'Exponential peeling' of ventilatory transients following inhalation of 5, 6 and 7% CO2

Nonlinearity identified by neural network models in Pco2 control system in humans

The nonlinearity included in the PCO2 control system in humans is evaluated using the degree of nonlinearity based on a difference of residuals. An autoregressive moving average (ARMA) model and neural networks (linear and nonlinear) are employed to model the system, and three types of network (Jordan, Elman and fully interconnected) are compared. As the Jordan-type linear network cannot approximate respiratory data accurately, the other two types and the ARMA model are used for the evaluation of the nonlinearity. The results of the evaluation indicate that the linear assumption for the PCO2 control system is invalid for three subjects out of seven. In particular, strong nonlinearity was observed for two subjects.

Dynamic response characteristics of CO2-induced air hunger

The time course of change in 'air hunger', the uncomfortable urge to breathe, was assessed following sudden increases and decreases in PETCO2. Healthy normal men and women were mechanically ventilated at constant tidal volume and frequency, and were required to rate the perceived intensity of air hunger every 10-15 sec. PETCO2 was changed by altering FICO2 unbeknownst to the subject. Air hunger changed to its new level following steps with a median time constant of about 50 sec during hyperoxia. Changes in air hunger following PETCO2 steps were slightly faster when background gas was slightly hypoxic. Although the present results are consistent with the hypothesis that air hunger and ventilatory drive share the same receptors and central neural processes, analysis of dynamic response is probably not sensitive enough to disprove the hypothesis.

Nonlinear identification of the PCO2 control system in man

Two approaches to identification of the PCO2 system in man are described. The first uses a nonlinear 'black box' NARMAX identification package, while the second method uses a structured two-compartment Belville model. The data were obtained from volunteers breathing either room air or a controlled gas mixture, controlled via a pseudorandom M-sequence. Measurements were made of respiratory gas flow and PCO2 content of inspired and expired gases. The identification results indicate that a low-order dynamic model with nonlinear polynomial expansion gave the best fit to the data. In contrast, the Belville model gave best results with a two-compartment linear model, mainly because of difficulties in the optimisation routines when the Belville model was not linear. Thus, modern systemic methods of excitation and identification appear to be appropriate for modelling this respiratory subsystem of humans.

Dynamics of the ventilatory response in man to step changes of end-tidal carbon dioxide and of hypoxia during exercise

1. Four human subjects exercised in hypoxia (end-tidal partial pressure of O2 (P(ET),O2) ca 55 Torr; heart rate ca 100-130 beats min-1), and the contribution to the respiratory drive of the peripheral and central chemoreflex pathways have been separated on the basis of the latencies and the time courses of the responses to sudden changes of stimulus. 2. The subjects were exposed to repeated end-tidal step changes in PCO2 of ca 3-3.5 Torr (at nearly constant P(ET),O2) and PO2 (between ca 55 and 230 Torr) at three regions along the expiratory ventilation VE-P(ET),CO2 response line (hypocapnia, eucapnia, hypercapnia). The dynamics of the ventilatory responses were calculated using a two-compartment non-linear least-squares optimization method. 3. The component of the response attributable to the peripheral chemoreflex loop may in some subjects contribute up to 75% of the ventilatory drive during mild hypocapnic hypoxic exercise and ca 72% of the total gain following steps of P(ET),CO2 during hypoxic exercise. These data support the notion that the effectiveness of the peripheral chemoreceptor pathway is enhanced in moderate exercise. 4. During hypoxic exercise, the time delays and time constants attributed to the peripheral chemoreflex pathways (ca 3.5 and 9 s respectively) and to the central chemoreflex pathways (ca 9.5 and 47 s respectively) are some of the shortest reported. 5. The dynamics of the peripheral and central chemoreflex pathways appeared to be largely independent of each other. 6. There was a notable absence of systematic change of inspiratory and expiratory durations during the step-induced transients.

