GATE 2021 BM – Question 54
A clinician estimates cardiac output by thermodilution. A 2.0 mL saline injection at 7°C enters the right atrium, and the pulmonary-artery temperature-change integral is −20 K s. The calculation is $F=Q/[\rho_b c_b\int\Delta T_bdt]$, with $Q=V_i\Delta T_i\rho_i c_i$ and equal blood/injectate density and specific heat. Which statements identify why the calculated output is too low?
Practise this question in The GATE Grind →
Show answer and explanation
Correct answer: (A) The cold-saline volume is too small; (B) The magnitude of the temperature-change integral is too large
Explanation
With the stated cancellations and a usual blood temperature near 37°C, the numerator magnitude is 2 mL×30 K=60 mL K. Dividing by twenty K s gives three mL/s=.18 L/min, far below an ordinary healthy cardiac output. The formula increases with injectate volume and decreases with the temperature-integral magnitude. Thus too small a volume or too large an integral biases the estimate low, A and B. The source’s signed temperatures must be used consistently to obtain positive flow.