The control system regulating breathing in man

A Mathematical Model of Human Respiration at Altitude

We developed a mathematical model of human respiration in the awake state that can be used to predict changes in ventilation, blood gases, and other critical variables during conditions of hypocapnia, hypercapnia and these conditions combined with hypoxia. Hence, the model is capable of describing ventilation changes due to the hypocapnic-hypoxia of high altitude. The basic model is that of Grodins et al. [Grodins, F. S., J. Buell, and A. J. Bart. J. Appl. Physiol. 22:260-276, 1967]. We updated the descriptions of (1) the effects of blood gases on cardiac output and cerebral blood flow, (2) acid-base balance in blood and tissues, (3) O2 and CO2 binding to hemoglobin and most importantly, (4) the respiratory-chemostat controller. The controller consists of central and peripheral sections. The central chemoceptor-induced ventilation response is simply a linear function of brain P(CO2) above a threshold value. The peripheral response has both a linear term similar to that for the central chemoceptors, but dependent upon carotid body P(CO2) and with a different threshold and a complex, nonlinear term that includes multiplication of separate terms involving carotid body P(O2) and P(CO2). Together, these terms produce 'dogleg'-shaped curves of ventilation plotted against P(CO2) which form a fan-like family for different values of P(CO2). With this chemical controller, our model closely describes a wide range of experimental data under conditions of solely changes in P(CO2) and for short-term hypoxia coupled with P(CO2) changes. This model can be used to accurately describe changes in ventilation and respiratory gases during ascent and during short-term residence at altitude. Hence, it has great applicability to studying O2-delivery systems in aircraft.

Carbon dioxide sensitivity during hypoglycaemia-induced, elevated metabolism in the anaesthetized rat

We have utilized an anaesthetized rat model of insulin-induced hypoglycaemia to test the hypothesis that peripheral chemoreceptor gain is augmented during hypermetabolism. Insulin infusion at 0.4 U kg (-1)min(-1) decreased blood glucose concentration significantly to 3.37 +/- 0.12 mmol l(-1). Whole-body metabolism and basal ventilation were elevated without increase in P(a,CO(2)) (altered non-significantly from the control level, to 37.3 +/- 2.6 mmHg). Chemoreceptor gain, measured either as spontaneous ventilatory airflow sensitivity to P(a,CO(2)) during rebreathing, or by phrenic minute activity responses to altered P(a,CO(2)) induced by varying the level of artificial ventilation, was doubled during the period of hypermetabolism. This stimulatory effect was primarily upon the mean inspiratory flow rate, or phrenic ramp component of breathing and was reduced by 75% following bilateral carotid sinus nerve section. In vitro recordings of single carotid body chemoafferents showed that reducing superfusate glucose concentration from 10 mM to 2 mM reduced CO(2) chemosensitivity significantly from 0.007 +/- 0.002 Hz mmHg(-1) to 0.001 +/- 0.002 Hz mmHg(-1). Taken together, these data suggest that the hyperpnoea observed during hypermetabolism might be mediated by an increase in the CO(2) sensitivity of the carotid body, and this effect is not due to the insulin-induced fall in blood glucose concentration.

Exercise breathing pattern during chronic altitude exposure

Breathing pattern in response to maximal exercise was examined in four subjects during a 7-day acclimatisation to a simulated altitude of 4247 m (barometric pressure, PB = 59.5 kPa). Graded exercise tests to exhaustion were performed during normoxia (day 0), and on days 2 and 7 of hypoxia, respectively. Ventilation was significantly augmented in the hypoxic environment, as were both the mean inspiratory flow (VT/TI) and inspiratory duty cycle (TI/TTOT) components of it. VI/TI was increased due to a significant increase in tidal volume (VT) and a corresponding decrease in inspiratory time duration (TI). Throughout a range of exercise ventilation, TI/TTOT was increased due to an apparently greater decrease in expiratory time duration (TE) with respect to TI. In all cases, the relation between VT and TI displayed a typical range 2 behaviour, with evidence of a range 3 occurring at very high ventilatory rates. There was essentially no difference observed in the VT-TI relation during exercise between the normoxic and hypoxic conditions. No significant changes were observed in the breathing pattern in response to exercise within the exposure period (from day 2 to day 7), although there was a discernible tendency to a higher stage 3 plateau by day 7 of altitude exposure.

