, therefore, that some mechanism other than central–peripheral © 1996 NRC Canada

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chemoreflex interaction was responsible for the augmented ventilatory response in protocol A. One alternative explanation was described by Michel et al. (1966). They reported that the in vitro relationship between the pH and bicarbonate in true plasma does not apply in vivo when the PaCO2 is altered by carbon dioxide inhalation. CO2 breathing gave way to a small metabolic as well as a respiratory acidosis due to distribution of bicarbonate across the extracellular fluid, and this persisted long after the inspired PCO2 had been lowered. Considering the time course of the recovery period, Robbins (1988) concluded that this effect could not have influenced his results. If the metabolic acidemia was significant, the augmentation of hypoxic sensitivity at the peripheral chemoreceptors would last throughout the hypoxic period, as seen in subject 1569. In this study, it was assumed that the 30-s interval was sufficient time for the discharge from the peripheral chemoreceptor to return to near resting levels. This interval was chosen based on the method used by Robbins (1988) and on previous studies of the kinetics of the ventilatory response to hypercapnia, in which the time constant of the fast component of the response (attributed to the peripheral chemoreceptor) averaged 8–10 s (Dahan et al. 1990; Berkenbosch et al. 1992). Assuming that peripheral chemoreceptor drive declines exponentially, an interval equal to three time constants would allow the process to be about 95% complete at the end of the 30-s interval, and would assure that the PCO2 at the central chemoreceptor was still high when hypoxia was introduced. However, if the CO2 at the carotid bodies remained high for the duration of the hypoxic step, the results could be complicated by the interaction between hypoxia and CO2–H+ at the peripheral chemoreceptor (Gabel and Weiskopf 1975). When a two-component exponential model (Bellville et al. 1979) was used to estimate the temporal parameters of the ventilatory response to the step decrease in CO2 in protocol B, the average (n = 5) time constant of the fast ventilatory component was 10.2 ± 2.8 s, indicating that the peripheral chemoreceptor response to a step down in PetCO2 would be nearly complete when the hypoxic step was introduced. While an interval equal to four time constants is needed to allow an exponential process to be functionally (98%) complete, a period of three time constants allows the process to be about 95% complete. In addition, the results in 4 of the 5 subjects showed no evidence for peripheral interaction between CO2 and hypoxia, suggesting that the 30-s interval was adequate to prevent such interaction from contaminating the results. It is also doubtful that peripheral interaction between hypercapnia and hypoxia could account for the increased hypoxic response to protocol A in subject 1569, as the time constant of the peripheral chemoreceptor response to a step down from hypercapnia was 10.7 s. In this subject, the response to hypoxia was augmented for the duration of the 5-min hypoxic step, whereas any interaction between hypoxia and H+ would have decayed by 12.8 s (4τ × 10.7 s = 42.8 s – 30 s, the time interval between the hypercapnic and hypoxic steps) into the hypoxic step, when the carotid body response to the step down in PetCO2 was functionally complete. A second critical assumption of this experimental design is that the PCO2 at the central chemoreceptors remains high for some time after the hypoxic stimulus is introduced. The time constant of the central chemoreceptor response to a step down from hypercapnia averaged 147 s. These results are similar to

Can. J. Physiol. Pharmacol. Vol. 74, 1996 Fig. 4. The upper panel shows ventilatory response to the step out of hypercapnia (protocol B) in a representative subject (1569). The best-fit model to the data is superimposed on the corresponding ⋅ averaged Ve response. The residuals are plotted in the lower panel.

those of previous studies (Dahan et al. 1990; Swanson and Bellville 1975) and suggest that the discharge from the central chemosensitive tissue was still significant for at least 2 min into the hypoxic step. In addition, the ventilation at 5.5 min post-hypercapnia in protocol B was 2.59 ± 1.56 L⋅min–1 higher (p < 0.05) than the prehypercapnia baseline, indicating that full recovery from the ventilatory response to hypercapnia had not yet occurred. The maintained increase in ventilation could be attributed either to slow changes in brain tissue or cerebrospinal fluid pH or to a continued neural afterdischarge (Eldridge and Gill-Kumar 1980; Millhorn et al. 1980). The literature does not support the possibility that afterdischarge following the removal of hypercapnia in protocol A would be attenuated by central hypoxia (Engwall et al. 1994). The effects of changes in PaCO2 and PaO2 on cerebral blood flow (CBF) complicate the interpretation of results of wholebody studies of respiratory control. Ventilation and cerebral blood flow are intimately related because of the central role played by the circulation in controlling the chemical environment © 1996 NRC Canada

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St. Croix et al. Table 1. Values for the estimated temporal parameters of the ventilatory response to a step down from hypercapnia. Subject

