Equations Describing the Derived Haemodynamic Parameters

Created on Mon, 06/29/2015 - 18:33
Last updated on Mon, 06/29/2015 - 18:33
These are the equations which are used to calculate the derived parameters (i.e. derived from the measurements produced by the PA catheter in the course of a thermodilution).

…Anything that called an INDEX is a variable which is corrected for body surface area.

Cardiac index (CI)

This is the thermodilution cardiac output corrected for body size. Normal is 2.8 to 4.2 L/min per m2

CI


 

Stroke Volume (SV)

This is the volume of blood ejected by the ventricles during systole.

SV


 

Stroke Volume Index (SVI)

This is the Stroke Volume, corrected for body size

SVI equation

Right Ventricular Ejection Fraction (RVEF)

This is the fraction of right ventricular volume ejected during systole.
It indicates the strength of ventricular contraction during systole.
It can be measured directly by using a specialized rapid-response thermistor.

RVEF equation

Right Ventricular End-Diastolic Volume (RVEDV)

This is volume of blood left behind after systole in the right ventricle.
This is a true measure of right ventricular preload. It is calculated by rearranging the above equation.

RVEF equation


 

Left Ventricular Stroke Work Index (LVSWI)

This is the work performed by the ventricle to eject the ejection fraction.

LVSVI equation

Stroke work is a function of the systolic pressure load, which is afterload minus preload.
Afterload is the mean arterial pressure (MAP), and preload is essentially the PAWP (pulmonary artery wedge pressure). The factor of 0.0136 converts pressure and volume to units of work.

Right Ventricular Stroke Work Index (RVSWI)

This is the work required to move the preload though the pulmonary circulation.
It is also a function of the systolic pressure load, which is right ventricular afterload (the MPAP, mean pulmonary arterial pressure) minus the preload (which is the CVP).

RVSVI equation

Systemic vascular resistance index (SVRI)

This is directly proportional to the pressure gradient from the aorta to the right atrium (MAP – CVP)
This is inversely proportional to blood flow, or cardiac index (CI)

SVRI equation

The 80 here is again a factor necessary to convert units.

One does not need the cardiac index; a "pure" unadjusted SVR can be calculated from the cardiac output alone: SVR = ( 80 x (mean arterial pressure - mean right atrial pressure) / cardiac output).

A normal SVR is 700-1600 dyn.s/cm5

A normal SVRI is 1970-2390 dyn·sec/cm5/m2

The CICM examiners have actually expected their fellowship candidates to perform this calculation in a previous SAQ paper (specifically, Question 19 from the first paper of 2006).

Pulmonary vascular resistance index (PVRI)

This works under exactly the same principles as the above equation.
The PVRI is directly proportional to the pressure gradient from the pulmonary artery to the left atrium (MPAP – PAWP). Again, its inversely proportional to blood flow, or cardiac index (CI)

PVRI equation


 

Oxygen delivery (DO2)

This is the rate of oxygen delivery in arterial blood.

DO2 equation


 

Mixed venous oxygen saturation (SvO2)

This is an indirect marker of systemic blood flow.

Because your tissues extract a known proportion of oxygen from the arterial blood, one can infer that if there is very little oxygen left over, the tissues are extracting more of it, and the reason is that the blood flow to the tissues is decreased. Decreased blood flow means the tissues have more time to extract capillary oxygen. All of this flies out the window in sepsis or cyanide poisoning, where tissue oxygen consumption is unrelated to blood flow.

Oxygen Uptake or Oxygen Consumption (VO2)

This is the rate at which oxygen is sucked out of the capillaries by the tissues.

VO2 equation

Oxygen Extraction ratio (O2ER)

This is the ratio of O2 uptake to O2 delivery; i.e. what fraction of the delivered oxygen is being consumed in the tissues.

O2ER equation

 

References

The PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007) is the source for most of this information.

The normal values, and all these queations (plus many many more) can be found here, at the LIDCO group "Normal Hemodynamic Parameters" page.