CORONARY BLOOD FLOW
Dr. David Lyness@Gas_Craic
- Blood flow to the heart occurs mainly during diastole.
- Coronary blood flow is mainly determined by local oxygen demand.
- The vascular endothelium is the final common pathway controlling vasomotor tone.
- When anaesthetising patients with coronary artery disease, maintain coronary perfusion pressure and avoid tachycardia.
The heart has the highest oxygen consumption per tissue mass of all human organs.
The resting coronary blood flow is ∼250 ml/min; 5% of cardiac output.
Ischaemia results when oxygen demand outstrips supply.
Sympathetic Stimulation - (Beta)
Parasympathetic stimulation - (Muscarinic)
Sympathetic Stimulation (Alpha)
TWO coronary ostia arise from the sinuses of Valsalva just above the aortic valve.
The LEFT CA divides into the left anterior descending artery and circumflex artery.
Supplies the lateral and anterior walls of the left ventricle, and the anterior two thirds of the interventricular septum.
The RIGHT CA supplies the right ventricle, the posterior wall of the left ventricle and posterior third of the septum.
The major CA's divide into epicardial arteries. Intramuscular arteries penetrate the myocardium perpendicularly to form subendocardial arterial plexuses.
Most of the blood from left ventricular muscle drains into the coronary sinus.
The anterior cardiac vein receives blood from the right ventricular muscle.
They both open into the right atrium.
Thebesian veins drain a small proportion of coronary blood directly into the cardiac chambers and account for true shunt.
- Arterial oxygen extraction is 70–80%, compared with 25% for the rest of the body.
- Increased oxygen consumption must be met by an increase in coronary blood flow
- CorBF may increase fivefold during exercise. Supply usually closely matches any change in demand.
- An increase in coronary blood flow can independently increase myocardial oxygen consumption (Gregg effect).
- Explained by full coronary arteries splinting the heart and increasing the end-diastolic fibre length and contractility.
Coronary perfusion pressure (CPP)
- During systole, IM vessels are compressed and twisted by the contracting muscle and blood flow to the LV is at its lowest.
- CPP = difference between the aortic diastolic pressure and LV end-diastolic pressure (LVEDP) (or preload).
- Phasic changes in blood flow to the RV are less pronounced because of the lesser force of contraction.
- Any increase in heart rate impinges on diastolic time more than systolic time and reduces the perfusion time.
Vessel wall diameter
- Vasomotor tone and deposits inside the vascular lumen determine the vessel wall diameter.
Vasomotor Tone Control
- Oxygen delivery is the product of arterial oxygen carrying capacity and myocardial blood flow.
Diastolic Time = (60 s/heart rate − 0.2 s)Note systole normally is around 0.2s
Ratio DPTI/TTI = endocardial viability ratio (EVR) = myocardial oxygen supply-demand balance.The EVR is normally 1 or more. A ratio <0.7 is associated with subendocardial ischaemia.
Flow down left coronary only occurs in diastole- hence tachycardia worsens ischaemia