About the Calculations & Formulas question bank
Calculation questions are the most reliably scoreable items on the exam, because the answer is not a matter of judgement. If you know the formula, plug in correctly, and keep your units straight, the mark is yours. The candidates who lose these marks almost always lose them on units or on which value goes in the numerator.
This bank drills each formula from several directions: compute the value, work backwards to a missing variable, and interpret the result clinically. That last step matters — the exam frequently asks not what the number is but what it means for the patient.
What this bank covers
- Cardiac output by the Fick method and by thermodilution
- Cardiac index and body surface area
- Stroke volume, stroke volume index, and ejection fraction
- Systemic and pulmonary vascular resistance
- Valve area by the Gorlin and Hakki equations
- Shunt calculations and Qp:Qs
- Mean arterial pressure and the shock index
- QT correction by Bazett and Fridericia, and heart rate from an ECG
How to use it
- Write the formula out before you touch the numbers. Almost every error is a setup error, not an arithmetic one.
- Watch the units — resistance in Wood units versus dynes·s·cm⁻⁵ differs by a factor of 80.
- After every calculation, ask whether the number is plausible for a human being. A cardiac index of 12 means you made a mistake, not that you found a remarkable patient.
Frequently asked questions
What is the Fick equation for cardiac output?
Cardiac output equals oxygen consumption divided by the arteriovenous oxygen difference — VO₂ ÷ [(CaO₂ − CvO₂) × 10]. In practice VO₂ is usually assumed at 125 mL/min/m² rather than measured, which is the method's main weakness.
How do I convert vascular resistance between units?
Multiply Wood units by 80 to get dynes·s·cm⁻⁵, and divide by 80 to go the other way. Normal SVR is roughly 10–20 Wood units, or 800–1600 dynes·s·cm⁻⁵.
Which QTc formula does the exam use?
Bazett's is the classic — QT divided by the square root of the RR interval — and it is what most questions expect. Fridericia's (dividing by the cube root of RR) is more accurate at extremes of heart rate, and the calculator below gives you both.