Question 19

College Answer

This is a core question. It was expected candidates could provide a definition (heart rate x 
stroke volume) and then move on to outline factors that affect it (afterload, preload, 
contractility). Additional marks were awarded for descriptions of the relationship to mean 
systemic filling pressure and other influences beyond this. 
Most candidates described a thermodilution cardiac output curve but almost all described the 
technique as based on the “Fick equation or method” (which is used to estimate cardiac 
output from oxygen consumption). Very few candidates correctly identified the Stewart 
Hamilton equation as the integration method used to relate cardiac output (flow) to 
temperature change as an example of indicator dye dilution.
Candidates seemed to lack depth and understanding on this topic.

Discussion

This is a difficult SAQ to approach, particularly with regards to the meat of it - where you have to discuss the determinants of cardiac output in six minutes. The additional marks which the examiners say were awarded for the discussion of such minutiae as mean systemic filling pressure might give trainees the impression that all such minutiae would be rewarded with marks. That is far from certain. If one decided to discuss each possible determinant of preload afterload and contractility, one would likely squander one's time and not elevate the mark much beyond a total of 6 or 7. Ergo, a lot of loose fatty material has been trimmed from this answer. For those readers who are interested in the full-far version, huge sprawling fields of wild digression stretch as far as the eye can see in the increasingly misnamed Required Reading section. 

Definition

• Cardiac output is defined as the volume of blood ejected by the heart per unit time.
• It is usually presented as [stroke volume × heart rate], in L/min

The determinants of cardiac output are:

• Heart rate
  • A higher heart rate increases cardiac output as it multiplies by stroke volume
  • An excessively high heart rate decreases cardiac output by decreasing preload
• Stroke volume, which is in turn determined by preload, afterload and cardiac output
• Preload
  • Increased preload leads to an increase in the stroke volume 
• Afterload
  • Increased afterload decreases stroke volume
• Cardiac contractility:
  • Increased contractility improves stroke volume at any given preload or afterload value

Thermodilution measurement of cardiac output:

• Rate of blood flow can be determined from the rate of change in the concentration of substance after a known amount of it has been added to the bloodstream
• This uses the equation, V̇ = m/Ct, where:
  • V̇ = flow, or cardiac output
  • m = dose of the indicator, 
  • C = concentration, and
  • t = time
• In the case of thermodilution, the "indicator substance" is a known volume of cold saline (or heated blood), and the equation 
• The use of thermodilution for cardiac output measurement requires a modified version of the abovestated equation, otherwise known as the Stewart-Hamilton equation, which incorporates correction factors for specific heat and specific gravity of both the indicator and the blood.

It is important at this stage to point out that the thermodilution (or any other indicator dilution) method relies on the Fick principle, but does not involve the Fick method. The Fick method is where you collect the exhaled oxygen in a bag to calculate the VO₂, and measure the arterio-venus oxygen difference to calculate the cardiac output. The Fick principle is the theoretical basis of this measurement, which simply points out the relationship between the cardiac output and the concentration difference of a marker substance between an upstream and downstream points in the blood flow. If you know the dose of injected marker, the principle states, you can calculate the cardiac output from the concentration difference - which is basically what indicator dilution does.

Vincent, Jean-Louis. "Understanding cardiac output." Critical care 12.4 (2008): 174.

Taylor, Henry Longstreet, and Kenneth Tiede. "A comparison of the estimation of the basal cardiac output from a linear formula and the “cardiac index”." The Journal of clinical investigation 31.2 (1952): 209-216.

Tanner, J. M. "The construction of normal standards for cardiac output in man." The Journal of clinical investigation 28.3 (1949): 567-582.

Grollman, Arthur. "PHYSIOLOGICAL VARIATIONS IN THE CARDIAC OUTPUT OF MAN: XI. The Pulse Rate, Blood Pressure, Oxygen Consumption, Arterio-Venous Oxygen Difference, and Cardiac Output of Man During Normal Nocturnal Sleep." American Journal of Physiology-Legacy Content 95.2 (1930): 274-284.

WIGGERS, CARL J. "Determinants of cardiac performance." Circulation 4.4 (1951): 485-495.

Åstrand, Per-Olof, et al. "Cardiac output during submaximal and maximal work." Journal of Applied Physiology 19.2 (1964): 268-274.