Dr James Rogers,
Frenchay Hospital, Bristol, UK.

Introduction		Systemic circulation
The heart		Blood flow: Hagen-Poisseuille formula
Electrophysiology of the heart		Control of systemic circulation
Cardiac cycle		Control of arterial pressure
Coronary circulation		Cardiovascular responses to anaesthesia
Cardiac output

Introduction

The cardiovascular system consists of the heart and two vascular systems, the systemic and pulmonary circulations. The heart pumps blood through two vascular systems - the low pressure pulmonary circulation in which gas exchange occurs, and then the systemic circulation, which delivers blood to individual organs, matching supply to metabolic demand. Blood pressure and flow is largely controlled by the autonomic nervous system ( The Autonomic Nervous System, Update in Anaesthesia 1995;5:3-6), and is also influenced by surgery and anaesthetic drugs. A good working knowledge of cardiovascular physiology is necessary to practice safe anaesthesia. 
 
The heart

The heart comprises four chambers, and is divided into a right and left side, each with an atrium and a ventricle. The atria act as reservoirs for venous blood, with a small pumping action to assist ventricular filling. In contrast, the ventricles are the major pumping chambers for delivering blood to the pulmonary (right ventricle) and systemic (left ventricle) circulations. The left ventricle is conical in shape and has to generate greater pressures than the right ventricle, and so has a much thicker and more muscular wall. Four valves ensure that blood flows only one way, from atria to ventricle (tricuspid and mitral valves), and then to the arterial circulations (pulmonary and aortic valves). The myocardium consists of muscle cells which can contract spontaneously, also pacemaker and conducting cells, which have a specialised function. 
 
Electrophysiology of the heart

Myocardial contraction results from a change in voltage across the cell membrane (depolarisation), which leads to an action potential. Although contraction may happen spontaneously, it is normally in response to an electrical impulse. This impulse starts in the sinoatrial (SA) node, a collection of pacemaker cells located at the junction of the right atrium and superior vena cava. These specialised cells depolarise spontaneously, and cause a wave of contraction to pass across the atria. Following atrial contraction, the impulse is delayed at the atrioventricular (AV) node, located in the septal wall of the right atrium. From here His-Purkinje fibres allow rapid conduction of the electrical impulse via right and left branches, causing almost simultaneous depolarisation of both ventricles, approximately 0.2 seconds after the initial impulse has arisen in the sinoatrial node. Depolarisation of the myocardial cell membrane causes a large increase in the concentration of calcium within the cell, which in turn causes contraction by a temporary binding between two proteins, actin and myosin. The cardiac action potential is much longer than that of skeletal muscle, and during this time the myocardial cell is unresponsive to further excitation. 
 
The cardiac cycle

The relationship between electrical and mechanical events in the cardiac cycle is summarised in Figure 1. There is a similar cycle on both sides of the heart, but the pressures in the right ventricle and pulmonary arteries are less than those in the left ventricle and aorta. Systole refers to contraction, while diastole refers to relaxation. Both contraction and relaxation can be isometric, when changes in intraventricular pressure occur without a change in length of the muscle fibres. The cycle starts with depolarisation at the sinoatrial node leading to atrial contraction. Until this time blood flow into the ventricles has been passive, but the atrial contraction increases filling by 20-30%.

Ventricular systole causes closure of the atrioventricular valves (1st heart sound), and contraction is isometric until intraventricular pressures are sufficient to open the pulmonary and aortic valves, when the ejection phase begins.

The volume of blood ejected is known as the stroke volume. At the end of this phase ventricular relaxation occurs, and the pulmonary and aortic valves close (2nd heart sound). After isometric relaxation ventricular pressures fall to less than atrial pressures. This leads to opening of the atrioventricular valves and the start of ventricular diastolic filling. The whole cycle then repeats following another impulse from the sinoatrial node. 

Teaching Point

The electrocardiogram (ECG) measures changes in skin electrical voltage/potential caused by electrical currents generated by the myocardium. The P wave reflects atrial depolarisation, the QRS complex ventricular depolarisation, and the T wave ventricular repolarisation (Figure 1). Repolarisation is a process that occurs in many cells where the electrical potential across the cell membrane returns from the value during the action potential to that of the resting state, the resting potential. Although the ECG shows heart rate and rhythm and can indicate myocardial damage, it gives no information on the adequacy of contraction. Normal electrical complexes can exist in the absence of cardiac output, a state known as pulseless electrical activity or electromechanical dissociation

(Continued ...)

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