Oxygen transport

Published on 13/02/2015 by admin

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Oxygen transport

David R. Mumme, MD

The amount of O2 delivered to tissues is equal to the amount contained in the blood (the arterial O2 content) times the cardiac output (CO): CO = stroke volume × the heart rate (Figure 21-1). The arterial O2 content is a product of the hemoglobin (Hb) concentration in grams per deciliter times the amount of O2 in each gram of Hb times the O2 saturation, expressed as a fraction, usually given as 1.39 mL of O2 for every gram of Hb. The affinity of Hb for O2 (Figure 21-1) determines the characteristics of the oxyhemoglobin (HbO2) dissociation curve, with pH, temperature, and concentration of 2,3-diphosphoglycerate (2,3-DPG) having the greatest impact on the affinity of Hb for O2. Five variables affect O2 delivery: (1) Hb concentration, (2) Hb affinity for O2 (P50), (3) percent O2 saturation of Hb (SaO2), (4) CO, and (5) the amount of O2 dissolved in blood (usually trivial amounts).

image
Figure 21-1 The oxyhemoglobin dissociation curve plots hemoglobin saturation (ordinate) at varying O2 tensions (abscissa). Note that hemoglobin is approximately 80% saturated at a PaO2 of 50 mm Hg (P50), 75% saturated at a PaO2 of 40 mm Hg (venous), and 30% saturated at a PaO2 of 20 mm Hg. The normal P50 of adult humans is 26.7 mm Hg. 2,3-DPG = 2,3-Diphosphoglycerate. (Reprinted, with permission, from Miller RD, ed. Anesthesia. 6th ed. Philadelphia, Elsevier Churchill Livingstone, 2005:1799-1827.)

The arterial O2 content (CaO2) is calculated as the sum of the O2 bound by Hb and the O2 dissolved in the plasma.

< ?xml:namespace prefix = "mml" />CaO2=(Hb×1.39×SaO2/100)+(PaO2×0.003)

image

For example, if Hb is 15 g/dL, SaO2 is 100%, and PaO2 is 100 mm Hg, then

CaO2=(15×1.39×1)+(100×0.003)=20.85+0.3=21.15 mL/dL(or 211.5 mL/L)

image

Note that dissolved O2 (PaO2 × 0.003) typically has little impact on CaO2. Notable exceptions occur when O2 carried by Hb is severely diminished, such as in severe anemia or carbon monoxide intoxication or if the PaO2 is very high.

The O2 content of mixed venous blood is usually 25% less than that of arterial blood due to O2 extraction by the tissues. At a mixed venous O2 saturation of 75% and mixed venous O2 tension of 40 mm Hg, mixed venous O2 content (CimageO2) is

Cv_O2=(15×1.39×0.75)+ (40× 0.003)=15.64+.12=15.76 mL/dL

image

Oxygen delivery (image) to the tissues is the product of CO and CaO2. For example, if the CO is 5.0 L/min in the first example, then the image is

D˙O2 = 21.15 dL/L × 50 dL/min=1057 mL/min (approximately 1 L/min)

image

Oxygen consumption (imageO2), approximately 250 mL/min for an adult, is the CO multiplied by the difference between arterial and venous O2 content (assuming no shunt). This calculation uses the Fick principle:

V˙O2=CO×C(a–v_)O2

image

Thus, for a constant imageO2, a decrease in the CO requires a proportionate increase in the C(a − image)O2, usually achieved by increasing tissue O2 extraction. Conversely, if imageO2 increases, CO, C(a − image)O2, or both CO and C(a − image)O2 must increase.

The oxyhemoglobin dissociation curve

The HbO2 dissociation curve is the measured relationship between PO2 and SO2 (see Figure 21-1). The position of this curve is best described by the position at which Hb is 50% saturated, which is normally 26.7 mm Hg in adult humans. Shifting this curve to the left or right has little effect on SO2 greater than 90%, at which point the curve is relatively horizontal; a much greater effect is seen for values in the steeper parts of the curve (SO2 < 90%).

Variables shifting the HbO2 dissociation curve are listed in Table 21-1. A left-shifted HbO2 dissociation curve indicates a higher affinity of Hb for O2 and, thus, a higher saturation at a given PaO2 (e.g., fetal hemoglobin). This increased affinity of Hb for O2 may require higher tissue perfusion to produce the same O2 unloading. Note that banked blood is markedly depleted of 2,3-DPG within 1 to 2 weeks, which can affect O2 delivery after massive transfusion.

Table 21-1

Variables That Shift the Oxyhemoglobin Dissociation Curve

Left Right
Alkalosis Acidosis
Hypothermia Hyperthermia
Decreased 2,3-diphosphoglycerate Increased 2,3-diphosphoglycerate
Abnormal hemoglobin (fetal) Abnormal hemoglobin
Carboxyhemoglobin Increased CO2
Methemoglobin  

A right-shifted HbO2 dissociation curve implies lower affinity and, thus, lower saturation at a given PaO2, but may permit lower tissue perfusion because the lower affinity in effect allows easier unloading of O2 to the tissues. Adding hydrogen ions, 2,3-DPG, and heat causes a rightward shift of the curve.

Chronic acid-base changes will cause a compensatory change in 2,3-DPG within 24 to 48 hours and restore the HbO2 dissociation curve back toward normal.

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