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Ask the Clinical Instructor
The registered cardiovascular invasive specialist (RCIS) exam does require the candidate to be familiar with a few different ways to calculate cardiac output. In reality, some are more reliable and accurate than others. It also appears that the emphasis on each varies slightly from hospital to hospital around the country.
The predominate ways to calculate cardiac output in the cath lab are:
1. Angiographic
2. Thermodilution Cardiac Output
3. Fick Cardiac Output
Angiographic
We essentially do this on each left ventriculogram (LV gram) when we figure out the ejection fraction (EF). Sometimes you can just look at the LV gram and take a good guess at the EF, and as you become more experienced, your guesses become more and more accurate.
However, to perform analysis of the cardiac output correctly, it requires a little bit of work, and unfortunately, a little bit of math. The basic calculation for determining cardiac output is:
CARDIAC OUTPUT = HEART RATE x STROKE VOLUME
To figure out what the stroke volume is angiographically, we can use a simple formula:
STROKE VOLUME = LEFT VENTRICULAR END DIASTOLIC VOLUME – LEFT VENTRICULAR END SYSTOLIC VOLUME
Some of our imaging systems will allow you to draw the systolic and diastolic outlines, and then provide some data for you. If you had to work it out on paper, and using the formulas and data provided above, the cardiac output for the above angiogram would be something like this:
SV (systolic volume) =
LVEDV (left ventricular end diastolic volume) –
LVESV (left ventricular end systolic volume)
SV = 148 – 84 (note: round numbers whenever possible) SV = 64ml
We will hopefully know what the heart rate is, and if it is, say, 80 beats per minute, we can finish the cardiac output calculation as follows:
CO (cardiac output) = HR (heart rate) x SV
CO = 80 x 64
CO = 5,120 ml or 5.12 liters per minute
To take it further and figure out the ejection fraction, the formula is:
Ejection Fraction (EF) =
(End Diastolic Volume [EDV] – End Systolic Volume [ESV]) / EDV x 100
If we use the numbers from the previous exercise, we have this calculation:
EF = (148-84)/148 x 100
EF = 64/148 x 100
EF = 0.43 x 100
EF = 43%
Thermodilution Cardiac Output (TDCO)
Unless you are a math whiz, you will likely rely on the computer equipment to provide the results you need from the data that you supply. The process is fairly simple, but the results are only as good as the accuracy of the technical aspects of the procedure. During a TDCO, an cool injectate of a known temperature is injected into the right atrium, and a temperature measurement occurs at the distal pulmonary artery (Figure 2). The time it takes for the temperature to return to normal is used to calculate the cardiac output. Essentially, the computer is looking for the CHANGE IN TEMPERATURE OVER TIME.
Think of it this way (and keep this in mind for the next section as well)…the lower (or slower the flow) the cardiac output, the longer it will take for the temperature to change, because the blood is not moving as fast. The opposite is true for a higher cardiac output: the temperature will change quickly (Figure 3).
This process is obtained by the use of a Swan-Ganz catheter specifically made for thermodilution cardiac outputs (Figure 4).
A simple thing like a temperature discrepancy between what the computer THINKS the injectate temperature is and what the injectate temperature ACTUALLY is can provide results that appear to be incompatible with sustaining life. The details and discussions of this procedure are planned for a future article.
Fick Cardiac Output
There is no way around it, the Fick Cardiac Output involves math. Yes, your hemodynamic monitoring system will figure it out for you at the time of the procedure. But what if the doctor changes his mind and wants all the results from the data he has on a piece of paper, and your terminals are all in use? Plus, it’s good practice for the RCIS exam do be able to work these calculations out on paper.
The Fick CO is a method that determines the uptake of oxygen passing through the tissues. As blood passes through tissue, oxygen is extracted from the red blood cells/hemoglobin. The slower the blood flow (more time in contact with the tissues and lower cardiac output), the more oxygen that is extracted. The faster the blood flow (less time in contact with the tissues and higher cardiac output), the less oxygen that is extracted (does that sound familiar?).
The formula may seem intimidating at first, but will make sense once explained. You may see different variations of the Fick formula, but this variation is the one that we should be using in the cath lab because we should have this data available.
CO = O2 consumption ml/min x body surface area m2
(SaO2% – SvO2%) x Hemoglobin x 1.36 x 10
Let’s look at some of these items:
O2 consumption: In the past, this value used to be obtained through the use of complicated devices to measure actual consumption. Today, that is seldom done. A ‘ given’ of 125 ml/min oxygen consumption is used. You should keep in mind, however, that this number is a normal value, and not one that adjusted to reflect the acuity of the patient.
Body Surface Area (BSA): This is usually calculated by your computer based upon the patient’s height and weight.
SaO2% = Oxygen saturation % of arterial blood
SvO2% = Oxygen saturation % of venous blood
Hemoglobin (Hgb): Should be readily available on your pre-procedural labs.
1.36: The theoretical oxygen carrying capacity of 1 gram of hemoglobin. (note: on the RCIS exam, you may see this as a value of 1.39. Make sure to always look at your data.)
10: A mathematical conversion in the formula.
When the patient is brought to the room, the oxygen should be removed for at least 10 minutes prior to obtaining oxygen saturation samples. This will allow the oxygen saturations to move to a more normal representation of the body’s condition. In the event that your patient has a medical condition that requires the use of oxygen, make sure to contact the cardiologist to see how they wish to proceed. Don’t take oxygen off of a patient that needs it.
We mentioned that the Fick CO is based upon the extraction of oxygen from the tissues. In order to properly calculate this, we would need to know the HIGHEST and LOWEST saturations in the body. The HIGHEST arterial saturation would be obtained from the pulmonary veins. However, we can not easily obtain a sample from that region. The closest we can realistically come to that is the aorta. The LOWEST saturations in the body would be right before the blood re-enters the lung. The distal pulmonary artery is the reasonable place for us to obtain that sample. They are then run via an oximetry device, or in the respiratory therapy department. Let’s assume that we have the following values for the variables:
O2 Consumption: 125
BSA: 2.0
Hgb: 12.6
SaO2%: 97%
SvO2%: 58%
The numerator is easy to figure out: 125 ml/min x 2.0 m2 = 250 ml/min/m2
We will have to work out the denominator.
With the oxygen saturations, we can not use them as a % in the formula. We will have to change them to the equivalent of the value of 1, i.e., 97% becomes 0.97 and 58% becomes 0.58. If you accidentally forget to do this, you will have a very ‘off’ CO and you will know something is wrong.
Working the denominator:
(SaO2% – SvO2%) x Hgb x 1.36 x 10 (0.97 – 0.58) x 12.6 x 1.36 x 10 0.39 x 12.6 x 1.36 x 10 4.9 x 1.36 x 10 6.7 x 10 67
What we then have is:
250 ml/min/m2
67 = CO = 3.73
If you work the same numbers, but change the venous saturation to 64% or 66%, or a higher number, then you can see a demonstration about the less extraction, the better the cardiac output (until it can become TOO high).
These three ways to calculate cardiac outputs can not only help you in the lab to obtain the necessary information for the physician to help them guide the patient’s medical management, but you can also be more prepared for the RCIS examination.
