to the AV node.
Right and left bundle branches and His‐Purkinje system.Conduct impulses throughout ventricles and ventricular septum.
III Unique features of the equine heart
Large SA node.A “wandering pacemaker” is common.Seen as variably shaped P waves on electrocardiogram (ECG).
Large atria that may depolarize slightly asynchronously.Result in biphasic P or bifid waves on the ECG.
Deeply penetrating His‐Purkinje system.Facilitates movement of electrical impulses throughout the large ventricular muscle. Often called Type II Purkinje system.
Ossa cordi.In all species there is a connective tissue “skeleton” that separates the atria from the ventricles.In cattle and in older horses, these structures may ossify and create two bones, the “ossa cordi.”
IV Circulatory systems
The systemic (high pressure) and pulmonary (low pressure) circulatory systems are separate but coupled (in series) and interdependent so that dysfunction of one will lead to dysfunction of the other.
The circulatory systems are not just mere conduits, but are active and, through dilation and constriction, control distribution of blood throughout the body and in localized tissue beds.
The lymphatic system is often included as a component of the circulatory system.
A Components of the systemic circulation
Aorta → Arteries → Arterioles → Capillaries → Venules → Great veins → Right atrium
The elastic wall of the aorta recoils following ventricular contraction, creating a force that maintains blood flow throughout systole and diastole.
Arterioles provide the greatest resistance to circulation and, via dilation or constriction, control blood flow to each tissue capillary bed.
Capillaries are the site of exchange of nutrients and waste products.
The majority of the circulating blood volume (approximately 80%) is generally “stored” in the venules and great veins.
B Components of the pulmonary circulation
Pulmonary artery → Arterioles → Capillaries → Pulmonary vein → Left atrium
The pulmonary artery is the only artery in the body that carries deoxygenated blood, and the pulmonary vein is the only vein that carries oxygenated blood.
Although the pulmonary circulation receives the same cardiac output as the systemic circulation, the pulmonary system remains a low‐pressure system due to the:Tremendous dispensability of the thin‐walled vessels.The large number of vessels that are not normally perfused but that can be recruited in times of increased output.
Distribution of pulmonary blood vessels is an important component of ventilation/perfusion (V/Q) distribution and gas exchange.
Unlike most tissues in the body, pulmonary tissues constrict when hypoxic (hypoxic pulmonary vasoconstriction) in an attempt to divert blood away from poorly ventilated alveoli. This phenomenon can contribute to V/Q mismatch, especially during anesthesia since atelectasis is common in anesthetized horses.
The lung also receives blood flow through the bronchial circulation, a branch of the systemic circulation that perfuses the tissues of the respiratory system.
C Blood
Consists of plasma and cellular components.
Normal equine hematocrit or packed cell volume (PCV) is approximately 35–45% and normal hemoglobin is approximately 15 g/dl.Most oxygen is transported bound to hemoglobin (see section on DO2)When saturated, equine hemoglobin binds 1.36–1.39 ml of oxygen per gram of hemoglobin (Hüfner's constant).
V Cardiovascular physiology
The cardiac cycle can be described as a period of ventricular contraction (systole) followed by ventricular relaxation (diastole).
The electrical, mechanical and audible events that occur during the cardiac cycle are depicted in the Wigger's diagram (see Figure 3.1) and described below.
Figure 3.1 The Wigger's diagram depicts the electrical, mechanical and audible events of the cardiac cycle. SLV, semilunar valve; MV, mitral valve; a, atrial contraction; c, tricuspid bulge; v, atrial filling.
A Events occurring during late diastole
The cardiac cycle begins with the spontaneous discharge of the pacemaker, the SA node.
Discharge is followed quickly by electrical activation of the right atrial muscle and then the left atrial muscle.This results in the P wave on the ECG.Passive filling of the ventricles occurs during this period.Because electrical activation always precedes mechanical activity (termed the electromechanical delay), the actual atrial contraction occurs shortly after the P wave is generated.
The rapid flow of blood from atrium to ventricle following atrial contraction generates the atrial or fourth heart sound (S4) and adds blood to the ventricles so that end‐diastolic blood volume (or preload) is reached.The atrial contribution to the ventricular blood volume is generally minimal and not affected by atrial arrhythmias such as atrial fibrillation.However, during high HRs when the diastolic filling time is shortened and in patients with impaired contractility and decreased stroke volume (SV) the atrial contribution becomes a significant percentage of the total ventricular volume and subsequent ejection fraction.
Atrial contraction causes a rise in atrial pressure (“a” wave) which is transmitted up the systemic venous system and often produces a normal jugular pulse.
The atrial excitation wave reaches the medial wall of the right atrium and is conducted slowly through the AV node.This results in the PR interval on the ECG.AV block occurs when the impulse from the atria is not conducted through the AV node to the ventricles.This is reflected on the ECG as a P wave that is not followed by a QRS.In the horse, AV block is generally normal due to inherently high vagal tone, and is considered benign if the block is abolished by exercise or excitement.
B Events occurring during systole
The impulse exits the AV node and electrical activation of the ventricles occurs.This
