Anesthesia can cause changes in the function of the cardiovascular system. Cardiovascular monitoring of patients under anesthesia
is necessary so that potentially harmful changes in the function of the cardiovascular system can be recognized and appropriate
action can be taken. The first step in successful monitoring is understanding how the cardiovascular system works. Methods
of monitoring the cardiovascular system include use of an ECG to identify potential arrhythmias, determining heart rate and
rhythm, measuring blood pressure and assessing volume status. Direct measurement of cardiac output in veterinary patients
is rarely done except in research settings. It is invasive and requires the placement of a Swan-Ganz catheter that is passed
from the jugular vein, through the right atrium into the pulmonary artery.
Anatomy and physiology of the heart
The heart is responsible for pumping blood around the body and consists of four chambers: the right atrium, the right ventricle,
the left atrium and the left ventricle. Arteries are the vessels that carry blood from the heart and veins carry blood to
the heart. Sodium, chloride, potassium and calcium are the electrolytes that are most important for normal cardiac function.
Depolarization of the cell happens when sodium channels in the cell membrane open increasing sodium permeability. Resting
membrane potential becomes less negative due to an influx of positive sodium ions. Cells begin repolarizing when the sodium
gates close and negatively charged chloride ions begin to move into the cell. This causes calcium channels to open allowing
an influx of these ions. Final repolarization happens when the calcium channels close and potassium permeability increases.
Any alterations of normal plasma concentrations of these electrolytes can affect cardiac muscle function.
The sino-atrial node in the wall of the right atrium initiates the heartbeat. Impulses from this node transmit to the atrioventricular
(AV) node. Other impulses in the heart are transmitted by the bundle of HIS, the bundle branches and the Purkinje fibers.
Any damage to the cardiac muscle can result in unsynchronized impulse transmission, irregular heart contractions and reduced
cardiac output. The sino-atrial (SA) node acts as an intrinsic pacemaker and controls the rate of contractions. Both parasympathetic
and sympathetic nervous systems innervate the SA node. Acetylcholine and noradrenaline are nervous system mediators that affect
sodium, calcium and potassium channels and can increase or decrease depolarization. Many drugs used for anesthesia purposes
can affect heart rate and therefore monitoring is strongly indicated.
The cardiac cycle is the complete series of events that happens in the heart during one heartbeat. Blood flows into the atria
from the vena cava and pulmonary veins. The cycle starts with depolarization at the sinoatrial node leading to atrial contraction.
The atrioventricular valves, called the mitral and tricuspid valves, open when atrial pressure exceeds ventricular pressure.
While the atria contract, blood flows into the relaxed ventricles. This is diastole-when the ventricles are relaxed and filling.
Next, the atria relax and the ventricles contract (systole) pushing blood out the aortic and pulmonary valves. Ventricular
systole causes closure of the atrioventricular valves and this action is the first heart sound heard on auscultation. The
second heart sound is generated when ventricular relaxation occurs and the pulmonic and aortic valves close.