The influence of oxygen on the ventilatory response to carbon dioxide in man

1. The ventilatory response to isoxic square-wave challenges in end-tidal PCO2 was investigated at three levels of end-tidal PO2 (PET, O2) in nine healthy male subjects. 2. Twenty-seven responses against a background of mild hypoxia (PET, O2 approximately 10 kPa), sixty-seven against a background of normoxia (PET, O2 approximately 14.5 kPa) and seventy-six against a background of hyperoxia (PET, O2 approximately 70 kPa) were collected. 3. The breath-to-breath data were partitioned into a fast and a slow ventilatory component using a two-compartment model. 4. In the normoxic and hypoxic experiments the CO2 sensitivity of the fast component averaged to about 30 and 40% of the total CO2 sensitivity, respectively. In the hyperoxic experiments three subjects had no fast component in their response while in three others the CO2 sensitivity of the fast component averaged to about 24% of the total CO2 sensitivity. In the remaining three subjects the presence of a fast component was doubtful. 5. We argue that the fast component is due to the peripheral chemoreflex loop and the slow component to the central chemoreflex loop. 6. The central CO2 sensitivity and the apnoeic threshold (extrapolated end-tidal CO2 at zero ventilation in the steady state) were 15% smaller in hyperoxia than those in normoxia and hypoxia. In normoxia and mild hypoxia the central CO2 sensitivities were not significantly different. 7. We argue, that apart from peripheral oxygen-carbon dioxide interaction, there is evidence for central oxygen-carbon dioxide interaction in human subjects. 8. We conclude that in general there is a contribution to ventilation of the peripheral chemoreceptors during hyperoxia in man.

A dynamic analysis of the ventilatory response to hypoxia in man

1. The dynamics of the ventilatory response to isocapnic hypoxia were studied in seven healthy subjects using four different levels of hypoxia, (inspired oxygen pressures, PI,O2 equal to 110, 100, 80 and 60 mmHg) successively increasing and decreasing stepwise. 2. Five such progressions were performed for each subject, corresponding to five different durations of the steps (t) ranging between 0.33 and 5.00 min. The overall duration of one test (T) was taken as the sum of the seven successive PI,O2 hypoxic steps (t) plus one step t of air breathing. Thus, the values of T ranged between 2.6 and 40.0 min. 3. End-tidal CO2 pressure was maintained constant (+/- 1 mmHg) throughout the test by manipulation of inspired CO2 pressure. 4. We measured, as a function of T, (i) the magnitude of the loops formed by the ventilatory response curves (PA,O2-VE) as measured by their surface area (S), (ii) the magnitude of ventilatory response to each rising hypoxic step, and (iii) the difference between resting VE and VE observed at PA,O2 equal to 50 mmHg (delta V50). On average, we found one maximum in absolute value of S at T = 8 min and one minimum at T = 12 min, along with two maxima of ventilatory response at T values of 8 and 24 min. 5. The same measurements were made on tidal volume response curves (PA,O2-VT) and ventilatory frequency response curves (PA,O2-f): on average we observed two non-significant peaks in the progression with T of VT and S(VT) and two significant peaks in that of delta VT,50 for T = 8 and T = 24 min. No significant peak was observed in the progression with T of f curve parameters. 6. These results are discussed together with the current dynamic model of the ventilatory control system, which includes a central neural controller with no dynamics of its own and a linear response to chemoreceptor inputs. We discuss the physiological meaning of a negative loop area in relation to the previously described depressant effect of hypoxia upon the brain stem. 7. We conclude that the dynamics of the controlling neuronal network are responsible for the observed singularities which result from differential sensitivity properties of the controller. We propose the existence of discrete excitatory states of the controller as a possible explanation of the shape of the steady-state response curve to hypoxia and of the loop variations.

A dynamic analysis of the ventilatory response to carbon dioxide inhalation in man

1. The dynamics of the ventilatory response to carbon dioxide inhalation were studied in ten healthy young men using four different inspired fractions of carbon dioxide (FI, CO2) in air (0.015, 0.030, 0.045 and 0.060) successively increasing and decreasing stepwise. 2. Seven such different progressions were performed for each subject and each of seven different durations of the steps (t) ranging between 0.1 (i.e. one ventilatory cycle) and 10 min ('steady-state' conditions). The overall duration of one test (T) was taken as the sum of the seven successive FI, CO2 steps (t) plus one step, t, of air breathing. Thus, the values of T ranged between 0.8 (i.e. eight ventilatory cycles) and 80 min. Three subjects were tested twice. 3. We measured, as a function of T, the magnitude of the loops formed by the curves PA, CO2-VE and the value of the highest ventilatory response (VE max) to each progression. For all ten subjects, both functions had two maxima, one for T values of 2.6 or 8.0 min and one for T values of 24 or 40 min, and one minimum at T equal to 12 min. 4. The same measurements were made on tidal volume-response curves (PA, CO2-VT) and ventilatory frequency-response curves (PA, CO2-f) and yielded the same results except for the ventilatory frequency-response curves, for which we only found a statistically insignificant single maximum for T values of 24 or 40 min. 5. The locations of the maxima in loop magnitude and VE max were similar in duplicate tests in three subjects, whereas the quantitative values of these variables showed wide differences. 6. We compared our results with what is expected from the current linear dynamic model of ventilatory control submitted to the same forcing function: the first maximum in the loop magnitude is predicted by the model, but the second is not. The model shows no peak in the evolution of VE max. 7. We conclude that controlled system dynamics, which are the only ones included in dynamic models of ventilatory control, cannot by themselves account for our observations, and that one should take into consideration the dynamics of the controlling neuronal network.

