Veterinary Clinical Medicine-I (General and Systemic)

UNDERSTANDING THE FUNCTIONAL RESPONSE TO HEART DISEASE

The importance of the components of the cardiovascular reserve becomes apparent in understanding the functional response to heart disease. The following factors must be considered (and their limitations).

1. Venus oxygen reserve
2. The maximus effective heart rate
3. The stroke-volume reserve
4. Cardiac work and efficiency
5. Coronary vascular reserve
6. Cardiac enlargement

Venous Oxygen Reserve

• Exercise taxes the cardiovascular reserve and serves as a model to define its component. The amount of exercise or activity that can be performed over any extended period of time is limited by the maximal capacity of the cardiovascular system to deliver oxygen to the tissues.
• The amount of oxygen consumed by the tissues is limited by the cardiac output and by the amount of oxygen extracted from each increment of blood. Therefore, oxygen availability to tissues is dependent upon cardiac output and the magnitude of the arterial-venous oxygen difference.
• Typically, each 100 ml of arterial blood contains l9cc of oxygen. At rest, mixed venous blood contains about 14cc of oxygen per l00ml. Oxygen extraction increases progressively with exercise so that at maximal levels of exertion, 75% of the oxygen may be removed from the blood passing through the systemic capillaries.
• The venous oxygen reserve can be depleted to this extent only if extraction increases in both active and inactive tissues. Vasoconstriction during exertion diverts blood flow from the skin, kidneys, gastrointestinal tract, and spleen, making more oxygen available for active muscles.

Heart Rate

• Heart rate is the most liable factor in the cardiac reserve an increase in heart rate in the principle means by which cardiac output is increased in response to an increased demand for blood flow. The maximal effective increase in heart rate is approximately two and one half times the normal rate for that animal.
• Example: In the dog the resting rate is about 70 beats per minute, however the heart rate in this species is capable of increasing its maximum cardiac output effectively up to 180 beats per minute. Beyond this rate. Tachycardia results.
• Increased heart rate usually do not occur as isolated events, but rather as part of a coordinated cardiovascular response of autonomic (sympathetic) nervous system origin.

Stroke Volume

• Mean the amount of blood the heart ejects with each beat. When the normal resting heart contracts, it empties by only one half. Thus it can theoretically increase its output by emptying to a greater degree or utilizing the systolic reserve volume.

Efficiency of the Myocardium

• The efficiency of the myocardium is the amount of useful work performed divided by the total energy expended. Thus, myocardial efficiency may improve in the normal animal during exercise since useful work is increased more than myocardial oxygen consumption. In congestive heart failure, the efficiency of energy conversion is reduced at rest and declines further during exercise.

Coronary Vascular Reserve

• The amount of oxygen available to the myocardium depends upon the coronary blood flow and the arterial-venous oxygen difference.
• The normal heart, even at rest extracts most of the oxygen available to it. (Therefore, the myocardium has very little coronary venous oxygen reserve).

Cardiac Enlargement

• Hypertrophy
• Dilatation
  • Both maybe due to a physiologic compensatory mechanisms which enables the heart to adapt to an increased work load.
• Compensatory mechanisms (most are self limiting)

a. Tachycardic

b. Chamber dilatation

c. Wall hypertrophy