7. Cardiac Output 2024

What is cardiac output (CO)?

Volume of blood pumped by the left ventricle per minute.

What is the formula for cardiac output?

CO = heart rate × stroke volume.

What is stroke volume (SV)?

Volume of blood ejected per heartbeat.

What is end-diastolic volume (EDV)?

Volume of blood in the ventricle at the end of diastole.

What is end-systolic volume (ESV)?

Volume of blood remaining in the ventricle after contraction.

How is stroke volume calculated?

SV = EDV – ESV.

What is ejection fraction (EF)?

Percentage of EDV ejected during systole.

How is ejection fraction calculated?

EF = SV / EDV.

What is a normal ejection fraction?

~55–70%.

What is the Fick method for measuring CO?

CO = O₂ uptake / (arterial – venous O₂ difference).

What device is used to measure O₂ uptake in the Fick method?

Spirometer.

Where is venous O₂ measured for the Fick method?

Right ventricle or pulmonary artery.

Why is the thermodilution method preferred over Fick?

Less invasive and simpler in practice.

What is preload?

The initial stretch of the ventricular myocardium before contraction.

What physiological value estimates preload?

End-diastolic volume (EDV) or end-diastolic pressure (EDP).

What other pressures approximate preload in a normal heart?

Atrial pressure and venous pressure.

How does increased preload affect stroke volume?

It increases stroke volume.

What law describes the relationship between preload and stroke volume?

Frank-Starling Law of the Heart.

What is the Frank-Starling Law?

Greater preload leads to stronger contraction and increased stroke volume.

Why does increased preload enhance contractility?

It optimizes sarcomere length, increasing force of contraction.

Does preload change ejection fraction?

Yes — EF increases with increased preload (assuming constant afterload and contractility).

What happens to stroke volume if preload is reduced?

Stroke volume decreases.

What is afterload?

The pressure the ventricle must overcome to eject blood.

What is used as an estimate of afterload in the left ventricle?

Arterial pressure.

How does increased afterload affect stroke volume?

It decreases stroke volume.

How does increased afterload affect ejection fraction?

It decreases EF.

Why does stroke volume decrease with higher afterload?

More energy is spent overcoming resistance, less for ejection.

What is contractility?

The intrinsic ability of cardiac muscle to contract at a given preload and afterload.

How is contractility assessed?

Using the slope of the Starling curve or dP/dt (rate of pressure rise during isovolumetric contraction).

How does increased contractility affect stroke volume?

It increases stroke volume.

How does increased contractility affect ejection fraction?

EF increases.

What stimulates increased contractility?

Sympathetic input (norepinephrine, epinephrine).

What effect does sympathetic stimulation have on Ca²⁺ handling?

Increases Ca²⁺ influx and release, enhancing contraction.

What does digitalis inhibit?

The Na⁺/K⁺ ATPase.

How does digitalis increase contractility?

Inhibits Na⁺/K⁺ ATPase → increases intracellular Na⁺ → reduces Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger → increases [Ca²⁺]i → stronger contraction.

What do pressure-volume (PV) loops plot?

Left ventricular pressure vs. volume over a single cardiac cycle.

What does the width of the PV loop represent?

Stroke volume.

What causes the bottom-right corner of the loop?

End-diastolic volume (mitral valve closes).

What event begins isovolumetric contraction on the loop?

Closure of the AV (mitral) valve.

What marks the start of ejection on the PV loop?

Opening of the aortic valve.

What causes the top-left corner of the loop?

End-systolic volume (aortic valve closes).

What marks the start of isovolumetric relaxation?

Closure of the aortic valve.

What causes ventricular filling to begin on the loop?

Opening of the AV (mitral) valve.

How does increased preload affect the PV loop?

Rightward shift → increased EDV → increased SV.

How does increased afterload affect the PV loop?

Taller, narrower loop → increased pressure, decreased SV.

How does increased contractility affect the PV loop?

Leftward shift of ESV point → increased SV, increased EF.