Articles |
Correspondence to David A. Kass, MD, Carnegie 538, Department of Cardiology, The Johns Hopkins Medical Institutions, 600 N Wolfe St, Baltimore, MD 21287.
Abstract Age-related arterial stiffening and widening of the
pulse pressure elevates ventricular systolic wall stress while it
lowers diastolic coronary perfusion pressure. These changes are thought
to adversely alter the balance between myocardial work load and blood
supply. To test this hypothesis, the native compliant thoracic aorta
was surgically bypassed by a stiff tube in reflex-blocked anesthetized
dogs. Ventricular outflow was directed into either native aorta or the
bypass; the latter resulting in an increase in arterial pulse pressure
from 37.8 to 107.5 mm Hg (P<.001), with minimal change in
mean pressure and flow. Cardiac work load was assessed by
pressure-volume area (PVA), which combines external and internal left
ventricular work and is linearly related to myocardial oxygen
consumption (M
O2), and by
M
O2 itself. Regional phasic and mean
coronary flow were measured in the left anterior descending coronary
artery, and global flow was assessed by radiolabeled microspheres.
Myocardial supply-demand balance was assessed by comparing flow at
matched PVA or M
O2, flow-PVA relations,
and endocardial-to-epicardial flow ratios. When blood flow was directed
into the stiff bypass tube, peak systolic pressure, wall stress, and
PVA all rose nearly 50%, yet diastolic perfusion pressure fell by 20
mm Hg (all P<.01). Rather than being compromised, however,
mean coronary flow rose by 34%, maintaining the same
endocardial-to-epicardial flow ratio (
1.1). Flow augmentation
persisted when data were compared at matched work load (PVA or
M
O2) and mean arterial pressure, as well
as over a range of work loads (P<.001 from ANCOVA of
flow-PVA relations). The increased flow resulted from enhanced systolic
perfusion, which nearly equaled diastolic flow when ejection passed
into the stiff bypass. These data counter the notion that cardiac
coupling with a stiff arterial system (as with aging) necessarily
compromises myocardial flow versus metabolic demand. However, the data
highlight a greater role of systolic flow under such conditions and
also raise the novel suggestion that enhanced pulsatility of the
arterial pressure waveform may itself augment coronary perfusion.
Key Words: coronary flow pressure-volume area arterial compliance systolic hypertension aging
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