Measurement of CO and Vascular Resistances

Question 1

Most accurate method in measuring cardiac output

O2 consumption may be assumed to be ____________ of body surface area (BSA).

Answer 1

True Fick Method

In the true Fick method, oxygen consumption is measured using an oxygen analyzer chamber placed around the patient’s head or face. True Fick cannot be performed in patients requiring O2 therapy. The Fick method is particularly useful in patients with low CO and a large arteriovenous O2 difference

125 mL/min/m2

Question 2

This method states that the higher the cardiac output, the more brief and sharp the temperature change will be with a small area under the curve

Answer 2

Thermodilution method

This method is valid in most cases, and has less than 5% to 10% error in comparison with the true Fick method, except in cases of TR, low cardiac output, and right-to-left or left-to-right shunt

Question 3

Normal SVO2 level

Answer 3

SvO2 greater than 60%

In case of normal hemoglobin level and normal arterial O2 saturation, and in the absence of O2 therapy, a normal SvO2 greater than 60% corresponds to an adequate cardiac output that matches the peripheral O2 demands.

Patients with inappropriately reduced CO increase their peripheral extraction of O2, thus reducing SvO2.

Question 4

Causes of errors in cardiac output measurement by thermodilution technique

TR
Low cardiac output
Right to left shunt
Left to right shunt

Answer 4

1. Falsely low CO
2. Falsely high CO
3. Falsely higher right sided flow
4. Falsely lower right flow

-TR: part of the injectate goes backward and keeps getting recycled, producing a prolonged, low-amplitude curve suggesting a falsely lower co.

-Low cardiac output <3.5 L/min (particularly <2.5 L/min): the injectate may warm up excessively as it is traveling slowly through the RA and the RV and will give the wrong impression of a slight and brief temperature change at the tip, hence the impression of a higher co (by up to 35%). The use of an iced injectate may compensate for this error.

-Right-to-left shunt: part of the injectate is not used by the forward RV output and thus creates an abbreviated thermodilution curve suggesting a falsely higher right-sided flow.

-Left-to-right shunt: the shunted blood dilutes the cold injectate and may abbreviate the thermodilution curve. However, practically, there is recirculation of the injectate producing a prolonged decay and thus suggesting a falsely lower right-sided flow.*

-Arrhythmia: beat-to-beat variability in stroke volume may affect the estimation of CO, since thermodilution estimates CO over few beats only.

Question 5

Give the Fick equation

Answer 5

CO = O2 consumption (ml/min) / AV difference in O2 content (ml O2/L blood)

CO = O2 consumption (ml/min) / (SaO2 - SvO2) x 1.36 x Hgb x 10

Question 6

Ohms law

SVR
PVR

Answer 6

Ohms law: Resistance = change in pressure / CO

SVR in dynes= (MAP - RA pressure) / CO x 80
PVR in dynes = (PA pressure - Mean PCWP or LA pressure) / CO x 80
PVR in Woods unit = PVE in dynes/80

In the presence of a shunt, PVR is measured using the Fick calculated pulmonary blood flow (Qp) rather than thermodilution CO, which is inaccurate in case of a shunt:

PVR = (mean PA pressure − PCWP) / Qp

Question 7

Equation for TPG

Answer 7

TPG = mean PA pressure - PCWP