Rad-97
Chapter 1: Technology Overview
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29
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Successful Monitoring for SpHb
A stable SpHb reading is associated with correct sensor placement, small physiological
changes during the measurement and acceptable levels of arterial perfusion at the
measurement site. Physiological changes at the measurement site are mainly caused by
fluctuations in the oxygen saturation, blood concentration and perfusion. See
Safety
Information, Warnings and Cautions
on page 13 and
Troubleshooting Measurements
on
page 137.
General Description for Total Arterial Oxygen Content (CaO2)
Oxygen (O
2
) is carried in the blood in two forms, either dissolved in plasma or combined with
hemoglobin. The amount of oxygen in the arterial blood is termed the oxygen content (CaO
2
)
and is measured in units of ml O
2
/dL blood. One gram of hemoglobin (Hb) can carry 1.34 ml of
oxygen, whereas 100 ml of blood plasma may carry approximately 0.3 ml of oxygen*. The
oxygen content is determined mathematically as:
CaO
2
= 1.34 (ml O
2
/g) x Hb (g/dL) x HbO
2
+ PaO
2
(mmHg) x 0.003 (ml O
2
/dL/mmHg)
Where HbO
2
is the fractional arterial oxygen saturation and PaO
2
is the partial pressure of
arterial oxygen.
For typical PaO
2
values, the second part of the above equation is approximately 0.3 ml O
2
/dL
based on PaO
2
being approximately 100 mmHg. Furthermore, for typical carboxyhemoglobin
and methemoglobin levels, the functional saturation (SpO
2
) as measured by a pulse oximeter
is given by:
SpO
2
= 1.02 x HbO
2
*Martin, Laurence. All You Really Need to Know to Interpret Arterial Blood Gases, Second
Edition. New York: Lippincott Williams & Wilkins, 1999.
General Description for SpOC
The above approximations result in the following reduced equation for oxygen content via the
Pulse CO-Oximeter:
SpOC (ml/dL*) = 1.31 (ml O
2
/g) x SpHb (g/dL) x SpO
2
+ 0.3 (ml O
2
/dL)
*When ml O
2
/g Hb is multiplied by g/dL of SpHb, the gram unit in the denominator of ml/g
cancels the gram unit in the numerator of g/dL resulting in ml/dL (ml of oxygen in one dL of
blood) as the unit of measure for SpOC. See
Safety Information, Warnings and Cautions
on
page 13.
General Description for Carboxyhemoglobin (SpCO)
Pulse CO-Oximetry is a continuous and noninvasive method of measuring the levels of
carboxyhemoglobin saturation (SpCO) in arterial blood. It relies on the same basic principles
of pulse oximetry (spectrophotometry) to make its SpCO measurement.
The measurement is obtained by placing a sensor on a patient, usually on the fingertip for
adults and the hand or foot for infants. The sensor connects either directly to the Pulse
CO-Oximetry device or through a device patient cable.
