Noninvasive monitoring of oxygen and carbon dioxide

Reviews

Noninvasive Monitoring of Oxygen and Carbon Dioxide AYMAN O. SOUBANI, MD Pulse oximetry and capnography are widely used in clinical practice. They provide quick and noninvasivemethodsto estimate arterial oxygen saturation and carbon dioxide tension in different situations including emergency departments, intensive care units, and during procedures. This article reviews the principles of surgery, accuracy, limitations, and clinical applications of these instruments. (Am J Emerg Med 2001;19: 141-146. Copyright ©2001 by W.B. Saunders Company) There is increasing interest in the use of noninvasive methods to monitor patients in the emergency departments, intensive care units, and during procedures. Pulse oximetry and capnography are widely used to estimate the arterial oxygen saturation and carbon dioxide tension respectively. This article reviews the principles of operation, accuracy, limitations, and clinical applications of these instruments. Although other methods such as transcutaneous oxygen and carbon dioxide probes are available, they are not widely used and will not be discussed here. PULSE OXIMETRY The introduction of pulse oximetry to clinical practice in the early 1980s offered a reliable, noninvasive, and easy to use tool of continuous monitoring of arterial oxygen saturation. In 1935 Kramer was the first to continuously measure the oxygen saturation of blood moving through an isolated artery of an animal using an instrument that transmits light through tissue. 1 However, he was unable to differentiate arterial from capillary or venous oxygen saturation. In 1941 Milikan introduced an ear oximeter that measures blood saturation using barrier large layer photocells and light with red and green filters. 2 Then in 1972 Takuo Aoyagi determined that the pulsatile components of the red and infrared light absorbances as they pass through tissue were related to arterial hemoglobin saturation. 1 Applying this observation to oximeters lead to the development

From the Division of Pulmonary, Critical Care and Sleep Medicine, Wayne State University School of Medicine, Detroit, MI. Manuscript received May 1, 2000, accepted July 1, 2000. Address reprint requests to Ayman O. Soubani, MD, Pulmonary and Critical Care Division, Harper University Hospital, 3 Hudson, 3990 John R, Detroit, MI 48201. E-mail: [email protected] Key Words: Noninvasive, monitoring, oxygen, carbon dioxide, pulse oximetry, capnography, end-tidal CO2. Copyright © 2001 by W.B. Saunders Company 0735-6757/01/1902-0012535.00/0 doi:l 0.1053/ajem.2001.21353

of the currently used pulse oximeters. The concept of pulse oximetry is based on 3 principles3: 1. Every substance has a unique absorbance spectrum. The different types of hemoglobin have different absorption spectra (Fig 1). For example oxyhemoglobin absorbs less red light (660 nm) and more infrared light (940 nm) than deoxyhemglobin. 4 2, The absorption of light as it passes through a clear nonabsorbing solvent is proportional to the concentration of the solute and the length of the path the light has to travel in that solvent (based on the LambertBeer Law). 3. The presence of pulsatile signal generated by the arterial blood that is relatively independent of the nonpulsatile venous blood in the tissues. The current pulse oximeter consists of a probe containing 2 small, high intensity monochromatic light emitting diodes (LEDs) that are activated alternatively, and emit light at 660 nm (red) and 940 nm (infrared) wavelengths. There is also a photodetector that measures the amount of light transmitted through the tissue (usually fingertip or earlobe). The measurements are plotted against a standard calibration curve (that is determined by direct measurements of arterial oxygen saturation (SaO 2) of normal resting healthy volunteers, and stored in a digital microprocessor within the oximeter) to give the estimated patients' oxygen saturation (SpO2). It should be noted that SpO 2 of the same patient may vary from one brand of pulse oximeter to another, because of technical factors and the calibration curves generated from the volunteers. 5 Accuracy and Limitations The accuracy of pulse oximeters is strongly influenced by the data collected from what are considered healthy volunteers, and stored during the initial programming of the device. So the reference data may be affected by factors such as the volunteers' skin color and hemoglobin concentration which in turn may affect the patients' SpO 2. Most manufacturers report accuracy to within ___2% for SpO 2 70% to 100%, and _+3% for SpO2 50% to 70%, with no reported accuracy below 50% saturation. 1 Many studies were done to determine the accuracy of pulse oximeters. 6,7,8 Chapman reported good correlation between SaO 2 and SpO 2 (0.09%) when SaO 2 was above 75%, however there 141

