Page 39 - Basic Monitoring in Canine and Feline Emergency Patients

An approximate calculation for MAP has been As blood travels away from the heart, the arterial
written as: walls become less compliant (increasingly stiff),
VetBooks.ir MAP = diastolic pressure +  1 × pulse pressure   which leads to an increasing systolic pressure as the
stiffer arterial walls can no longer distend as easily

with the pressure wave. As mentioned above, since

 3
the diastolic pressure does not exhibit as large a
A healthy aorta is relatively compliant and wall change as blood travels peripherally, the increasing
distension along with reflective waves maintains a systolic pressure leads to an increasing pulse pressure
lower pulse pressure in the aorta. Although the in the peripheral vessels versus the aorta. Visually this
primary role of the aorta and large arteries is to act leads to a taller and narrower peripheral arterial
as a conduit for blood delivery to the tissues, a pressure waveform versus the wider and less tall
secondary role is to dampen the pressure changes aortic pressure waveform shown in Fig. 2.2.
that occur in the cardiac cycle. Approximately 10% Therefore, the peripheral non-invasive blood pres-
of the energy produced during left ventricular con- sure measurements that are routinely obtained may
traction is directed toward distension of the aortic overestimate the aortic systolic and pulse pressures.
walls. During diastole, the distended arterial walls However, the MAP measured in the aorta versus the
recoil and reflect energy back into the aorta and MAP in a peripheral vessel should be similar since
this energy continues to squeeze blood forward the area under the curve remains the same despite the
even while the heart is not contracting. This in turn alterations in the waveform as the systolic pressure
helps ensure forward blood flow throughout both and pulse pressures increase (Fig. 2.5). The consistent
systole and diastole. The more compliant or disten- MAP pressure is more important from a cardiac
sible a vessel wall, the greater the wall recoil and cycle perspective (compared to changes in systolic
return of energy to the vessel during diastole. and diastolic pressure) as tissue perfusion is more
Therefore, a more distensible vessel will have a aligned with the MAP than the systolic pressure.
diastolic pressure closer to the systolic pressure and Normal blood pressure readings for dogs and cats
a lower pulse pressure. are listed in Table 2.1; however, as discussed later,

Systolic pressure

Pressure (mm Hg) MAP

Aorta Diastolic pressure Peripheral artery

Fig. 2.5. A diagrammatic representation of how the pressure wave varies from the aorta to the peripheral arteries
(where non-invasive blood pressure monitoring typically occurs). As the walls of the aorta are more compliant, these
walls distend during systole and then recoil during diastole. This property causes some of the energy created during
systole to be stored and then released by the walls of the aorta during diastole and has the effect of dampening the
pressure changes in the aorta. The peripheral artery walls are less compliant and do not absorb as much of the
systolic pressure. This leads to taller and more narrow pressure profiles in the peripheral arteries. However, since
mean arterial pressure (MAP) is the area under the curve, this value remains relatively constant as blood travels from
the aorta to the peripheral arteries.

Blood Pressure Monitoring 31