Studies on arterial chemoreceptors in man

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Ventilatory responses to hypocapnic vertebral artery perfusion in intact and carotid body denervated dogs

The ventilatory responses to step changes in vertebral artery PCO2 were investigated in intact and carotid body denervated dogs. The steady-state ventilatory responses of the denervated dogs were less than those of intact dogs. However, when expressed as a ratio to the control ventilation there was no difference between the two groups. While the arterial PCO2 was held at 56 mm Hg by adding CO2 to the inspired air the perfusion of the vertebral arteries was switched from the dog's own arterial supply to hypocapnic blood. The ventilation of the denervated dogs decreased at a faster rate (half time = 130 +/- 9 sec) to a level less than the room air control ventilation. The ventilation in the intact dogs decreased at a slower rate (half time = 184 +/- 23 sec) and was maintained above the room air control level after ten minutes of hypocapnic perfusion. Increasing the medullary blood flow, as measured with radiolabeled microspheres, augmented the rate of decline of ventilation in intact dogs. We conclude, (1) the influence of the peripheral chemoreceptors appears to increase as central drive is decreasing, and (2) the remaining time course of the decrease in ventilation is related to the rate of brain stem perfusion.

Inspiratory-expiratory responses to alternate-breath oscillation of PACO2 and PAO2

Breath-by-breath respiratory responses of three healthy adults to imposed alternate-breath oscillation of end-tidal PCO2 (between +5 and +15 torr above the eupnoeic level) and/or PO2 (between 80 and 45 torr) were studied at rest and during mild cycle ergometer exercise. There was often alternation in inspiratory and expiratory tidal volumes and mean flows, and in expiratory duration, but not in inspiratory duration. The latency of responses, estimated by cross-correlation, corresponded closely to the lung-ear transport delay (measured by oximetry). There were two general patterns of response: in-phase, with inspiratory responses leading expiratory, and, more often, out-of-phase, with expiratory responses leading inspiratory. These patterns were associated with arrival of the onset of the alternating signal at the ear in inspiration and expiration, respectively. It is concluded that the timing of alternating humoral signals at the carotid bodies in relation to the phase of respiration determines the pattern of inspiratory-expiratory response, and that expiratory events can be independent of the previous inspiration.

Medullary and carotid chemoreceptor interaction for mild stimuli

The interaction of medullary and carotid chemoreceptors during mild stimulation was investigated in 44 experiments on 6 chloralose-urethane anesthetized mongrel dogs using a donor-perfused, bilateral carotid sinus preparation. The donor dog breathed hypoxic mixtures (average PaO2 of 78 mm Hg) and the experimental animal breathed hypercapnic mixtures (average PACO2 of 49 mm Hg) in order to separately or simultaneously stimulate both chemoreceptor areas at low levels. After 4 min, the changes of tidal volume (VT), respiratory rate (f) and minute ventilation (VI), as a percentage of the control value, were compared to test whether the sums of the changes for separate stimuli were the same as for simultaneous stimuli, i.e. additive chemoreceptor effects. The simultaneous stimuli had significantly (P less than 0.05) greater responses for VT (19% greater than 5%) and VI (42% greater than 13%), but not for f (23% = 9%). Stepwise multiple regression studies of the response/control ratios on the blood gas values showed that multiplicative interaction terms accounted for more of the variance than additive terms for VT, f and VI and yielded equations which had overall significant slopes. We conclude that this evidence demonstrates that the two chemoreceptor effects combine synergistically at low levels of stimulation.

The relation between hypoxia and CO2-induced reflex alternation of breathing in man