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FIGURE l. Absorption spectra for the different types of hemoglobin in the wavelength range between 600 and 1000 nm. The wavelengths for monochromic light emitting diodes (LEDs) used in pulse oximetry are shown (660 and 940 nm). Reprinted with permission from Wahr JA, Temper KK, Diab M: Pulse oximetry. Respir Care Clin N Am 1995;1:77-105.1 were significant difference (11.2%) when those subjects were exposed to hypoxia and their SaO 2 decreased to 50% to 60%. 9 In another study Hannhart et al examined the accuracy of 14 oximeters in 16 healthy adults at 4 steady state levels of hypoxia (FiO 2 0.21, 0.1, 0.08, 0.07 lasting 20 minutes at each level). The SaO 2 ranged between 99% and 55%. The difference between SpO z and SaO 2 was less than 3% at high SaO2 (83% to 99%), but increased to 8% -+ 5% at SaO 2 levels below 78%. 1° In the critically ill patients, oximeters have also been shown to be accurate. In a study of 54 ventilator-dependent patients, the difference between SpO 2 and SaO 2 was 1.7% + 1.2% at SaO 2 greater than 90%, that increased to 5.1% -+ 2.7% when the SaO 2 was less than 90%. 11 Collectively the studies show that pulse oximeters are reasonably accurate under steady state conditions and when the SaO 2 is above 70%. The accuracy deteriorates significantly with SaO 2 below 70%. Another factor that will influence the accuracy of oximeters is the response time. There is a delay between a change in S p Q and the display of that change, and this depends on the signal averaging time, circulation time, and the site of the oximeter probe. In general finger probes are slower (24-35 sec) than earlobe probes (10-20 sec). 12 Oximeters have several limitations that may lead to inaccurate readings, and these need to be considered when interpreting SpO 2. One of the most important limitations is that it estimates the arterial oxygen saturation (SaO 2) and not the arterial oxygen tension (PaO2). It is known that because of the sigmoid shape of the oxygen dissociation curve, large changes in the PaO 2 may occur in the upper and lower horizontal portions of the curve with minimal changes in the SaO,. As a result, the patient, especially while on supplemental oxygen, may have serious drop in his or her PaO 2 with only mild drop in SpO 2. On the other hand, PaO 2 may increase to toxic levels without significant changes in SpO 2. Equally important is that pulse oximeters give us no information about PH or CO 2 level, so the patient may experience serious changes in PH and PaCO e (such as

hypercabnic acidosis) that may go undetected if the health provider totally depends on the pulse oximeter in assessing the patient's respiratory status. Low perfusion states such as low cardiac output, vasoconstriction, vasoactive therapy, and hypothermia affect the SpO2 because of impaired peripheral perfusion that make it difficult for the sensors to detect arterial pulsations. 13,14 The presence of dyshemoglobins affects the accuracy of pulse oximeters. Carboxyhemoglobin has an absorbance similar to oxyhemoglobin at 660 nm, so it will overestimate the SpO> leading to falsely high readings? 5 The presence of methemoglobin, that may be generated by oxidizing agents like nitrites and sulfonamides, may also lead to falsely high or low SpO2 .16 Dyes used for diagnostic purposes like methylene blue and indocyanine green affect SpO~ readings leading to falsely low values, whereas flouresem injections have not been shown to affect SpO2 .17 Nail polishes, specifically blue, green or black colors have light absorbance near 660 nm and affect the measured SpO 2 leading to falsely low readings, is As for skin color, it was suggested that dark skin may affect the ability of pulse oximeter to pick the arterial pulsations, thus affecting the SpO 2. In one study of patients in the intensive care unit (ICU), the difference between SpO 2 and SaO 2 was more frequent in black patients when compared with white patients. ~1 Other studies did not find a significant difference/9 The effect of anemia on SpO 2 is controversial. Most studies indicate that the concentration of hemoglobin does not affect the pulse oximeter reading until the hematocrit reaches values less than 10%. 2°,21 Polycythemia, on the other hand, does not affect SpO> Hyperbilirubinemia has not been shown to affect SpO 2 and in one study, bilirubin levels up to 30.6 mg/dL did not affect the accuracy of pulse oximeter. 2~ Motion artifacts is a common problem in using pulse oximeters. This is especially true in the ICU and exercise laboratories. 23 Several modifications are being considered to minimize this problem. Clinical Applications

Pulse oximeters are widely used in clinical practice. They provide quick and noninvasive method of evaluating the oxygen saturation in different situations. They assist clinicians and health care providers in identifying subtle hypoxemia especially in acute situations because clinical evaluation of hypoxemia was found to be inaccurate54 In a study of 14,059 patients evaluated in an emergency department, the patients were monitored by pulse oximetry and the information was given to physicians after their initial assessment. The information resulted in significant changes in the management (diagnostic tests and/or therapy), and these changes were most frequent when SpO 2 was around 89%. 25 Pulse oximeters are also helpful in monitoring patients during procedures like bronchoscopy, endoscopy, cardiac catheterization, exercise testing, and sleep studies. They are commonly used during labor and delivery for both the mother and infant. It is widely accepted to use pulse oximeters to monitor patients before and during anesthesia and postoperatively in the recovery room. In a trial of 20,802 patients scheduled

