Hb consists of four o2-binding heme molecules, which are iron-containing porphyrin rings, as well as four globin
protein chains. The molecular weight of Hb is 66,500 d.
There are three important physiologic properties of its chemical binding with o2.
Hb binds reversibly with o2 to form
oxyhemoglobin (Hb-o2). Although there are
four heme molecules, oxyhemoglobin saturation is not limited to
25%, 50%, 75%, and 100%. Measured
oxyhemoglobin saturation reflects the average of all Hb
molecules, in which some have all four saturated and others may
have three, two, or one.
o2 associates and dissociates from Hb very rapidly in the order of milliseconds. This is very important,
as erythrocytes spend less than 1 second in capillaries.
The sigmoidal shape of the oxyhemoglobin dissociation curve (Figure 20-1) is governed by the molecular interaction of the molecule, allowing it to load o2 in
the lungs and deliver it to the end-organ tissues in a very efficient manner.
The flat upper portion of the curve shows that saturation is
minimally affected by reasonable decreases in Pao2.
For example, a Pao2 of 100 mm Hg equals
a saturation of about 98%, while a Pao2 of
70 mm Hg equals a saturation of approximately 94%. This
allows us to tolerate some (V̇/Q̇) mismatching,
hypoventilation, and high altitudes and still be able to load o2.
The steep lower part of the curve allows the tissues to extract
a large amount of o2 with only a small decrease
in Pao2 (Table 20-5).
Also, there are many factors that regulate and affect the concentration of Hb in the blood, including dietary factors like iron, folate,
and vitamin B12, as well as erythropoietin levels, not
to mention toxins, hemorrhage, and hemoglobinopathies (see Chapter 226, Anemia).
Although the first two-wavelength oximeter was developed in 1935, the first pulse oximeter was invented in 1972 by
Takuo Aoyagi.3 The device measures the ratio of
red to infrared (660 nm to 940 nm) light absorption of the pulsating components at the measuring site. As deoxygenated Hb and oxyhemoglobin absorb
light at these different wavelengths, respectively, the device detects
Hb o2 saturation extremely effectively.
By the mid-1980s, this technology became the standard of care in
operating arenas and quickly spread to postanesthesia care units,
intensive care units, and EDs as it became more compact and affordable.
Now it is considered a vital sign at triage.
Pulse oximetry is not a substitute for blood gas analysis, as it is solely a measure of the percentage of Hb that is saturated
with o2 (Sao2) using
infrared light absorption. It gives no information about pH, Paco2,
bicarbonate measurements, Hb, or o2 content
(Pao2) of the blood. A patient with severe
anemia could have a normal Sao2 yet very
low o2 content.
Limitations associated with pulse oximetry include falsely elevated saturation readings in the presence of toxic hemoglobins4,5 and cyanide (see Chapter 65, Respiratory Distress, Chapter 217, Carbon Monoxide, and Chapter 198, Industrial Toxins)
and falsely low readings in the presence of impaired local perfusion,
arteriovenous grafts, hypothermia, vasopressor use, fluorescent
light, and a few nail polishes. Fetal Hb absorbs light at the same
spectrum as normal Hb and gives accurate values.
Factors Affecting Oxyhemoglobin Dissociation
The standard o2-Hb dissociation curve shown above in Hemoglobin and the Oxyhemoglobin Dissociation Curve
applies under the exact following conditions: human Hb type A, pH = 7.4
([H+] = 40 nmol/L),
Pco2 = 40 mm Hg, temperature = 37°C
(98.6°F), and 2,3-diphosphoglycerate (2,3-DPG) concentration = 15
micromoles/gram Hb. When any of the last four factors increase,
the affinity of Hb for o2 decreases and
shifts the curve to the right. The entire curve is shifted proportionately
without changing its shape. Conversely, when any of the factors
decrease the curve will shift to the left (Table 20-6). This has certain physiologic advantages. Shifting the
curve to the right facilitates o2 delivery
(Do2) to the tissues, whereas shifting the curve
to the left increases Hb saturation of o2 (Tables 20-6, 20-7, and 20-8).
Table 20-6 Factors Affecting Oxyhemoglobin Dissociation |Favorite Table|Download (.pdf)
Table 20-6 Factors Affecting Oxyhemoglobin Dissociation
|Left Shift||Right Shift|
|Decreased Pco2||Increased Pco2|
|Decreased 2,3-DPG||Increased 2,3-DPG|
|Abnormal and fetal hemoglobins|
Table 20-7 Changes in PaO2 Related to pH |Favorite Table|Download (.pdf)
Table 20-7 Changes in PaO2 Related to pH
|Pao2 (mm Hg*)||80||90||100||111||122||134||148|
Table 20-8 Changes in PaO2 Related to Temperature |Favorite Table|Download (.pdf)
Table 20-8 Changes in PaO2 Related to Temperature
|Pao2, mm Hg*||117||111||105||100||90||76|
The more acidic the blood, the more readily Hb gives up its o2 and the
higher the Pao2 (the partial pressure of o2 dissolved
in blood) for a particular oxyhemoglobin saturation. In contrast,
alkalosis makes Hb hold on to its o2 more
tightly, lowering the Pao2 present at a
particular oxyhemoglobin saturation. In general, a rise or fall
in pH of 0.10 causes a fall or rise (i.e., an opposite change) in
the Pao2 of about 10% (Table 20-7).
Partial Pressure of Carbon Dioxide
A shift of the oxyhemoglobin dissociation curve,
as a result of changes in the blood levels of co2 (Haldane effect)
and hydrogen ions (Bohr effect), enhances oxygenation of the blood
in the lungs and promotes release of o2 from
the blood in the tissues. As the blood passes through the lungs, co2 diffuses from the blood into the alveoli. This reduces the blood Pco2 and decreases the hydrogen ion concentration because of the resulting
decrease in the blood carbonic acid level. Both changes shift the
oxyhemoglobin dissociation curve to the left. With a shift to the
left, the quantity of O2 binding to Hb at
any given Pao2 is increased, allowing greater o2 transport to the tissues. Then, when the blood reaches the tissue capillaries, the opposite effect occurs. co2 entering
the blood from the tissues shifts the curve to the right. This displaces o2 from the Hb and delivers o2 to the tissues at
a higher Po2 than would otherwise occur.
As blood temperature increases, Hb gives up o2 more readily, raising the Po2 in the plasma. The opposite occurs during cooling. For each 1°C (1.8°F) rise in temperature, the Pao2 rises about 5% (Table 20-8). With hypothermia, the Pco2 falls by about the same amount.
2,3-DPG is the most plentiful compound present in red blood cells except for Hb. A normal concentration of 2,3-DPG in a red blood
cell keeps the oxyhemoglobin dissociation curve shifted slightly
to the right all the time. Under hypoxic conditions lasting longer
than a few hours, as in high altitudes, the quantity of 2,3-DPG
increases considerably, shifting the oxyhemoglobin dissociation
curve even farther to the right. This can cause the Po2 in
the plasma to be as much as 10 mm Hg higher. Conversely, if the
concentration of 2,3-DPG falls, as it does in banked blood or in
critical illness, the Hb binds o2 more tightly
as the curve shifts to the left. This is an important consideration during
Most hemoglobinopathies cause a left shift of the curve. Both
carboxyhemoglobin and methemoglobin toxicities severely shift the
curve to the left. The physiology of carboxyhemoglobin binding is
discussed in Chapter 217, Carbon Monoxide Poisoning. Fetal
Hb shifts the curve to the left and alters its shape, which allows
for Hb binding at very low Pao2.