CARDIAC WORK:

A measurement of ventricular power.

Commonly used cardiac physiologic measurements:

• Left ventricular stroke work
• Cardiac minute work, which is a measurement of volume work and pressure work.

Cardiac work is stroke work

• The work the heart performs in each beat to eject blood.
  — Work = distance x force.

Left ventricular stroke work:

• Stroke Volume x Aortic Pressure
• Stroke volume is the distance; aortic pressure is the force

Cardiac Minute Work:

Cardiac work per unit time.

• Cardiac minute work = Heart Rate x Left Ventricular Stroke Work.
  – Heart rate introduces the element of time.
• Left ventricular stroke = Stroke Volume x Aortic Pressure (as we’ve previously shown).
• Thus:
  Cardiac minute work = Heart Rate x Stroke Volume x Aortic Pressure.
  – Recall that Heart Rate x Stroke Volume = Cardiac Output
  – Cardiac output is key to evaluating heart performance.

• So, we can re-write the equation again to show that:
  Cardiac Minute Work = Cardiac Output x Aortic Pressure.
  — With this re-arrangement, we can see that cardiac minute work is the product of both volume work and pressure work.
  Thus, increased cardiac output and/or aortic pressure causes increased cardiac work.

MYOCARDIAL OXYGEN CONSUMPTION:

Myocardial oxygen consumption correlates directly with cardiac minute work.
— Of the two components (volume work and pressure work), pressure work is the primary driver of myocardial oxygen consumption.
– This is because pressure work is more metabolically costly than volume work.

Cardiac Hypertrophy:

• Characterized by thickened myocardium:
  Increased pressure work raises myocardial oxygen consumption, and, in response to increased demand, the myocardium hypertrophies.
• In both aortic valve stenosis and systemic hypertension, increased left ventricular pressure work causes left ventricle hypertrophy.
• In pulmonary hypertension, increased right ventricular pressure work causes
  right ventricular hypertrophy.

FICK PRINCIPLE

Used to calculate cardiac output by measuring myocardial oxygen consumption; the calculation is based on the principle of the conservation of mass.

Cardiac output = (Total oxygen consumption) / (Oxygen content of the pulmonary vein – Oxygen content of the pulmonary artery)

Example problem:

• A patient’s total body oxygen consumption is 200 ml of oxygen per minute;
• The oxygen content of the pulmonary vein is 0.15 ml oxygen per ml blood.
• The oxygen content of the pulmonary artery is 0.10 oxygen per ml blood.
  — This gives us a cardiac output of 4000 ml per minute.