Cardiac Output

Cardiac output (CO) is the volume of blood ejected by a ventricle each minute: CO = heart rate (HR) × stroke volume (SV) Stroke volume is the amount ejected with each contraction. It changes with preload, afterload, and contractility. Preload reflects ventricular filling and myocardial fibre stretch before contraction. Within physiological limits, greater preload increases stroke volume through the Frank–Starling mechanism. Afterload is the resistance the ventricle must overcome to eject blood; systemic vascular resistance is a major component of left-ventricular afterload. Contractility is the intrinsic strength of myocardial contraction, independent of preload. The formula explains why a fast pulse does not necessarily mean adequate output. A modest increase in heart rate may preserve CO, but extreme tachycardia shortens diastolic filling time, reduces stroke volume, and increases myocardial oxygen demand. Severe bradycardia reduces CO by removing too many contractions per minute. A failing ventricle may also eject a small stroke volume despite an adequate heart rate. Cardiac index (CI) adjusts CO for body surface area: CI = CO ÷ body surface area CI is more useful than an isolated CO...