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64 PA R T I / Anatomy and Physiology
maximum amount of oxygen carried by 1 gram of Hgb. This ex- Oxygen consumption is affected by several factors. Blood flow de-
pression is depicted in the following equation: pends on the cardiac output and on the degree of constriction of
the vascular bed in the tissue (vasoregulatory mechanisms). Low
(Hgb 1.36 Sa O2 )
(0.0031 Pa O2 )
97% 3% Hgb decreases the amount of available oxygen to be delivered to
the tissues. Reduced Pa O2 can affect the driving force needed to
equal to 100 mm
load the oxygen molecule on the Hgb. Decreased Sa O2 affects the
Assuming a normal Hgb of 15 g, arterial Pa O 2
Hg, and 98% saturation, the arterial oxygen content (Ca O2 ) is 20
affinity between oxygen and Hgb, enhancing the release of oxygen
mL/dL. The equation can also be used to determine venous oxy-
to the tissues. The metabolic rate of the tissues also affects the
). Assuming no change in the Hgb and a ve-
affinity of oxygen to be released.
gen content (Cv O 2
nous Pa O2 of 40 mm Hg and 75% saturation, the venous oxygen
content is 15 mL/dL. Oxygen Extraction Ratio
The percentage of oxygen extracted by the tissues is a useful indi-
Measurement of Oxygen Delivery cator of the balance between oxygen delivery and consumption.
˙
˙
Q
Q
Measurement of oxygen delivery (DO 2 ) or transport (QO 2 ) is cal- Oxygen extraction represents the difference between arterial and
culated by multiplying total arterial oxygen content by cardiac venous oxygen contents (normal 5 mL/dL or 25%) and is known
output (CO): as the C(a–v) O2 difference or oxygen extraction ratio (O 2 ER). This
˙ ratio increases in pathological conditions characterized by an im-
DO 2 CO Ca O 2 10
balance between oxygen delivery and VO 2 . O 2 ER is increased by
) factors such as decreased cardiac output, increased oxygen con-
where Ca O 2 (Hgb 1.36 Sa O 2 )
(0.0031 Pa O 2
13.6 sumption (e.g., shivering), anemia, and decreased arterial oxygena-
CO Hgb Sa O 2
tion. O 2 ER is decreased in conditions where VO 2 is relatively low
of 20 mL/dL,
In patients with cardiac output of 5 L/min and a Ca O 2 in proportion to oxygen delivery, such as in sepsis, hypothermia,
˙
arterial oxygen delivery (DO 2 ) is 1,000 mL of oxygen/min or on av- high-flow states, peripheral shunting, or cytopathic hypoxia. 180,187
2
erage 500 to 650 ml/min/m . Critical oxygen delivery, which reflects
the point where oxygen delivery fails to satisfy metabolic needs for
˙
oxygen has not been definitively identified. Critical DO 2 has been es- R EFERENCES
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2
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VO 2 CO Hgb 13.6 (Sa O2 Sv O2 ) Hypertension, 20, 846–852.
v v
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˙
VO 2 234 mL/min Pharmacology and Physiology, 27, 826–829.
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