CARDIO-RESPIRATORY INTERACTION AND ITS CONTRIBUTION IN SYNCOPE

A hypothetical causal link between ventilatory regulation of carbon dioxide anddevelopment of syncope during orthostatic challenges is reduction in arterial partialpressure of carbon dioxide and resultant reduction in cerebral blood flow. We performedtwo experiments to investigate the ventilatory sensitivity to carbon dioxide and factorsaffecting cerebral autoregulation (CA). We also studied the nonlinear phase couplingbetween cardio-respiratory parameters before syncope.For experiment one, in 30 healthy adults, we stimulated chemo and baro reflexesby breathing either room-air or room-air with 5 percent carbon dioxide in a pseudorandom binary sequence during supine and 70 degree head up tilt (HUT). Six subjectsdeveloped presyncope during tilt.To determine whether changes in ventilatory control contribute to the observeddecrease in PaCO2 during HUT, we assessed ventilatory dynamic sensitivity to changesin PaCO2 during supine and 70 degrees HUT. The sensitivity of the ventilatory controlsystem to perturbations in end tidal carbon dioxide increased during tilt.To investigate nonlinear phase coupling between cardio-respiratory parametersbefore syncope, bispectra were estimated and compared between presyncopal andnon-presyncopal subjects. Our results indicate that preceding presyncope, nonlinearphase coupling is altered by perturbations to baro and chemo reflexes.To investigate the effects of gender in CA, we selected 10 men and 10age-matched women and used spectral analysis to compare differences in CA betweenmen and women. Our results showed that gender-related differences in CA did exist andgender may need to be considered as a factor in investigating CA.To investigate the influence of induced hypocapnia on CA in absence ofventilatory variability, we performed experiment two in which subjects were randomlyassigned to a Control (under normocapnia) or Treatment (under hypocapnia) group. Bothgroups voluntarily controlled their breathing pattern yet two groups breathed in air withdifferent levels of carbon dioxide. Our results show that changes in mean blood pressureat middle cerebral artery level were less transferred into mean cerebral blood flow in theTreatment group than in the Control group, suggesting better CA under hypocapniarelative to under normocapnia