SOUBANI • OXYGEN AND CO2 MONITORING

for surgery, the patients were randomized to receiving pulse oximeter monitoring or not. Hypoxemia was detected 19 times more frequently during anesthesia in the pulse oximetry group, and the same observation was noted in the recovery rooms. Bronchospasm, atelectasis, and bradycardia were detected more frequently in the group monitored by pulse oximeters. However there was no differences between the 2 groups in the incidence of cardiovascular, neurologic, infectious complications or death. 6,26 In the ICU, pulse oximeters are routinely used especially during mechanical ventilation. They are used to assist in titrating the FiO 2 and during weaning. However there are very few studies to document the usefulness of pulse oximeters in these settings and to show their effect on outcome. In a study of 24 patients on mechanical ventilation after elective cardiac surgery, those who were monitored by pulse oximeters had fewer arterial blood gases than the control group (5.9 --- 2.7 v 10.5 _+ 1.8). The duration of intubation was not different between the 2 groups. 27 Another study found no difference in the duration of mechanical ventilation, ICU stay, or the need for supplemental oxygen between a control group and another monitored by pulse oximeters. 28 There are no significant complications related to using pulse oximeter. It's main limitation is the false-negative and positive results of SpO 2 that may lead to inappropriate decisions in the treatment of patients. Minor complications have been reported such as pressure sores from prolonged application of pulse oximeter probes. Burns underneath the probes have been described with the use of pulse oximeters in magnetic resonance imaging (MRI) scanners. 23

CAPNOGRAPHY Capnometry is the measurement of C O 2 concentration in a gas mixture. Continuous waveform display of the capnometer data throughout the ventilatory cycle is called capnography. The measurement of CO 2 in respiratory gases was first accomplished in 1865 using the principles of infrared absorption. Capnography was developed in 1943 and introduced to clinical practice in the early 1950s. 29 The most commonly used methods of capnography use infrared light absorption (infrared spectrometry) and mass spectrometry. 3° The first method depends on the concept that CO 2 strongly absorbs infrared light with a wavelength of 4280 /xm. So infrared light is emitted from a hotwire and filtered to obtain the desired wavelength. This infrared radiation passes through the sample chamber where it is absorbed by CO 2 and the remaining unabsorbed radiation is focused onto a detector with a semiconductor that creates an electrical signal. The concentration of CO 2 is directly proportional to the amount of infrared light absorbed; and the higher the CO 2 concentration in the gas mixture the more infrared radiation is absorbed and less arrives to the detector. This method allows for real time, continuous measurement, and display of PCO 2 with a delay time of approximately 0.25 second. Nitrous oxide, which absorbs infrared light at a relatively close wavelength, may interfere with the measurement of CO 2 concentration. In mass spectrometry, the sample enters a large chamber with a 1 0 - 4 mmHg vacuum and is exposed to an electronic beam that ionizes gases. The ionized particles then pass

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through a magnetic field that deflects these particles into arches, the diameter of each arch depends on the mass to charge ratio of each gas. Lighter gas particles tend to be deflected more, resulting in a smaller radius arch. A detector is positioned at the other end of these arches, and depending on the frequency with which the ions strike the detector, the concentration of that gas is determined. Although the mass spectrometer is very accurate and the delay time is 0.1 second, it is more expensive and not readily portable. The gas to be analyzed reaches the sample chamber in 1 of 2 ways31: a mainstream analyzer that resides within the breathing circuit, usually between the end of the endotracheal tube and the Y connection. This method is usually used for infrared spectrometers. Because the sample chamber is part of the breathing circuit, the delay time is minimized and there are no problems with increased work of breathing or clogging by pulmonary secretions. However the analyzer is heavy and may cause kinking or dislodgement of the endotracheal tube. The second method is sidestream analyzer, in which the gas sample is aspirated from the breathing circuit through a small bore tube to a remote analyzer. This method is mainly used for mass spectrometry and some infrared analyzers. It adds little weight to the breathing circuit but the narrow lumen of the sampling tube is more likely to be clogged by pulmonary secretions and the delay time is much longer than the mainstream method. Also there may be some loss of the expired tidal volume caused by continuous sampling. The capnogram during expiration is normally divided into 3 phases (Fig 2): the first phase is zero and represents the gas from the anatomical dead space (trachea) that is free of CO2. In the second phase the curve rises sharply as the alveolar gas that contains CO 2 mixes with the dead space. As expiration continues, more and more of the alveoli empty and CO 2 concentration rises rapidly until a plateau, that essentially represents alveolar gas, is reached (phase 3). When inspiration starts the CO 2 level drops sharply and

