Ribcage contribution to CO2 response during rebreathing and steady state methods

Cerebral blood flow sensitivities to CO2 measured with steady-state and modified rebreathing methods

It is well established that the ventilatory response to carbon dioxide (CO(2)) measured by modified rebreathing (Sr(VE)) is closer to that measured by the steady-state method (Ss(VE)) than is the response measured by Read's rebreathing method. It is also known that the value estimated by the steady-state method depends upon the combination of data points used to measure it. The aim of this study was to investigate if these observations were also true for cerebral blood flow (CBF), as measured by steady-state (Ss(CBF)) and modified rebreathing (Sr(CBF)) tests. Six subjects undertook two protocols: (a) steady state: PET(CO2) was held at 1.5 mm Hg above normal (isocapnia) for 10 min, then raised to three levels of hypercapnia, (8 min each; 6.5, 11.5 and 16.5 mm Hg above normal, separated by 4 min isocapnia). End-tidal PO2 was held at 300 mm Hg; (b) modified rebreathing: subjects underwent 6 min of voluntary hyperventilation to PET(CO2) approximately 20 mm Hg, and then rebreathed via a 6l bag filled with 6.5% CO(2) in O(2). We confirmed that the value for Ss(VE) depended upon the combination of data points used to calculate it, and also confirmed that Ss(VE) and Sr(VE) were similar. However, this was not the case with CBF. Estimates of Ss(CBF) were the same, regardless of the data points used in calculation, and Ss(CBF) was 89% greater than Sr(CBF) (P<0.05). We interpret these findings as consistent with the notion that the specific CO(2) stimulus differs for CBF and ventilatory control. The data also indicate that prior hypocapnia in the modified rebreathing protocol may have a persistent effect on both cerebral vessels and central ventilatory control.

Cerebral blood flow sensitivity to CO2 measured with steady-state and Read's rebreathing methods

The ventilatory response to carbon dioxide (CO2) measured by the steady-state method is lower than that measured by Read's rebreathing method. A change in end-tidal P CO2 (PET CO2) results in a lower increment change in brain tissue P CO2 (Pt CO2) in the steady-state than with rebreathing: since Pt(CO2) determines the ventilatory response to CO2, the response is lower in the steady-state. If cerebral blood flow (CBF) responds to Pt CO2, the CBF-CO2 response should be lower in the steady-state than with rebreathing. Six subjects undertook two protocols, (a) steady-state: PET CO2 was held at 1.5 mmHg above normal (isocapnia) for 10 min, then raised to three levels of hypercapnia, (8 min each; 6.5, 11.5 and 16.5 mmHg above normal, separated by 4 min isocapnia). End-tidal P O2 was held at 300 mmHg; (b) rebreathing: subjects rebreathed via a 6 L bag filled with 6.5% CO2 in O2. Transcranial Doppler-derived CBF yielded a higher CBF-CO2 sensitivity in the steady-state than with rebreathing, suggesting that CBF does not respond to Pt CO2.

Measuring central-chemoreflex sensitivity in man: rebreathing and steady-state methods compared

We compared the central-chemoreflex sensitivities estimated from steady-state tests with those estimated from rebreathing tests in five subjects. In one laboratory, each subject underwent nine dynamic end-tidal forcing experiments. Three repetitions of 3, 6 and 9 mmHg step changes in the end-tidal partial pressure of carbon dioxide, from a pre-step partial pressure 1.5 mmHg above resting, were used to establish four points of the steady-state ventilatory response to carbon dioxide. In another laboratory, each subject underwent two rebreathing experiments, one using Read's rebreathing technique and the other a modified rebreathing method which included a prior hyperventilation. The central-chemoreflex sensitivities, estimated from the slopes of the ventilatory responses to carbon dioxide using different combinations of the four steady-state points. were compared to those estimated from the slopes of the rebreathing responses. The steady-state sensitivities were significantly lower than the Read rebreathing sensitivities. The ratio of modified rebreathing sensitivities to steady-state sensitivities was closest to one when steady-state sensitivities were estimated from the two middle points of the ventilatory responses. The mean (SE) ratio of the sensitivities was 1.22 (0.21) in this case. We identify a number of factors that may affect the estimation of central-chemoreflex sensitivity using each technique. These include a maximum limit of the ventilation response at high partial pressures of carbon dioxide, an inability to sustain high ventilation for the duration of the steady-state tests and the inclusion of parts of the ventilatory response whose carbon dioxide partial pressures lie below the central-chemoreflex threshold. We conclude that the modified rebreathing method provides the best estimate of central-chemoreflex sensitivity of the three methods.

Association of chest wall motion and tidal volume responses during CO2 rebreathing

The purpose of this study is to investigate the effect of chest wall configuration at end expiration on tidal volume (VT) response during CO2 rebreathing. In a group of 11 healthy male subjects, the changes in end-expiratory and end-inspiratory volume of the rib cage (delta Vrc,E and delta Vrc,I, respectively) and abdomen (delta Vab,E and delta Vab,I, respectively) measured by linearized magnetometers were expressed as a function of end-tidal PCO2 (PETCO2. The changes in end-expiratory and end-inspiratory volumes of the chest wall (delta Vcw,E and delta Vcw,I, respectively) were calculated as the sum of the respective rib cage and abdominal volumes. The magnetometer coils were placed at the level of the nipples and 1-2 cm above the umbilicus and calibrated during quiet breathing against the VT measured from a pneumotachograph. The delta Vrc,E/delta PETCO2 slope was quite variable among subjects. It was significantly positive (P < 0.05) in five subjects, significantly negative in four subjects (P < 0.05), and not different from zero in the remaining two subjects. The delta Vab,E/delta PETCO2 slope was significantly negative in all subjects (P < 0.05) with a much smaller intersubject variation, probably suggesting a relatively more uniform recruitment of abdominal expiratory muscles and a variable recruitment of rib cage muscles during CO2 rebreathing in different subjects. As a group, the mean delta Vrc,E/delta PETCO2, delta Vab,E/delta PETCO2, and delta Vcw,E/delta PETCO2, slopes were 0.010 +/- 0.034, -0.030 +/- 0.007, and -0.020 +/- 0.032 1/Torr, respectively; only the delta Vab,E/delta PETCO2, slope was significantly different from zero. More interestingly, the individual delta VT/delta PETCO2 slope was negatively associated with the delta Vcr,E/delta PETCO2 (r = 0.68, P = 0.021) and delta Vcw,E/delta PETCO2 slopes (r = 0.63, P = 0.037) but was not associated with the delta Vab,E/delta PETCO2 slope (r = 0.40, P = 0.223). There was no correlation of the delta Vrc,E/delta PETCO2 and delta Vcw,E/delta PETCO2 slopes with age, body size, forced expiratory volume in 1 s, or expiratory time. The group delta Vab,I/delta PETCO2 slope (0.004 +/- 0.014 1/Torr) was not significantly different from zero despite the VT nearly being tripled at the end of CO2 rebreathing. In conclusion, the individual VT response to CO2, although independent of delta Vab,E, is a function of delta Vrc,E to the extent that as the delta Vrc,E/delta PETCO2 slope increases (more positive) among subjects, the VT response to CO2 decreases. These results may be explained on the basis of the respiratory muscle actions and interactions on the rib cage.