~ .,111. -=--
----=-
critical care Pulse Oximetry* Uses and Abuses Lynn M. Schnapp, M.D.; and Neal H. Cohen, M.D.
Pulse oximetry has made a significant contribution to noninvasive monitoring in a wide variety of clinical situations. It allows for continuous reliable measurements of oxygen saturation while avoiding the discomfort and risks o! arterial puncture. As the extent of hypoxic episodes during various procedures and clinical settings is better appreciated, the role of continuous noninvasive monitoring
will undoubtedly expand. An understanding of the principles and technology of pulse oximetry will allow physicians to obtain maximal clinical benefit from its use. (Chest 1990; 98:1244-50)
of Pa02 has been the standard M easurement method for evaluating oxygenation in the clini-
PRINCIPLES
cal setting. Pulse oximetry is now a widely available technology that provides an easy, noninvasive, and reliable method to monitor oxygenation. Pulse oximetry has become the standard of care in the operating room and is quickly becoming routine in the recovery room, intensive care units, and other clinical settings. 1 The basic principles of pulse oximetry will be reviewed, and various clinical and technical considerations affecting the accuracy of pulse oximetry will be discussed. HISTORY
Although the clinical use of pulse oximetry has become popular only recently, the technology has existed for over 50 years. The measurement of oxygen saturation using light absorption was first proposed in the 1930s. The development of the Clark electrode in the 1950s, which allowed for easy measurement of Pa02 , slowed further work on oximetry. The ear oximeter was introduced in the 1960s for noninvasive monitoring of oxygenation, but it was bulky and required heating the ear to arterialize it in order to obtain reliable measurements of oxygen saturation. Several technologic advances, including the development of LEDs and the microprocessor, and the merging of two ideas, optical plethysmography and spectrophotometry, have made the current era of pulse oximetry possible. 2 In order to appreciate the appropriate clinical uses and limitations of pulse oximetry, the principles of oximetry must be understood. *From the Departments of Medicine and Anesthesia. University of California. San Francisco. Reprint requests: Dr. Cohen, Critical Care Medicine. 513lbmassus, San Francisco 94143-0648
1244
LED = Iight-emitting diode
I
The principle of oximetry is based on Beer's law, which states that the concentration of an unknown solute dissolved in a solvent can be determined by light absorption (Fig 1). This relationship is expressed mathematically by Beer's equation (equation 1): ~t =
Lm e -
(D C a)
(1)
where ~n is the amount of incident light, D is the distance through which light travels, C is the concentration of a substance (ie, hemoglobin); a is the extinction (absorption) coefficient, and Lout is the intensity of transmitted light through a substance. The extinction coefficient is a constant for a given substance at a specific wavelength. Beer's law explains what happens when the probe is placed and light is passed through a finger (Fig 2). The majority of light is absorbed by connective tissue, skin, bone, and venous blood. The amount of light absorbed by these substances is constant with time and does not vary during the cardiac cycle. With each heartbeat, there is a small increase in arterial blood, which results in an increase in light absorption. By
Lin
Lout
FIGURE 1. Beer's law, where ~II is amount of incident Ii~ht. C is concentration of substance, D is distance through which light travels. and Lout is intensity of transmitted light through substance.
Pulse Oximetry; Uses and Abuses (SChepp, Cohen)
Light Source
Photodetector
comparing the peak to the trough absorbances, the contribution from nonarterial sources becomes irrelevant. Therefore, the thickness of the skin, fat, skin pigment, do not influence the accuracy of the pulse oximeter. The probe is made up of a light source and a sensor (photodetector) which are placed across a pulsatile vascular bed (Fig 3). The light source consists of two LEDs that emit light at known wavelengths, generally 660 nm (red) and 940 nm (infrared). These particular wavelengths are used because the absorption characteristics of oxyhemoglobin and reduced hemoglobin are quite different at the two wavelengths (Fig 4). By comparing the ratio (R) of pulsatile and baseline absorption at these two wavelengths, the ratio of oxyhemoglobin to reduced hemoglobin is calculated as follows (equation 2):
FIGURE 2. Pulse oximetry probe placed across vascular bed of digit (reprinted with permission from Principles of Pulse Oxime~ Nellcor, Inc., 1988).