Gc

Gp

τc

τp

Tc

Tp

Drift

RSS

1643 2374 2007 2402 1569 Average

–3.14 –1.72 –3.12 –1.43 –1.92 –2.27 (0.81)

–0.90 –0.50 –2.07 –1.02 –1.48 –1.19 (0.60)

105.10 134.90 176.30 148.48 170.12 146.98 (28.71)

14.39 10.14 9.35 6.64 10.65 10.23 (2.79)

4.05 5.79 12.63 12.68 3.45 7.72 (4.59)

4.00 3.01 4.64 5.36 2.99 4.00 (1.03)

2.18 1.05 2.02 1.17 1.54 1.59 (0.50)

264.23 80.01 360.56 267.90 62.30

Note: Gc and Gp, central and peripheral gain terms in L⋅min–1⋅torr–1; τc and τp, central and peripheral time constants in s; Tc and Tp, time delays of the central and peripheral chemoreflex loops in s; Drift, drift term in mL⋅min–1. RSS, residual sum of squares.

of the brain. Ventrolateral medullary surface blood flow is CO2 sensitive (Feustal et al. 1984). Increases and decreases in PCO2 cause vasodilation and vasoconstriction of the arteriolar channels, respectively, and the associated changes in peripheral vascular resistance are responsible for the changes in cerebrovascular circulation time and the velocity of flow (Markwalder et al. 1984). The CBF response to step changes in PCO2 are rapid with time constants in the order of 20 s (Severinghaus and Lassen 1967). The faster ventilatory responses to a step increase in PCO2 than to a step decrease in PCO2 (Bellville et al. 1979; DeGoede et al. 1985) have been attributed to a lower blood flow in the recovery phase than in the hypercapnic phase (Bellville et al. 1979; Feustal et al. 1984). CBF has also been reported to be sensitive to changes in PO2 in unanesthetized humans (Ellingsen et al. 1987; Kety and Schmidt 1948). In this study, the increase in CBF associated with a step down in PO2 could facilitate the washout of CO2 from the central chemosensitive area in the experimental protocol A, reducing the amount of time the central chemoreceptors would be exposed to high CO2. In contrast to the rapid CBF response to changes in PCO2, however, the response to hypoxia has a slow time course. While the time constant in experimental animals has been described to be 35– 40 s (Doblar et al. 1979; Van Beek et al. 1986), the time constant in humans has been estimated to be approximately 6 min (Ellingsen et al. 1987). Therefore changes in CBF are unlikely to mask the presence of central–peripheral interaction in this study. The results of this study are consistent with the most widely accepted model describing the interactions between chemical respiratory feedback stimuli (Bellville et al. 1979; Berkenbosch et al. 1992; Cunningham et al. 1986; Dahan et al. 1990). In this model, hypoxia and the CO2–H+ complex interact at the level of the peripheral chemoreceptor, and the drives from the periphery and from the central chemosensitive area add together in their effects on ventilation. The appropriateness of this model has been demonstrated in cats, using the artificial brainstem perfusion technique (Van Beek et al. 1983). The evidence in humans is not as definitive as a result of the difficulty in isolating respiratory stimuli to a single chemosensitive site. The results of experiments using the technique of dynamic end-tidal forcing (Swanson and Bellville 1974) and attempts to fit the ventilatory response to CO2 during euoxia, using a two-compartment exponential model, in which the equation was extended to incorporate the interaction parameter (Dahan et al. 1990) introduced by Robbins (1988), have failed to demonstrate any significant central–peripheral interaction. Duffin (1989) used an adaptation of the Read rebreathing technique

(Read 1966), modified for use with mild hypoxia and prior hyperventilation, or hyperoxia and prior hyperventilation, to separate the peripheral, central, and combined peripheral and central CO2 drives. The ventilatory response to the combined chemoreceptor drives was found to be the sum of the central and peripheral drives, indicating that there was no significant multiplicative interaction between the signals within the central nervous system. The results of this study were contrary to those of Robbins (1988), which appeared to advance the possibility of interaction between the central and peripheral chemoreceptors in man. However, further work done in the Oxford laboratory, by Clement et al. (1992), using metabolic acidosis, generated by a brief bout of hard exercise, to selectively stimulate the peripheral chemoreceptors, and CO2 inhalation as a stimulus common to both sets of chemoreceptors, also failed to support this theory. It was reported that the ventilatory sensitivity to hypoxia at matched arterial pH values was not significantly different between conditions of high (CO2 inhalation) and low (metabolic acidosis) central chemoreceptor activity. In conclusion, this study failed to detect the presence of any interaction between the central and peripheral chemoreflexes.

Acknowledgements This study was funded by grants from the Natural Sciences and Engineering Research Council of Canada and the Ontario Lung Association. The authors thank Brad Hansen for technical assistance and the volunteers for their participation in this study.

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