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FIGURE 2. Normal capnograph consisting of 3 phases: Phase I at zero baseline representing anatomic dead space CO2. Phase II shows rapid sharp rise caused by progressive alveolar CO2. Phase III represents the alveolar CO, plateau. The end point of alveolar plateau just before the start of inhalation is called end tidal CO2 (PetCO2). The normal disparity between PaCO 2 (arterial CO2 tension) and PetCO 2 is 1 to 2 mmHg. Reprinted with permission from Shapiro BA, Harrison RA, Cane RD, et al: Clinical Application of Blood Gases (ed 4). Chicago, Year Book Medical Publishers Inc, 1989.29

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there is a rapid down stroke of the curve to zero. The point at which the plateau ends just before inspiration is referred to the end tidal CO 2 concentration (PetCO2). PetCO2 is commonly used to estimate PaCO 2 quantitatively, however in normal healthy and awake people the PetCO 2 is usually slightly less than PaCO2, and the difference between PaCO 2 and PetCO 2 is called CO 2 gradient (P(a-et) CO2). In normal individuals P(a-et) CO 2 gradient ranges between 2 and 5 mmHg. 32 The gradient is higher in situations that increase the physiological dead space (which is the sum of anatomical dead space that does not participate in gas exchange and alveolar dead space because of nonfunctioning alveolar units), or cause ventilafion/perfusion (V/Q) mismatch or increased shunt. Table 1 describes the different clinical conditions that affect PetCO.. Capnographic waveform changes are also significant and may be characteristic of certain clinical situations (Fig 3), so it is essential to monitor the waveform tracing in addition to the numerical value of PetCO 2. Clinical Applications

PetCO 2 can be used reliably to estimate the PaCO, as long as the capnogram waveform remains unchanged,-the temperature remains constant and the ventilatory and cardiovascular functions are stable. In addition capnography is helpful in ensuring adequacy of pulmonary ventilation, assessing changes in pulmonary blood flow and physiological dead space, and detecting the addition of CO 2 to the systemic circulation. 33

Confirming Endotracheal Intubation Capnography has been shown to be useful during intubation to ensure tracheal rather than esophageal intubation. CO 2 is primarily eliminated from the lung and the concentration of CO 2 in the stomach is negligible, so with tracheal intubation the PetCO 2 is high (20-45 mmHg) and remains so, whereas with esophageal intubation the PetCO 2 is usually zero and the waveform disappears. If ventilation by mask took place before intubation, CO 2 may be detected with esophageal intubation from exhaled gas in the stomach, however this is usually very low (3-5 mmHg) and deTABLE 1. Conditions that Affect PetCO 2 Increase in PetCO2 Increase in cardiac output Administration of bicarbonate Addition of CO2 (eg, during laparoscopy) Hypoventilation Hyperthermia Decrease in PetCO2 Esophageal intubation Decrease in cardiac output and pulmonary blood flow Cardiac arrest Pulmonary embolism Air embolism Hyperventilation Hypothermia Disruption of ventilation system (eg, disconnection from ventilator, circuit leak, obstruction of endotracheal tube, or accidental extubation)

Plateau

Normal

g

Obstructive Disease

Restrictive Disease

Time

FIGURE 3. Normal capnograph pattern showing sharply rising expiration slope and a clear alveolar plateau. Obstructive pulmonary disease causes prolonged expiration slope with difficult to identify alveolar plateau. Restrictive pulmonary disease causes irregularity in the plateau due to uneven alveolar emptying. Reprinted with permission from Shapiro BA, Harrison RA, Cane RD, et al: Clinical Application of Blood Gases (ed 4). Chicago, Year Book Medical Publishers Inc, 1989.29

creases rapidly to zero after 4 to 5 breathes. 34 Capnography is also helpful in recognizing disconnection from ventilation system, accidental extubation and airway obstruction. 3~ It is important to mention at this point that capnography should be viewed as an adjunct to01, and clinicians should continue to rely on standard clinical signs to ensure proper intubation and ventilation.