Four healthy young volunteers, selected for the responsiveness and steadiness of their breathing, were studied in rest and mild exercise while receiving alternate inspirates of low and high PCO2 (0 and 8.6 kPa). PACO2, oscillated between ca. 6 and 7.5 kPa (45-55 torr). PAO2 was held steady at 4-7 levels between 6 and 28 kPa (45-210 torr). Thirteen separate inspiratory and expiratory variables (volumes, times, flows) were recorded and tested for reflex alternation. Matched controls were performed. Responses were generally small in relation to the scatter. Reflex alternation of any one variable was not always evident. The incidences of the responses were, in descending order, inspiratory flows and volumes, expiratory flows and volumes, expiratory duration; inspiratory duration alternated seldom, and then with only small amplitude. Reflex alternation was more likely to be observed in hypoxia than in euoxia or hyperoxia. A tendency for the incidences to be greater in exercise than at rest was not significant, but the amplitudes of alternation showed a significant difference in favour of exercise. In a substantial minority of experiments the amplitude of reflex alternation was significantly and positively correlated with hypoxia (1/(PAO2--C)). Alternation also occurred frequently in another substantial minority of experiments in which, however, there was no significant amplitude-hypoxia correlation. It was concluded that these two groups probably differed not so much in the form of the amplitude-hypoxia relation as in respect of the extent of the scatter in the observations. The results are consistent with interaction of non-steady-state with steady-state signals at the arterial chemoreceptors.

Normalization of the blunted ventilatory response to acute hypoxia in congenital cyanotic heart disease

Patients with congenital cyanotic heart disease have a blunted ventilatory response to hypoxia, but the permanence of the blunting is disputed. To determine how early the blunted ventilatory response develops and whether it is reversible, we studied three groups of children and young adults: five (seven to 13 years of age) with acyanotic heart disease, eight (seven to 16) with cyanotic congenital heart disease (arterial oxygen saturation, 55 to 83 per cent), and 13 (seven to 17) whose cardiac defects were repaired (arterial oxygen saturation, 93 to 98 per cent). The ventilatory response to acute hypoxia was subnormal in the hypoxemic children in that their ventilation showed little increase when arterial oxygen saturation fell by 10 to 20 per cent, compared to a 150 to 300 per cent increase in the control subjects. This characteristic even appeared in a seven-year-old patient, indicating that the disorder occurs in early life. The appearance of blunted ventilatory response is delayed when hypoxia from birth is less severe. After operation, with return of the arterial hypoxemia to normal, the response was in the normal range. We conclude that the blunted response is reversible.

Posthyperventilation apnea associated with severe hypoxemia

We studied a 14-year-old girl who suffered fractures of her mandible and tegmen following a fall from a balance beam. Thirteen days after hospitalization, she developed severe, protracted, recurrent episodes of hyperventilation; subsequently, she suffered posthyperventilation apnea, which at times was prolonged and association with severe hypoxemia with an arterial oxygen pressure as low as 25 mm Hg. The patient was treated with added dead space and chlorpromazine hydrochloride (Thorazine). Postulated mechanisms for her disorder are discussed. The importance of close clinical and laboratory observation in similar cases is stressed.

[Mathematical simulation of the respiratory system (author's transl)]

The respiratory system is described as a feedback control system. The controller consists of the peripheral chemoreceptors and the central chemosensitive structures, the respiratory centre in the medulla oblongata and the thorax-lung pump which they drive. The controlled system is comprised of three compartments (lung, brain and the remaining tissue) connected by the blood circulation. The controlled values are arterial pH and arterial O2 partial pressure and cerebral extracellular pH. Earlier models have been improved by: (1) the dead space description, (2) the thermodynamic formulation of the CO2 dissociation equation and the simple but accurate O2 dissociation equation of the blood, (3) the alteration of the CO2 dissociation equation for the brain and the remaining tissue to accommodate recent results, (4) the application of the one-receptor-theory of central chemosensitivity, (5) the pH dependence of brain circulation, (6) the bicarbonate exchange between blood and extracellular fluid of the brain and (7) the introduction of variable circulation times. Respiratory and metabolic disturbances of the respiratory system are analyzed. The mathematical formulation of the respiratory system is a differential difference equation system. In the steady state the experimental results are reproduced fairly well. A slight discrepancy is found in the simulation of metabolic acidosis. Apparently we have assumed the sensitivity of the peripheral chemoreceptors to be too large so that the respiratory response is not correctly predicted. In the numerical solution there is an overshoot in the on-transient and a damped oscillation in the off-transient of the alveolar CO2 partial pressure during respiratory acidosis. We have varied the parameters to make deviations small. The best agreement seems to result, if the central threshold is near the normal extracellular pH of the brain. A further deviation from experimental findings is that the cerebral CO2 and H+ concentration, the blood circulation of the brain, the alveolar O2 partial tension and the ventilation show a slight oscillation in the off-transient. Except for these discrepancies the experimental results, especially the stability of the extracellular pH of the brain, are reproduced fairly well. During hypoxia there are deviations form the experimental results if the central residual activity is constant and the central threshold deviates from the normal extracellular pH of the brain. But if the central residual activity is pH dependent and if the central threshold is equal to the normal extracellular pH of the brain, then the time course of VE and the other variables agree fairly well with experimental results. There is also a good correspondence between the theoretical and experimental data during hyperoxia. During metabolic acidosis the time constant of the bicarbonate exchange between blood and extracellular fluid of the brain is important. If a time constant of one minute is assumed, then the predicted and the experimental results correspond sufficiently well.

