Cardiac Output Determinants

Cardiac Output Determinants

🟢 Lite — Quick Review (1h–1d)

Cardiovascular physiology describes how the heart pumps blood, how vessels resist flow, and how the body regulates pressure to perfuse tissues. The core equation is CO = SV × HR, where cardiac output (L/min) equals stroke volume (mL/beat) multiplied by heart rate (beats/min). Mean Arterial Pressure is calculated as MAP = (CO × SVR) + CVP, or more practically as diastolic BP plus one-third of pulse pressure. Ejection fraction (EF) quantifies pump efficiency: EF = (EDV − ESV) / EDV × 100%, where normal exceeds 55%.

Three high-yield exam pointers for Saudi GP Board: (1) A Killip Class III patient presents with pulmonary edema — mortality risk jumps sharply compared to Class I. (2) Wenckebach block shows progressive PR prolongation before a dropped beat; Mobitz Type II does not. (3) MAP thresholds guide hypertension treatment — aim ≥65 mmHg in sepsis, maintain SBP ≥90 mmHg in shock.

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🟡 Standard — Regular Study (2d–2mo)

Cardiac Output Determinants

Cardiac output (5 L/min at rest) results from the interaction of stroke volume and heart rate. Stroke volume itself depends on three factors: preload (venous return filling the ventricle via Frank-Starling), afterload (arterial resistance opposing ejection), and contractility (intrinsic inotropic state). The relationship is unidirectional for preload (more filling → more output) but inverse for afterload (greater resistance → reduced output). This distinction is a frequent trap in exam questions.

Blood Pressure Regulation

Systemic Vascular Resistance (SVR) opposes flow through arterioles and is calculated as SVR = (MAP − CVP) / CO × 80 (units: dyn·s·cm⁻⁵). Baroreceptors in the carotid sinus and aortic arch sense pressure changes and modulate sympathetic/parasympathetic outflow within seconds. The renin-angiotensin-aldosterone system provides slower hormonal regulation over minutes to hours.

The Cardiac Cycle

Systole (contraction, ~0.3s) precedes diastole (relaxation, ~0.5s) in each cycle. During diastole, ventricular filling produces the S3 gallop in heart failure or the S4 from stiff, non-compliant ventricles. Murmurs require characterization by timing (systolic/diastolic), location, radiation, and quality — innocent murmurs lack associated symptoms and do not impair cardiac function.

Conduction Blocks

AV nodal blocks are classified by severity. Wenckebach (Mobitz Type I) demonstrates progressive PR interval prolongation culminating in a non-conducted P wave, then repeats cyclically. Mobitz Type II shows fixed prolonged PR intervals with intermittent dropped beats and carries greater progression risk to complete heart block.

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🔴 Extended — Deep Study (3mo+)

The Fick Principle

Oxygen consumption measurement underpins the gold-standard cardiac output calculation: CO = V̇O₂ ÷ (CaO₂ − CvO₂), where V̇O₂ is total body oxygen consumption (mL O₂/min), CaO₂ is arterial oxygen content, and CvO₂ is mixed venous content. A patient with V̇O₂ = 250 mL/min, CaO₂ = 20 mL O₂/dL, and CvO₂ = 15 mL O₂/dL yields CO = 250 ÷ 5 = 50 dL/min = 5 L/min. This method is reserved for research and catheterization labs — not clinical floor calculations.

Common Mistakes in Hemodynamic Calculations

1. Units mismatch: CO in L/min, SV in mL/beat produces HR in beats/min when multiplied, but requires dividing SV by 1000 to maintain L/min units.
2. MAP misconceptions: MAP ≈ diastolic + (pulse pressure ÷ 3) works when heart rate is normal; the full formula is essential in bradycardia or tachycardia.
3. EF assumptions: Ejection fraction assumes normal left ventricular dimensions — it becomes unreliable in restrictive cardiomyopathy where small end-systolic volume produces normal EF despite severe dysfunction.

Killip Classification in Acute MI

This prognostic stratification directly influences management and mortality estimates:

Class	Clinical Finding	Approximate In-Hospital Mortality
I	No heart failure signs	~6%
II	Rales, S3, jugular venous distension	~17%
III	Frank pulmonary edema	~30–40%
IV	Cardiogenic shock	>50%

NYHA Functional Classification

Beyond Killip, the NYHA classification guides chronic heart failure management: Class I (ordinary activity causes no symptoms) through Class IV (symptoms at rest). Saudi GP Board frequently pairs this with drug selection questions — ACE inhibitors for Classes I–III, aldosterone antagonists added for Class III–IV.

Practice Prompts

1. A 60-year-old man has EDV 140 mL, ESV 60 mL, HR 80 bpm. Calculate CO, SV, and EF. Classify his ejection fraction status.
2. An ECG shows progressive PR prolongation from 200ms to 400ms before a P wave fails to conduct, then the pattern restarts. What is the diagnosis, and does this require permanent pacing?

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