Ch 31 CV Assessment.docx - Chapter 31: Assessment of the CV...

Chapter 31: Assessment of the CV System
Factors Affecting CO
- Stroke Volume: amount of blood ejected from left ventricle per contraction
- Factors affecting SV:
o Preload
 The volume of blood in the ventricles at the end of diastole, before the
next contraction
 Determines the amount of stretch placed on myocardial fibers
 Can be increased by conditions such as hypertension, aortic valve disease,
hypervolemia
 Frank-Starling law : the more the myocardial fibers are stretched, the
greater their force of contraction
o Contractility
 Can be increased by epinephrine and norepinephrine released by SNS
o Afterload
 Peripheral resistance against which the left ventricle must pump
 Affected by the size of the ventricle, wall tension and arterial BP
 If arterial BP is elevated, the ventricles meet increased resistance to
ejection of blood, increasing the work demand
 Eventually results in ventricular hypertrophy, an enlargement of the heart
muscle without an increase in CO or the size of chambers
Cardiac Reserve
- Ability of CV system to respond to demands by altering CO in numerous situations in
health and illness (exercise, stress, hypovolemia)
Arteries & Arterioles
- Thick walls composed mainly of elastic tissue (cushions the impact of the pressure
created by ventricular contraction and provides recoil that propels blood forward into
circulation)
Capillaries
- Exchange of cellular nutrients and metabolic end products
- Connects arterioles and venules
Veins & Venules
- Large-diameter
- Thin-walled
- Low-pressure, high-volume system
- Larger veins contain semilunar valves at intervals to maintain the blood flow toward the
heart and prevent backward flow

Baroreceptors
- Aortic arch and carotid sinus (at the origin of the internal carotid artery)
- Sensitive to stretch or pressure within arterial system
- Stimulation (e.g., volume overload) sends information to the vasomotor center in brain
o Results in temporary inhibition of SNS and enhancement of PNS causing
decreased HR and peripheral vasodilation
Chemoreceptors
- Aortic and carotid bodies and the medulla
- Cause changes in respiratory rate and BP in response to increased arterial CO2 pressure
(hypercapnia) and to a lesser degree decreased plasma pH (acidosis) and arterial O2
pressure (hypoxia)
- When chemoreceptors in the medulla are triggered, they stimulate the vasomotor
center to increase BP
Blood Pressure
- Measurement can be
o Invasive: catheter insertion into an artery
o Noninvasive
- Auscultatory gap: sometimes occurs; loss of sound between SBP and DBP
Pulse Pressure
- Difference between SBP and DBP
- Normally about 1/3 of SBP
- Increased PP due to an increased SBP may occur during exercise or in individuals with
atherosclerosis of large arteries
- Decreased pulse pressure may be found in HF or hypovolemia
Mean Arterial Pressure (MAP)
- Average pressure within the arterial system that is felt by the organs in the body
- NOT the average of DBP and SBP
- > 60mmHg needed to adequately perfuse and sustain the vital organs of an average
person under most conditions