Ventilatory response to CO2 at rest and during positive and negative work in normoxia and hyperoxia

Ventilation versus alveolar PCO2 relationships were determined by the steady-state method in 6 normal male subjects at rest and during positive and negative work at one load in both normoxic and hyperoxic condition. In 5 subjects the slopes of the VE-PACO2 lines during positive and negative work increased in normoxia as compared with rest. This effect was less evident in hyperoxia. It was also found that the slopes of the VE-PACO2 lines in positive and in negative work were about the same in both normoxic and hyperoxic conditions. Oxygen uptake and CO2 production during positive work is higher than during negative work. These results suggest that: 1) the disagreement between various authors on the change of the slope of the VE-PACO2 line may be due to the differences in the method of calculation of the slope or the method of the determination of VE-PACO2 lines; 2) the stimuli from the muscle spindles in the working muscle during exercise probably do not contribute to the increase in ventilatory response to CO2; 3) the increased slope of the normoxic VE-PACO2 line during exercise may be due to the interaction of several factors such as impulses from working muscles, chemosensitivity of central or peripheral chemoreceptors, adrenal-sympathetic pathways or temperature; 4) respiratory oscilations of PAO2 or PACO2 do not seem to influence the respiratory response to CO2.

Relationship between carotid chemoreceptor activity and ventilation in the cat

The steady-state stimulus-response relations between arterial P02 and PCO2 and the mean activity of carotid chemoreceptors (single and multi-fiber) and ventilation were simultaneously recorded in 48 anesthetized cats. The carotid chemoreceptor activity varied linearly with the increase of arterial PCO2, below and above the normal value, at any given level of arterial P02. A decrease in arterial P02 increased the activity of the carotid chemoreceptors and increased its sensitivity to changes in arterial PCO2, showing multiplicative stimulus interaction. The authors also found that the response in ventilation during hypoxia to changes in arterial PCO2 below the normal value was smaller than that to changes above it, unlike the response of carotid chemoreceptors. This arterial PCO2 quasi-threshold for ventilation was, therefore, not due to a corresponding threshold for the activity of the carotid chemoreceptors but to a central mechanism. Above the central PaCO2 threshold, the ventilatory response to changes in PaCO2 and Pa02 resembled that of chemoreceptors but the ventilation dependent on hypoxia was greater than that could be directly accounted for by the activity of peripheral chemorecepors. A multiplicative interaction between the activity of peripheral chemoreceptors and central CO2 excitation appears to play a role in the regulation of ventilation.

Abnormal breathing patterns

In health, breathing is regular and the respiratory rate is sufficiency constant to be useful as a vital sign of health and disease. This regularity depends on a complex interplay of chemical and neural control systems that operate automatically to reset the rate and depth of breathing as changes occur in posture and activity, to adjust the level of ventilation so that changes in gas tensions and pH in the blood and in the brain intersitial fluid are exceedingly modest despite wide swings in metabolic rate and in environmental conditions, and to coordinate ventilation and circulation so that the requirements of individual tissues for O2 delivery and CO2 removal are satisfied. Two broad categories of disorders can result from malfunction of these systems (Table 1): (1) disproportionate ventilation (too high or too low) for the level of metabolic activity, thereby producing severe abnormalities in blood gas tensions or in acid-base balance, and (2) an irregular breathing pattern without eliciting gross changes in blood gas tensions or in acid-base balance. Because of the complexity of the control system, each of these categories represents a final common pathway that can be produced in different ways. In this presentation, we will attempt to describe the general features that characterize the operation of the control system and some new technics that make it possible to trouble-shoot the malfunctioning system in order to identify the mechanism(s) responsible for the abnormality in breathing pattern.