Interpretive Complications Underlying Cerebrovascular Response To Hypercapnia; Significance Of The Central Respiratory Chemoreflex Transient

Abstract

Cerebral blood flow (CBF) is sensitive to changes in the arterial partial pressure of carbon dioxide (CO2) with cerebrovascular reactivity an established risk factor for stroke and neurodegenerative disease. However, its interpretation can be complicated given subtle differences in cerebrovascular and central respiratory chemoreflex response transients. PURPOSE: To examine to what extent exposure time to CO2 influences CBF response. METHOD: We measured CBF response to hypercapnia (FICO2 = 5%) in five healthy participants for 10 min in the supine position. End-tidal partial pressures of CO2 (PETCO2), minute ventilation (VE), and middle cerebral artery blood velocity (MCAv) were assessed during both the early (3~4 min) and late phases (9~10 min) of exposure. RESULT: We observed elevated VE larger during the late compared to the early phase of exposure (from 11 ± 2 to 29 ± 8 vs. 23 ± 4 L/min, P = 0.047) despite no differences in PETCO2 (P = 0.304). The corresponding increase in MCAv during the late phase was suppressed compared to the early phase (from 48 ± 11 to 58 ± 14 vs. 63 ± 13 cm/sec, P = 0.029). Thus, the response of CBF to change in PETCO2 at late phase was lower than that of early phase (1.1 ± 1 %/mmHg vs. 1.7 ± 1 %/mmHg, P=0.07). CONCLUSION: These findings highlight the importance of considering the central respiratory chemoreflex transient during the clinical assessment of cerebrovascular reactivity.