Cardiac Output
The volume of blood pumped by the heart per minute, calculated as heart rate multiplied by stroke volume. Cardiac output reflects the heart's ability to meet the body's metabolic demands and declines in conditions such as dilated cardiomyopathy and congestive heart failure.
Cardiac output (CO) is the total volume of blood the heart ejects into systemic circulation per minute. It is the product of two variables: heart rate (beats per minute) and stroke volume (the volume of blood ejected per heartbeat). A typical medium-sized dog at rest has a cardiac output of roughly 2 to 4 liters per minute, scaled to body size and metabolic demand.
Determinants of Cardiac Output
Four primary factors govern cardiac output:
- Preload: the volume of blood returning to the heart (venous return). Higher preload stretches the ventricle, increasing stroke volume up to a point (Frank-Starling mechanism).
- Afterload: the resistance the heart must overcome to eject blood. Elevated systemic vascular resistance (e.g., from hypertension) increases afterload and can reduce stroke volume.
- Contractility: the intrinsic strength of myocardial contraction, independent of preload and afterload. Contractility is reduced in dilated cardiomyopathy (DCM).
- Heart rate: higher rates increase CO to a point, but excessively rapid heart rates reduce diastolic filling time, ultimately decreasing stroke volume and CO.
Measurement in Dogs
Cardiac output is not routinely measured in general practice but is assessed in referral settings using:
- Echocardiography: Doppler-derived stroke volume (velocity-time integral at the aortic valve multiplied by aortic cross-sectional area) multiplied by heart rate
- Thermodilution: the gold standard in research and ICU settings, using a pulmonary artery catheter
- Bioimpedance: emerging noninvasive technique, not yet standard in veterinary medicine
When Cardiac Output Falls
Reduced cardiac output triggers compensatory mechanisms: increased heart rate, vasoconstriction, and fluid retention (via the renin-angiotensin-aldosterone system). These compensations maintain blood pressure temporarily but create a harmful feedback loop. In congestive heart failure (CHF), the heart can no longer compensate, and fluid accumulates in the lungs (left-sided failure) or abdomen (right-sided failure).
Breeds at highest risk for cardiac output compromise include Doberman Pinschers and Boxers (DCM), Cavalier King Charles Spaniels (mitral valve disease), and Great Danes (DCM and aortic stenosis).
Longevity Relevance
Maintaining adequate cardiac output throughout life depends on preserving myocardial health. Strategies include early cardiac screening in predisposed breeds, monitoring cardiac biomarkers (proBNP, troponin), maintaining lean body condition to reduce cardiac workload, and appropriate exercise protocols that support cardiovascular conditioning without excessive strain.