Patients are usually transferred to the intensive care unit (ICU) due to their hemodynamic instability. Interpretation of hemodynamic monitoring is crucial to delivery of prompt and appropriate intervention.
One of the goals of the cardiovascular system is to ensure optimum oxygen delivery to peripheral tissues. Oxygen delivery to the tissues (DO2, mL O2/min) is defined as the arterial oxygen content (CaO2, mL O2/dL) times the cardiac output (CO, L/min), described as
DO2 (mL O2/min) = CO (L/min) × CaO2 (mL O2/dL) × 10
Where CO (L/min) = Heart Rate (HR, beats/min) × Stroke Volume (SV, L/beat), and CaO2 (mL O2/dL) = (1.34 × [Hgb (g/dL)] × SaO2) + (0.0031 × PaO2 [mmHg]), where Hgb is hemoglobin, SaO2 the arterial saturation of oxygen, and PaO2 is arterial pressure of oxygen.1,2
Oxygen consumption (VO2) is the amount of oxygen utilized by the tissues and is defined by the following equation:
Given its negligible contribution, the dissolved oxygen in plasma can be omitted and the equation simplified, using the arterial concentration of carbon dioxide (CaCO2), the venous concentration of carbon dioxide (CvCO2), and the venous saturation of oxygen (SvO2):
VO2 = (CO × CaCO2 × 10) – (CO × CvCO2 × 10)
VO2 = CO × Hgb × 13.4 × (SaO2 – SvO2)
Normal oxygen consumption is 250 mL of oxygen per minute. When there is a stress or imbalance, the body compensates by increasing cardiac output or by increasing oxygen extraction at the tissue level. Increased oxygen extraction can be seen when there is a larger arterial-venous oxygen saturation difference and when the mixed venous oxyhemoglobin saturation (SvO2) is lower than normal. The body also tries to compensate during these times of stress by redistributing blood to these areas of greatest extraction and with selective vasoconstriction of other capillary beds.1,2
Several of these parameters can be measured using various techniques that are static, intermittent, or continuous. In an acute critical illness, it is preferable to perform hemodynamic monitoring at least intermittently, if not continuously. Not only does this type of monitoring help determine progression of the disease process, but it also can evaluate the success or failure of the interventions performed.3-5
As mentioned above, cardiac output is the heart rate times stroke volume. Stroke volume is the amount of blood ejected per beat and ...