Page 36 - Basic Monitoring in Canine and Feline Emergency Patients

Systolic
pressure
VetBooks.ir

Aortic pressure (mm Hg) Diastolic

pressure

1 cardiac cycle
Time

Fig. 2.2. A diagrammatic representation of the aortic pressure tracing over two left ventricular contraction cycles. The
pressure in the left ventricle increases as the left ventricle contracts until the pressure in the left ventricle exceeds
the diastolic aortic pressure and the aortic valve opens. Blood rapidly flows into the aorta, causing a rapid increase
in aortic pressure until it reaches its peak pressure (also known as the systolic pressure). As the blood leaves the
aorta and the left ventricle relaxes, the pressure in the aorta gradually declines back to baseline (diastolic pressure).
The mean arterial pressure is represented by the shaded area under the curve – due to the wider base and narrower
peak, the mean arterial pressure lies closer to the diastolic pressure than the systolic pressure.

and, therefore, cardiac output. In addition to the arteries and arterioles are the component of the
effects on heart rate and inotropy, the sympathetic network most responsible for SVR and the dis-
nervous system also plays a major role in vascular tribution of vascular tone. Beyond this network,
tone and systemic vascular resistance (SVR). capillaries, venules, and veins all exist but they
Increasing SVR will serve to increase blood pres- are low-resistance vessels with little to no mus-
sure. Resistance of an individual vessel is inversely cular tone, meaning that the vast majority of the
related to its radius; Poiseuille’s equation explains aortic systolic pressure has dissipated by the
that resistance is related to the radius raised to the time the blood reaches this part of the circula-
fourth power. If the radius doubles, then the resist- tion. The systolic blood pressure decreases by
ance will decrease by a factor of 16, and if the approximately 50–70% as it travels through the
radius is halved, then the resistance increases by a small arteries and arterioles. This leads to a
factor of 16. Vascular tone is maintained by the blood pressure at the arterial side of the capillar-
smooth muscle surrounding the vessel. This muscle ies that has dropped to 25–30 mmHg from the
layer can relax (or dilate) to increase blood flow starting pressure of approximately 120 mmHg
downstream of the vessel or can constrict to limit in the aorta.
downstream blood flow; small changes in vessel A multitude of factors are involved in establish-
diameter will have large changes in resistance and ing a vessel’s vascular tone. Extrinsic factors origi-
blood flow to tissues. nate outside of the tissue in which the blood vessel
is located. These factors include hormones such as
angiotensin II and sympathetic nervous system
Regulation of mean arterial pressure:
Systemic vascular resistance stimulation, which tend to cause a more global
change in vascular tone and therefore, SVR.
The systemic arterial network includes large, Intrinsic factors arise from the vessel itself or local
medium and small arteries and arterioles, all of tissues and have a greater role regulating local
which have a smooth muscular layer that allows blood flow; such factors include histamine, arachi-
for maintenance of vascular tone. The small donic acid metabolites, and oxygen tension.

28 D.S. Foy