Cardiac Output & Performance in Dogs

The term cardiac output (CO) is used to describe the cardiac performance of a dog and is the volume of blood a dog's heart is capable of pumping within the span of one minute. A dog's CO can also be derived by multiplying the heart rate by the stroke volume, termed the cardiac index (CI) when tied to body weight. A dog's cardiac performance can be subject to a number of factors.

Factors That Determine Cardiac Performance

• In addition to heart rate and myocardial oxygen demand (MVO2), factors determining cardiac performance in dogs include preload, where the myocardial cells stretch during diastole and before contraction; afterload, or the resistance encountered by the left ventricle as it attempts to eject blood; contractility, or the innate ability of the heart muscle to shorten and contract; dispensability, which describes the myocardium following contraction as it stretches to accommodate ventricular filling during diastole; and synergy of contraction, or the coordinated, efficient contraction process of all chambers in a normal heart to produce optimal fluid ejection.

Effect of Heart Rate on Cardiac Output

• An increase in heart rate because of increased cardiac performance has the direct effect of increasing cardiac output and cardiac index. At the same time, an increased heart rate leads to higher MVO2 and reduces the time available for preloading. As a result, the heart contains a lower volume of blood following diastole. A higher heart rate also reduces the time available for coronary perfusion, leading to a lower supply of blood to the heart muscle, or myocardium.

Factors That Affect Heart Rate

• Autonomic control is the norm, but sympathetic and parasympathetic stimulation can increase or decrease the heart rate. Factors such as cephalic stimulation, or periods from rest to excitement, can affect autonomic stimulation. Also, watch for increased or decreased respiration and baroreceptor activity, which controls and detects changes in blood pressure.

  Influences on heart rate include the Bainbridge reflex, which refers to how increased cardiac volume leads to higher left atrial pressure and heart rate; mechanoreceptors between the right atrium and cava veins or the left atrium and pulmonary veins; and conditions like tachyarrhythmia (excessively high heart rates) or bradyarrhythmia (excessively low heart rates).

How MVO2 Affects Cardiac Performance

• When a normal dog is at rest, supply and demand for myocardial oxygen exists in approximately equal volumes. A reasonable increase in MVO2 meets with a corresponding increase in cardiac output and supply (coronary flow). If the MVO2 remains unmet, cardiac performance becomes less efficient due to reduced contractility and dysrhythmias can occur.

  A further decline in cardiac output may follow, with a corresponding drop in coronary flow, which in turn perpetuates the anaerobic conditions unless MVO2 itself decreases.

How Contractility Affects Cardiac Performance

• Increased contractility allows for increased stroke volume, where more blood is pumped from the heart chamber with each beat. It also reduces preload, where emptying of the chamber is more complete. Higher contractility also results in higher myocardial oxygen consumption and increased cardiac output that should compensate for the increased MVO2. Failure of cardiac performance to compensate for this demand can lead to detrimental anaerobic conditions. Effects the opposite of what has been described can be expected when contractility decreases.