The myocardium (or heart muscle) is continuously beating and requires a reliable supply of blood delivering oxygen to fuel the muscle contractions. The left and right coronary arteries arise from just above the leaflets of the aortic valve. The left coronary artery quickly branches into two and is the main supply for the left ventricle (Figure 23.1). During systole, when the ventricle is contracting, the pressure in the left ventricular myocardium is so great that coronary artery flow is squeezed, almost halting it. The majority of myocardial perfusion (about 85%) occurs during diastole, the resting stage of the cardiac cycle. An increase in heart rate (HR) can be bad news for myocardial perfusion, as diastole shortens progressively as HR rises. The HR should normally be below 100 beats/min. The myocardium is able to extract 70–80% of the oxygen delivered to it, compared with only 25% extracted by tissues in the rest of the body. The right coronary artery supplies the right ventricle and as this does not generate such high pressure, flow is present throughout the cardiac cycle. The myocardium is damaged if the blood supply is interrupted, reduced or even stopped. This causes the chest pain of acute coronary syndrome (ACS, Chapter 29), possible altered heart rhythms (Chapter 31), or even cardiac arrest and death.

The cardiac conduction system (Figure 23.2) comprises autorhythmic cells, able to generate their own impulses and transmit electrical waves across the heart, stimulating the myocardium to contract. The sinoatrial node (SAN