maneuver to the shortest R-R interval during straining. Finally,
the centroperipheral Valsalva ratio (CPVR) was derived
from the velocity and arterial blood flow data for
Valsalva phase IIb and defined as the quotient of CVR to the
corresponding peripheral blood pressure acceleration (dv/dt/
dp/dt) in cm/mm Hg ท s. This is deemed valid because cerebral
perfusion pressure varies with arterial blood pressure,
and intracranial pressure is practically constant during phase
II of VM and cerebral blood flow velocity varies with cerebral
blood flow.36,37 pCO2 retention was determined by computing
the difference between values before and after the experiment.
Demographic parameters between groups were analyzed using
the v2 test. Comparisons of Doppler parameters were performed
using Student’s t-test. Differences of clinical parameters
were analyzed using Mann Whitney U test and Wilcoxon
test. For all tests, the level of significance was set at p < 0.05.