R
pulsatile absorbance (red) baseline absorbance (red) pulsatile absorbance (infrared) baseline absorbance (infrared)
(2)
Oxygen saturation is calculated from this ratio based on experimental data; there is no known mathematic relationship between oxygen saturation and R. The manufacturers studied a group of healthy nonsmoking male subjects, with measured methemoglobin and carboxyhemoglobin less than 2 percent and had them breathe various hypoxic gas mixtures to obtain oxygen saturations between 70 and 100 percent. 3 A sample of arterial blood was obtained, and the oxygen saturation was measured using a spectrophotometric heme oximeter (CO-oximeter). The value obtained by the COoximeter and the ccR" value measured by the pulse
Variable absorption due to pulse added volume of arterial blood Absorption due to arterial blood Absorption due to venous blood c
o .;::;
c. ~
o
'"
..0
«
Absorption due to tissue
Time FIGURE 3. Factors inHuencing light absorption through pulsatile vascular bed (from McGough EK, Boysen PC. Benefits and limitations of pulse oximetry in the ICU. J Crit Illness 1989; 4:23-31). CHEST I 98 I 5 I NOVEMBER, 1990
1245
(Red) 660nm
(Infrared) 940nm
o c
o
"ec "~
)(
w
-1
Hb
- 2 L . . . . . -. . . .- . . . - -......- ......--r--...,...-......--, 600 700 800 900 1,000
calibrated. An early study by Yelderman and NeW' examined the accuracy of pulse oximeters over the range of 70 to 100 percent saturation and found excellent correlation between the pulse oximetry and CO-oximetry measurements (correlation coefficient of 0.98; slope, 1.03; p
----=-
critical care Pulse Oximetry* Uses and Abuses Lynn M. Schnapp, M.D.; and Neal H. Cohen, M.D.
Pulse oximetry has made a significant contribution to noninvasive monitoring in a wide variety of clinical situations. It allows for continuous reliable measurements of oxygen saturation while avoiding the discomfort and risks o! arterial puncture. As the extent of hypoxic episodes during various procedures and clinical settings is better appreciated, the role of continuous noninvasive monitoring
will undoubtedly expand. An understanding of the principles and technology of pulse oximetry will allow physicians to obtain maximal clinical benefit from its use. (Chest 1990; 98:1244-50)
of Pa02 has been the standard M easurement method for evaluating oxygenation in the clini-
PRINCIPLES
cal setting. Pulse oximetry is now a widely available technology that provides an easy, noninvasive, and reliable method to monitor oxygenation. Pulse oximetry has become the standard of care in the operating room and is quickly becoming routine in the recovery room, intensive care units, and other clinical settings. 1 The basic principles of pulse oximetry will be reviewed, and various clinical and technical considerations affecting the accuracy of pulse oximetry will be discussed. HISTORY
Although the clinical use of pulse oximetry has become popular only recently, the technology has existed for over 50 years. The measurement of oxygen saturation using light absorption was first proposed in the 1930s. The development of the Clark electrode in the 1950s, which allowed for easy measurement of Pa02 , slowed further work on oximetry. The ear oximeter was introduced in the 1960s for noninvasive monitoring of oxygenation, but it was bulky and required heating the ear to arterialize it in order to obtain reliable measurements of oxygen saturation. Several technologic advances, including the development of LEDs and the microprocessor, and the merging of two ideas, optical plethysmography and spectrophotometry, have made the current era of pulse oximetry possible. 2 In order to appreciate the appropriate clinical uses and limitations of pulse oximetry, the principles of oximetry must be understood. *From the Departments of Medicine and Anesthesia. University of California. San Francisco. Reprint requests: Dr. Cohen, Critical Care Medicine. 513lbmassus, San Francisco 94143-0648
