Marino The ICU Book 4e, IE

147

The Pulmonary Artery Catheter

= CI × (CaO 2 – CvO 2 )

VO 2

(8.12)

If the CaO 2 the VO 2

and CvO 2

are each broken down into their component parts,

equation can be rewritten as: VO 2

= CI × 1.3 × Hb × (SaO 2 – SvO 2 )

(8.13)

(where SaO 2

and SvO 2

are the oxyhemoglobin saturations in arterial and

mixed venous blood, respectively). VO 2 is expressed as mL/min/m 2 (when the cardiac index is used instead of the cardiac output), and the nor- mal range is shown in Table 8.1. An abnormally low VO 2 ( < 100 mL/min/m 2 ) is evidence of impaired aerobic metabolism.

Oxygen Extraction Ratio The oxygen extraction ratio (O 2

ER) is the fractional uptake of oxygen from

the systemic microcirculation, and is equivalent to the ratio of O 2 to O 2 delivery. Multiplying the ratio by 100 expresses it as a percent. O 2 ER = VO 2 / DO 2 ( × 100) (8.14) The O 2 ER is a measure of the balance between O 2 delivery and O 2 uptake. It is normally about 25%, which means that 25% of the oxygen delivered to the systemic capillaries is taken up into the tissues. uptake

APPLICATIONS

Hemodynamic Patterns Most hemodynamic problems can be identified by noting the pattern of changes in three hemodynamic parameters: cardiac filling pressure (CVP or PAWP), cardiac output, and systemic or pulmonary vascular resist- ance. This is demonstrated in Table 8.2 using the three classic forms of shock: hypovolemic, cardiogenic, and vasogenic. Each of these condi- tions produces a distinct pattern of changes in the three parameters. Since there are 3 parameters and 3 possible conditions (low, normal, or high), there are 3 3 or 27 possible hemodynamic patterns, each representing a distinct hemodynamic condition.

Table 8.2

Hypovolemic Cardiogenic Vasogenic Shock Shock Shock Hemodynamic Patterns in Different Types of Shock

Parameter

CVP or PAWP

Low

High

Low

Cardiac Output

Low

Low

High

Systemic Vascular

High

High

Low

Resistance