Monitoring Alveolar Ventilation Capnography and PetCO 2 are useful in monitoring alveolar ventilation (PaCO2) in relatively healthy patients without significant pulmonary disease or hemodynamic instability and normal capnographic waveform. For example capnography is helpful in patients undergoing elective surgery under general anesthesia, patients receiving hyperventilation to decrease intracerebral edema, and those requiring apnea monitoring. Capnography is also useful in selected patients with no significant parenchymal lung disease in the intensive care unit during mechanical ventilation and the process of weaning. 36

Assessing Cardiopulmonal T Resuscitation (CPR) Several studies addressed the role of capnography during CPR. 37,3s,39 The decease in cardiac output and pulmonary blood flow during cardiac arrest result in decreased elimination of CO 2 by the lungs and low PetCO 2. Successful resuscitation results in increase in the cardiac output that will in turn lead to an increase in PetCO2. 3s Capnography has also been shown to help as a prognostic tool during CPR. A study of 35 cardiac arrests in 34 patients monitored by capnography during CPR, 9 patients who were successfully resuscitated had a high average PetCO 2 during CPR

SOUBANI • OXYGEN AND CO2 MONITORING

(15 -+ 4 mmHg) than 26 nonsurvivors (7 -+ 5 mmHg). None of the patients with PetCO2 less than 10 mmHg during CPR survived. 4° In addition capnographic monitoring is a useful guide to the adequacy of closed cardiac compressions during CPR. 4j

Monitoring Changes in Dead Space PetCO 2 is significantly affected by changes in the physiological dead space that in turn reflect changes in pulmonary blood flow and cardiac output. Sudden decrease in cardiac output, decreases the pulmonary blood flow, leading to increase in physiological dead space and a drop in PetCO 2 (widening of the Pa-etCO 2 gradient). 42 The same concept has been used to diagnose pulmonary embolism, that is associated with abrupt decease in pulmonary blood flow leading to a drop in PetCO> 43 Capnography is also useful in calculating the physiological dead space using mixed expired CO 2 tension (PeCO2). 29 The expired gas is collected in a Douglas bag over 2 to 3 minutes and an arterial blood sample is drawn at the same time. Dead space to tidal volume ratio (VD/VT) is calculated according to the Bohr alveolar equation: VD/VT = ( P a C t 2 - PeCOz)/PaCO2" It has also been suggested that capnography can be helpful in choosing the best positive end-expiratory pressure (PEEP) during mechanical ventilation. The hypothesis is PEEP decreases the dead space by recruiting alveoli, whereas higher PEEP leads to overdistention of the alveoli that will increase the dead space. So Pa-etCO 2 gradient is smallest when there is maximal recruitment of the alveoli without overdistention, and the PEEP level can be adjusted while monitoring the Pa-etCO 2 gradient in addition to other parameters. 44,45 In humans investigators showed this concept was useful only for patients whom PEEP produced alveolar recruitment, and the initial inflection point on the pressure-volume curve could be identified. 43 In conclusion capnography is a noninvasive tool of measuring the CO2 concentration in expired gas. It is not a substitute for measuring PaCt2, however it is helpful in optimizing patient care including detecting misplacement of endotracheal tube, adequacy of CPR, and changes in dead space. It is also useful in reflecting alveolar ventilation (PaCt2) provided the patient is hemodynamically stable with constant body temperature, and in the absence of severe pulmonary disease. Capnographic readings should be interpreted with caution in critically ill patients and those with acute respiratory failure.

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37. Trevino RP, Bisera J, Weil MH, et al" End-tidal CO2 as a guide to successful cardiopulmonary resuscitation: A preliminary report. Crit Care Med 1985;13:910-11 38. Garnett AR, Ornato JP, Gonzalez ER, et al: End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation. JAMA 1986;257:512-515 39. Falk J, Rackow EC, Weil MH: End-tidal carbon dioxide concentration during cardiopulmonary resuscitation. N Engl J Med 1988;318:607-611 40. Sanders AB, Kern KB, Otto CW, et al: End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation: A prognostic indicator for survival. JAMA 1989;262:1347-1351

41. Schmitz BD, Shapiro BA: Capnography. Respir Care Clin N Am 1995;1:107-117 42. Weil MH, Bisera J, Trevino RP, et al: Cardiac output and end-tidal carbon dioxide. Crit Care Med 1985;13:907-909 43. Hatle L, Rockseth R: The arterial to end-expiratory carbon dioxide tension gradient in acute pulmonary embolism and other cardiopulmonary diseases. Chest 1974;66:352-357 44. Blanch L, Fernandez R, Benito S, et al: Effect of PEEP on the arterial minus end-tidal carbon dioxide gradient. Chest 1987;92:451-454 45. Murray IP, Modell JM, Gallagher TJ, et al: Titration of PEEP by the arterial minus end-tidal carbon dioxide gradient. Chest 1984;85: 100-104