Page 34 - Basic Monitoring in Canine and Feline Emergency Patients

VetBooks.ir 2 Blood Pressure Monitoring

Daniel S. Foy*

College of Veterinary Medicine, Midwestern University, Glendale, Arizona, USA

2.1 Basic Physiology and Anatomy Many factors are incorporated into the MAP,
which is physiologically defined by the following
When discussing and evaluating blood pressure, the equation:
reference is almost always to systemic blood pres-
sure. However, the two sides of the heart (more MAP = cardiacoutput systemicvascularresistance
×
specifically the two ventricles) each create their own
pressure; the right ventricle generates the pulmo- There are individually many components that can
nary blood pressure, while the left ventricle gener- affect cardiac output. Cardiac output is defined as
ates the systemic (arterial) blood pressure (Fig. 2.1). stroke volume (amount of blood ejected by the left
The pulmonary circulation is a lower pressure sys- ventricle per beat) multiplied by the heart rate (con-
tem with systolic pressures normally ~25 mmHg. tractions per minute). Therefore, any factor that
The systemic circulation is a far higher pressure increases the heart rate (providing all other varia-
system with the systolic pressures normally ~120 bles remain constant) will increase MAP, and any
mmHg. This chapter will focus on the systemic factor that increases the stroke volume (providing
blood pressure generated by the left ventricle. all other variables remain constant) will increase
Systemic blood pressure is first generated by the MAP, and vice versa. Heart rate and stroke volume
contraction of the left ventricle. Isovolumetric contrac- are both affected by variables ranging from blood
tion increases pressure within the left ventricle until the volume to neurohormonal factors affecting the
pressure exceeds the resting (diastolic) pressure in the return of blood to the heart and/or the passage of
aorta. At that point, blood is ejected from the left ven- blood out of the heart into the vessels (Fig. 2.3).
tricle into the aorta; the maximal pressure in the aorta
achieved during the contraction phase of the left ven-
tricle is called the systolic pressure. As the contraction
phase of the left ventricle ends, the aortic pressure Regulation of mean arterial
begins to drop and the aortic valves close, preventing pressure: Stroke volume
retrograde flow of blood back into the left ventricle. As the blood volume is increased in the body, the
The lowest aortic pressure achieved during the left veins will generally increase the volume of blood
ventricular relaxation phase is the diastolic pressure. within their walls. In most physiologic states, as the
The difference between the systolic pressure and dias- volume of blood in the veins increases, the pressure
tolic pressure is the pressure that may be felt when within the vena cava returning blood to the heart
palpating pulses and is termed the pulse pressure. increases. The volume of returned blood will affect
A pressure tracing can be created which illus- the degree of ventricular myocardial distension
trates the pressure in the aorta throughout the prior to contraction (i.e. increased return of blood
entire contraction and relaxation cycle of the left causes increased ventricular distension). It is this
ventricle (Fig. 2.2). The area under this curve is the myocardial distension which defines the preload.
mean arterial pressure (MAP). The wide base and Therefore in most instances, as the blood volume
relatively narrow peak of this curve illustrates the increases, the return of blood to the right side of the
MAP is almost always closer to the diastolic pres- heart increases and myocardial distension (i.e. pre-
sure than the systolic pressure. load) increases.

* Corresponding author: dfoy1@midwestern.edu

26 © CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)