Systemic Oxygenation

One of the fundamental goals of critical care management is to promote tissue oxygenation, yet it is not possible to monitor tissue oxygen levels in a clinical setting. This chapter describes the measures of “systemic” oxygenation that are available, and how they can be used to evaluate tissue oxygenation.

I. Measures of Systemic Oxygenation

A. Oxygen Content of Blood

The concentration of O₂ in blood (called the O₂ content) is the summed contribution of the O₂ that is bound to hemoglobin (Hb) and the O₂ that is dissolved in plasma.

1. Hemoglobin-Bound O₂

The concentration of hemoglobin-bound O₂ (HbO₂) is determined as follows (1):

where Hb is the hemoglobin concentration in g/dL (grams per 100 mL), 1.34 is the O₂ binding capacity of hemoglobin (mL/g), and SO₂ is the O₂ saturation of Hb, expressed as a ratio (HbO₂/Total Hb).

Equation 6.1 states that, when Hb is fully saturated with oxygen (SO₂ = 1), each gram of Hb binds 1.34 mL O₂.

2. Dissolved O₂

The concentration of dissolved O₂ in plasma is determined as follows (2):

where PO₂ is the partial pressure of O₂ in blood (in mm Hg), and 0.003 is the solubility coefficient of O₂ in plasma (mL/dL/mm Hg) at normal body temperature.

Equation 6.2 states that, at normal body temperature (37°C), each 1 mm Hg increment in PO₂ will increase the concentration of dissolved O₂ by 0.003 mL/dL (or 0.03 mL/L) (2). This highlights the poor solubility of oxygen in plasma (which is why hemoglobin is needed as a carrier molecule).

3. Total O₂ Content

The total O₂ content in blood (mL/dL) is determined by combining Equations 6.1 and 6.2:
Table 6.1 shows the normal concentrations of O₂ (bound, dissolved, and total O₂) in arterial and venous blood. Note that the contribution of dissolved O₂ is very small; as a result, the O₂ content of blood is considered equivalent to the Hb-bound fraction.

B. Oxygen Delivery

The rate of O₂ transport in arterial blood, also known as oxygen delivery (DO₂), is a function of the cardiac output (CO) and the O₂ content of arterial blood (CaO₂) (3).

(The multiplier of 10 is used to convert the CaO₂ from mL/dL to mL/L.) If the CaO₂ is broken down into its components, Equation 6.5 can be rewritten as:

Note: The SaO₂ is monitored continuously with pulse oximeters, and the cardiac output can be measured with a pulmonary artery catheter (described in pages 88–91), or it can be measured noninvasively using techniques described in Reference 4.

Table 6.1 Normal Measures of Blood Oxygenation

Measure	Arterial Blood	Venous Blood
Partial Pressure of O₂	90 mm Hg	40 mm Hg
% Oxygenated Hb	98%	73%
Hb-bound O₂	19.7 mL/dL	14.7 mL/dL
Dissolved O₂	0.3 mL/dL	0.1 mL/dL
Total O₂ Content	20 mL/dL	14.8 mL/dL
Blood Volume^†	1.25 L	3.75 L
Total Volume of O₂	250 mL	555 mL
Values shown are for a body temperature of 37°C and a hemoglobin concentration of 15 g/dL. ^† Volume estimates based on a total blood volume (TBV) of 5 Liters, arterial blood volume = 25% of TBV, and venous blood volume = 75% of TBV. Abbreviations: Hb = hemoglobin; dL = deciliter (100 mL).

The normal range of values for DO₂ are shown in Table 6.2. Note that the DO₂ (and VO₂) are expressed in ab-solute and size-adjusted terms; the body size adjustment is based on body surface area in square meters (m²).

Table 6.2 Measures of Systemic Oxygen Balance

Measure	Normal	Tissue Hypoxia
DO₂	900–1100 mL/min or 520–600 mL/min/m²	Variable
VO₂	200–270 mL/min or 110–160 mL/min/m²	<200 mL/min or <110 mL/min/m²
O₂ER	20–30%	≥50%
SvO₂	65–75%	≤50%
ScvO₂	70–80%	?
Lactate	1–2.2 mmol/L¹	>1–2.2 mmol/L¹
¹Normal lactate levels vary from 1.0 to 2.2 mmol/L in individual laboratories. Abbreviations: DO₂ = O₂ delivery; VO₂ = O₂ consumption; O₂ER = O₂ extraction ratio; SvO₂ = mixed venous O₂ saturation; ScvO₂ = central venous O₂ saturation.

C. Oxygen Consumption

The rate of O₂ uptake into tissues is equivalent to the oxygen consumption (VO₂) because O₂ is not stored in tissues There are two methods for determining the VO₂.

1. Calculated VO₂

The VO₂ can be calculated as the product of the cardiac output (CO) and the difference between arterial and venous O₂ contents (CaO₂ – CvO₂).

(The multiplier of 10 is explained for the DO₂.) The CaO₂ and CvO₂ share a common term (1.34 * Hb), so Equation 6.7 can be rewritten as:

Note: Three of the four measurements used to calculate the VO₂ are also used to calculate the DO₂. The one ad-ditional measurement is the SvO₂, which is described later in the chapter.

The normal range of values for VO₂ is shown in Table 6.2. Note that the VO₂ is much smaller than the DO₂; the significance of this discrepancy is described later.
Each of the measurements used to calculate the VO₂ has an inherent variability, and the summed variability of the 4 measurements is ±18% (5,6,7). Therefore, the calculated VO₂ must change by at least 18% for the change to be considered significant.

2. Calculated vs. Whole Body VO₂

The calculated VO₂ is not the whole body VO₂ because it does not include the O₂ consumption of the lungs. Normally, the VO₂ of the lungs represents <5% of the whole body VO₂ (8), but it can account for as much as 20% of the whole body VO₂ when there is an inflammatory process in the lungs (9).

3. Measured VO₂

The whole body VO₂ can be measured with an O₂ analyzer that measures the fractional concentration of O₂ in inhaled and exhaled gas. The VO₂ is then derived as follows: