Determining differences between critical closing pressure and resistance-area product: responses of the healthy young and old to hypocapnia.

Healthy ageing has been associated with lower cerebral blood flow velocities (CBFVs); however, the behaviour of hemodynamic parameters associated with cerebrovascular tone (critical closing pressure, CrCP) and cerebrovascular resistance (resistance-area product, RAP) remains unclear. Specifically, evidence supports ageing being associated with greater cerebrovascular tone and resistance during exercise with elevated CrCP and RAP in older individuals at rest and during exercise. Comprehensive hemodynamic assessment of CrCP and RAP during hyperventilation-induced hypocapnia in two distinct age groups (young ≤ 49 and old > 50) has not been described. CBFV in the middle cerebral artery (CBFV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO2 (EtCO2, capnography) were recorded in 104 healthy individuals (43 young [age 33.8 (9.3) years], 61 old [age 64.1 (8.5) years]) during a minimum of 60 s of metronome-driven hyperventilation-induced hypocapnia. Autoregulation index was calculated as a function of time, using a moving window autoregressive-moving average model. CBFV was reduced in response to age (p < 0.0001) and hypocapnia (p = 0.023) (young 57.3 (14.4) vs. 44.9 cm s-1 (11.1), old 51.7 (12.9) vs. 37.8 cm s-1 (9.6)). Critical closing pressure (CrCP) increased significantly in response to hypocapnia (young 37.6 (18.5) vs. 39.7 mmHg (16.0), old 33.9 (13.5) vs. 39.3 mmHg (11.4); p < 0.0001). Resistance-area product was increased in response to age (p = 0.001) and hypocapnia (p = 0.004) (young 1.02 (0.40) vs. 1.09 mmHg cm s-1 (11.07), old 1.16 (0.34) vs. 1.34 mmHg cm s-1 (0.39)). RAP and not CrCP mediates differences in cerebrovascular resistance responses to hypocapnia between the healthy young and old individuals.