Estimating medullary chemoreceptor blood flow from ventilatory-CO2 response transients: theory and data from anesthetized dogs

We examined the tracer wash-in technique for measuring blood flow for a specific experimental model in which medullary chemoreceptor blood flow is estimated using carbon dioxide as the tracer. Using a mathematical model, we calculated the flow values that would be estimated when blood flow is a function of the tracer concentration, CO2. The estimate was compared with three indices of time-varying blood flow: the steady-state blood flow before and after the step change of CO2 and the time-average blood flow. Estimated blood flow was found to be most closely related to the final value of flow. Perfusion estimates from five pentobarbital anesthetized dogs were 102 ml/100 g/min (S.D. 35) at an average of PaCO2 of 60.7 Torr.

Dynamics of brain extracellular fluid pH and phrenic nerve activity in cats after end-tidal CO2 forcing

Ventilation is influenced by the interstitial [H+] of the brain. The pHecf, which in turn is determined largely by ventilation (via PaCO2) is sensed by the central chemoreceptors. In order to investigate the dynamics of both pHecf and neural tidal volume, we measured in cats with cut vagi and sinus nerves the dynamic medullary pHecf changes and the associated changes in integrated phrenic nerve activity after end-tidal CO2 forcing. The medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. After CO2 up-steps, the pHecf changed with a time constant of about 43 sec, after down-steps 30 sec was found. The central time constant of the neural tidal volume response was 50 sec (mode) in both cases, whereas the overall response had a (modal) time constant of 80 sec. The results indicate that pHecf dynamics and the dynamic characteristics of the central neural respiratory organization are about equally important in determining the dynamic neural tidal volume response. It is argued that when PaCO2 changes, the dynamic pHecf change is perfusion limited and macroscopically homogeneous within the brainstem. Therefore, in our view it seems that the location of the central chemoreceptors within the brainstem is of minor importance in determining the dynamic neural tidal volume response to PaCO2 changes.

Transient and steady state responses of pulmonary ventilation to the medullary extracellular pH after approximately rectangular changes in alveolar PCO2

The extracellular pH (pHe) either on the ventral surface of the medulla oblongata or the parietal cortex, the tidal volume, the expiratory PCO2 and the arterial blood pressure were continuously recorded in anaesthetized or unanaesthetized decerebrate cats. The concentration of the inspired CO2 was manipulated in order to obtain a nearly rectangular increase in the end-tidal PCO2. The responses of VT.f, VE and pH to such a change in PCO2 were observed. The observations from such a preparation were: 1. pHe responded with a delay of 5-7 s to a rectangular variation in end-tidal PCO2. 2. The time constant of the change in the medullary extracellular pH was in the range of 50 s and a similar value was found for VT and VE. 3. The response of VT or VE to a change in the medullary pHe was approximately linear in anaesthetized as well as in unanaesthetized decerebrate cats. The slope of the respiratory response of VT to pHe in decerebrate cats was about 3 times greater than that in anaesthetized cats. There were only slight differences in the relation of VT to pHe between the transient and steady state responses. This means that the "upstroke" of the on-transient of VT was approximately the same as the 'downstroke' of the off-transient. On the other hand, a slight delay was observed when VE was plotted against pHe. Pronounced delay occurred when respiratory frequency was plotted against pHe for on- and off-CO2 transients. 4. A marked hyteresis was observed when VT or VE was plotted against the cortical pHe for on- and off-CO2 inhalation. 5. Such a precise time correlation of the medullary surface pH and VT changes could only be possible if the pH on the ventral medullary surface is representative for the pH at the sensor.

Relative contribution of central and peripheral chemoreceptors to the ventilatory response to CO2 during hyperoxia

Using the technique of artificial ponto-medullary perfusion, the steady state ventilation during hyperoxia was measured in 15 anaesthetized cats as a function of the central PaCO2 (PaCO2) and peripheral PaCO2 (PapCO2). To a first approximation the ventilatory response was linearly related to both the central and peripheral arterial carbon dioxide pressures, viz. VE=SC . PacCO2 + Sp . PapCO2 - K where Sc and Sp represent the overall central and peripheral sensitivity to carbon dioxide. The mean ratio Sp/Sc was 0.48 (range 0.21 to 1.08). In carotid sinus denervated cats Sp was zero, while the values of Sc in these cats were in the range of Sc of cats with intact carotid sinus nerves. It is concluded that the peripse to CO2 under steady-state conditions. Chemodenervation experiments revealed that the carotid bodies play an essential role in this contribution.