7th February 2024

Summary

Cardiac output is the volume of blood ejected by the left or right ventricle per minute. It is dependent on stroke volume and heart rate.

Cardiac output = stroke volume x heart rate

The following describes how each factor affects cardiac output:

Preload

Preload refers to the degree to which the cardiac muscle fibres are stretched during the diastolic phase. Increased stretch in the cardiac myocytes will increase contractility up to a certain point. Beyond this, further increased stretch will reduce cardiac contractility. The Frank-Starling curve defines this law.

Upwards movement of the curve is seen when there is an overall positive inotropic effect, including exercise, increased sympathetic drive and digoxin. Downwards movement of the curve is seen in overall negative inotropic states, including beta-blockers, calcium channel blockers and cardiac failure. 

Cardiac contractility

Muscle contractility is seen as the force that the heart generates in a single heartbeat. Factors that affect this can be divided into positive or negative inotropes

• Positive inotropes:
  • Catecholamines and the sympathetic nervous system
  • Dopamine and isoproterenol. These stimulate the influx of calcium ions into myocytes
  • Digoxin increases intracellular calcium ion levels by reducing calcium movement out of the cell.
  • Hormones, including glucagon and thyroid hormones
• Negative inotropes
  • Beta-blockers
  • Calcium channel blockers
  • Hypoxia
  • Acidosis
  • Cardiac failure. 

Afterload

Afterload is the force resisting the contraction of cardiac myocytes. Mean arterial pressure and systemic vascular resistance reflect left ventricular afterload. Pulmonary vascular resistance is used to reflect right ventricular afterload. Factors that increase the systemic vascular resistance will increase afterload and reduce stroke volume. 

Heart rate

The heart rate is usually affected by the autonomic nervous system and medications that affect heart rate directly ( e.g. bisoprolol and ivabradine). On the usual spectrum, an increase in heart rate will increase the cardiac output. Heart rate also impacts the heart’s ability to fill during diastole during extremely high heart rates, affecting stroke volume.