Cardiac output measured by thermodilution technique is unreliable in all of the following situations EXCEPT: (DNB Jun-2008)
|A||Ventricular septal defect|
|C||Low cardiac output|
Methods of Measurement of cardiac out:
- In experimental Animal- With an Electromagnetic flow meter placed on the ascending Aorta.
- In Human - by
a. Doppler combined with Echocardiography
b. Direct Fick method
c. Indicator Dilution method ego thermodilution - most popular.
- "CO is m.c. measured by the thermodilation technique but the standard method, against which this technique and others are calibrated, remain the direct Fick oxygen method" –
a. Direct Fick method Q: The principle can be used to determine CO by measuring the amount of O2 consumed by the body in a given period and dividing this value by the A-V difference across the lung
b. Thermodilution technique Q
i. Method - cold saline is injected into the right Atrium. Temperature change in the blood is then recorded in the pulmonary artery.
ii. Principle - The temperature change is inversely proportional to the amount of blood flowing through the pulmonary artery
iii. Facts about estimating CO by thermodilution technique :
- The Indicator dilution (thermo-dilution) method is least reliable when the cardiac out is low and transit of cold bolus through right is delayed. Q
- Indicator-Dilution techniques
a. "If for example, there is severe valvular regurgitation or a low cardiac output state in which the washout of the indicator is prolonged and recirculation begins well before an adequate decline in the indicator curve occurs, determinations are erroneous. Intracardiac shunts may also greatly affect the shape of curve.
b. Thermo dilution method has several advantages: - it obviates the need for withdrawal of blood from an arterial site- and Less affected by recirculation. However a significant error occurs in patients with severe tricupsid regurgitation. Also, in patients with low outputs (esp< 2.5L/rnin), thermodilution tends to overestimate the cardiac out
c. Thermodilution method is used to measure cardiac output in the catheterization laboratory in patients without shunts.
Here, according to Nelson and Braunwald answer should be Pulmonary regurgitation, because they clearly mention - about - Tricupsid regurgitation, low cardiac output and shunt.
If Tricuspid regurgitation and pulmonary regurgitation is present or significant left to right shunt present - the thermodilution technique is less reliable.
Thermal dilution method is used except low cardiac ouput states where Fick's principle is preferred . Q