1244
LED = Iight-emitting diode
I
The principle of oximetry is based on Beer's law, which states that the concentration of an unknown solute dissolved in a solvent can be determined by light absorption (Fig 1). This relationship is expressed mathematically by Beer's equation (equation 1): ~t =
Lm e -
(D C a)
(1)
where ~n is the amount of incident light, D is the distance through which light travels, C is the concentration of a substance (ie, hemoglobin); a is the extinction (absorption) coefficient, and Lout is the intensity of transmitted light through a substance. The extinction coefficient is a constant for a given substance at a specific wavelength. Beer's law explains what happens when the probe is placed and light is passed through a finger (Fig 2). The majority of light is absorbed by connective tissue, skin, bone, and venous blood. The amount of light absorbed by these substances is constant with time and does not vary during the cardiac cycle. With each heartbeat, there is a small increase in arterial blood, which results in an increase in light absorption. By
Lin
Lout
FIGURE 1. Beer's law, where ~II is amount of incident Ii~ht. C is concentration of substance, D is distance through which light travels. and Lout is intensity of transmitted light through substance.
Pulse Oximetry; Uses and Abuses (SChepp, Cohen)
Light Source
Photodetector
comparing the peak to the trough absorbances, the contribution from nonarterial sources becomes irrelevant. Therefore, the thickness of the skin, fat, skin pigment, do not influence the accuracy of the pulse oximeter. The probe is made up of a light source and a sensor (photodetector) which are placed across a pulsatile vascular bed (Fig 3). The light source consists of two LEDs that emit light at known wavelengths, generally 660 nm (red) and 940 nm (infrared). These particular wavelengths are used because the absorption characteristics of oxyhemoglobin and reduced hemoglobin are quite different at the two wavelengths (Fig 4). By comparing the ratio (R) of pulsatile and baseline absorption at these two wavelengths, the ratio of oxyhemoglobin to reduced hemoglobin is calculated as follows (equation 2):
FIGURE 2. Pulse oximetry probe placed across vascular bed of digit (reprinted with permission from Principles of Pulse Oxime~ Nellcor, Inc., 1988).
R
pulsatile absorbance (red) baseline absorbance (red) pulsatile absorbance (infrared) baseline absorbance (infrared)
(2)
Oxygen saturation is calculated from this ratio based on experimental data; there is no known mathematic relationship between oxygen saturation and R. The manufacturers studied a group of healthy nonsmoking male subjects, with measured methemoglobin and carboxyhemoglobin less than 2 percent and had them breathe various hypoxic gas mixtures to obtain oxygen saturations between 70 and 100 percent. 3 A sample of arterial blood was obtained, and the oxygen saturation was measured using a spectrophotometric heme oximeter (CO-oximeter). The value obtained by the COoximeter and the ccR" value measured by the pulse
Variable absorption due to pulse added volume of arterial blood Absorption due to arterial blood Absorption due to venous blood c
o .;::;
c. ~
o
'"
..0
«
Absorption due to tissue
Time FIGURE 3. Factors inHuencing light absorption through pulsatile vascular bed (from McGough EK, Boysen PC. Benefits and limitations of pulse oximetry in the ICU. J Crit Illness 1989; 4:23-31). CHEST I 98 I 5 I NOVEMBER, 1990
1245
(Red) 660nm
(Infrared) 940nm
o c
o
"ec "~
)(
w
-1
Hb
- 2 L . . . . . -. . . .- . . . - -......- ......--r--...,...-......--, 600 700 800 900 1,000
calibrated. An early study by Yelderman and NeW' examined the accuracy of pulse oximeters over the range of 70 to 100 percent saturation and found excellent correlation between the pulse oximetry and CO-oximetry measurements (correlation coefficient of 0.98; slope, 1.03; p
