Читать книгу Manual of Equine Anesthesia and Analgesia - Группа авторов - Страница 83

 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.