Pulse oximetry and capnography in emergency and intensive care
Technological advances over the last 20 years now enable the rapid and continuous monitoring of an animal’s physiological parameters. These developments include pulse oximetry and capnography, and both can now play an important role to monitor companion animals in intensive care situations. Their use enables the clinician to assess and adjust the oxygen supply to the tissues and to maintain the blood pH within values compatible with good tissue function. This article outlines the advantages and limitations of pulse oximetry and capnography in emergency and intensive care to help the practitioner fully utilize these technologies in everyday practice.
Pulse oximetry is a non-invasive method that enables the continuous monitoring of variations in hemoglobin oxygenation (Figure 1). Pulse oximeters were first developed in 1935 but did not become commercially available until the 1970’s (1); the technique essentially involves an optical device that measures the difference in absorption of a wave of light between oxygenated hemoglobin (HbO2) and non-oxygenated hemoglobin (Hb); HbO2 absorbs more light in the infrared range (850-1000 nm) than Hb, which absorbs more light in the red wavelength (600-750 nm) (1). The pulse oximeter emits both red and infrared lightwaves across the measurement site (earlobe, interdigital spaces, tongue, etc.) to a photodetector, which then transmits the signal to the monitor, which in turn uses an algorithm to deliver a numerical value (2).
Pulse oximetry measures the oxygenation percentage of the hemoglobin (SpO2), which is a reliable approximation of arterial oxygen saturation (SaO2) (3). This value is then extrapolated to produce a value for the arterial oxygen partial pressure (PaO2) from the dissociation curve of hemoglobin (Figure 2). However, it is important to remember that the PaO2 values corresponding to the read SaO2 are influenced by the concentration of 2.3 diphosphoglycerate within the erythrocytes, the blood pH, and the body temperature. Normal SpO2 values (and thus SaO2) are 96-98%, which under normal physiological conditions corresponds to a PaO2 of 80-100 mmHg (4).
The signal obtained depends not only on the oxygen saturation of the hemoglobin but also the amplitude of the pulse, which reflects peripheral perfusion. Thus the signal may be influenced by cardiovascular and/or respiratory dysfunction, and it is sometimes difficult to distinguish between them everyday practice (4). Many pulse oximeters now use the plethysmography technique and thus display the pulse amplitude (Figure 3), which helps the practitioner interpret the numerical values.
Pulse oximetry was first used in healthy anesthetized animals and has now become part of the standard minimal anesthetic monitoring protocol (4). However its use is now more widespread, being employed to monitor animals under mechanical ventilation, for the evaluation of oxygenation when admitting animals in emergencies, or for the early detection of hypoxemia in animals hospitalized in intensive care.
There are different types of sensors (clip, cylinder, flatprobes, etc.), but clips are the most commonly used and most practical for veterinary medicine (4) (Figure 4). The sensor should be placed on a smooth, light-coloured zone of the patient, and the preferred areas are therefore the tongue, interdigital area, earlobe, axillary or inguinal folds, prepuce, or vulva (4).
To obtain the most accurate signal, it is important to follow a few guidelines (5):
• Choose a site with minimal pigmentation which is warm, thin-skinned, and without any fur. Mucosa is ideal; if skin is used, the area can be shaved and cleaned with alcohol if necessary.
• Protect the probe from ambient light.
• Keep the animal in a quiet environment.
• Always ignore the first value displayed in pulse oximetry; perform continuous monitoring or make several measurements.
• Ensure that the heart rate given by the pulse oximeter corresponds to the actual heartbeat of the patient.
• If the displayed values do not correlate with the clinical examination, confirm the values with an arterial blood gas analysis and repeat the measurements.
The most reliable values are obtained from well-perfused mucosa such as the tongue and the preputial or vaginal mucosa. These sites are very easy to access in an anesthetized animal, but such measurements become more difficult in conscious animals, especially if aggressive or painful. In such cases, it is advisable to use the axillary or inguinal skin folds and remove the probe between each measurement to avoid damage.
If the values obtained are highly variable, if the displayed heart rate is different to the animal’s actual heart rate, or if the plethysmography trace has reduced amplitude, it is important to resite the probe.
Advantages and indications
Arterial blood gas measurements provide a more precise measure of blood oxygenation, but pulse oximetry offers the advantage of continuous monitoring (2). Its ease of use and good tolerance, the complete absence of risk for the animal, the low cost, the possibility of bedside monitoring, and the instantaneous display of results all make it very useful for emergency medicine and intensive care (6).
To use pulse oximetry correctly, it is important to understand its limitations.
a. Device limitations
The size and shape of the probe can sometimes cause problems, especially in small animals such as cats (4). If the probe is left in place for several days (for example with an animal on ventilation), the resulting heat and pressure can cause tissue necrosis. The probe site, and movements of the animal, can both have a major influence on the values obtained, and as noted above its use can be complicated in conscious animals (6).
b. Technological limitations
The absorption of the light beam is altered by ambient light and by the color of the mucosa. The values obtained are not therefore very reliable in animals with black mucosa.
The technique is not very sensitive for evaluating PaO2 in patients on oxygen therapy because of the relationship between PaO2 and the inspired oxygen fraction (FiO2): in animals with no alteration in gas exchange, the PaO2 should be five times greater than the FiO2 (Figure 2) so that an animal, intubated and ventilated with 100% oxygen, has a PaO2 of 500 mmHg. Now, according to the dissociation curve of hemoglobin, as long as the PaO2 is more than 100 mmHg, the SaO2 is 100%. Thus, the SpO2 will not detect alterations in gas exchange if the PaO2 value varies between 100 and 500 mmHg. The SaO2 and SpO2 are only affected if the PaO2 falls below 100 mmHg (6). It is therefore important to complete the monitoring of blood oxygenation with blood gas measurements for animals on oxygen therapy. As explained earlier, the oximetry signal is highly dependent on tissue perfusion. The signal is therefore often of poor quality and potentially uninterpretable in hypovolemic and/or hypothermic animals with significant peripheral vasoconstriction. The supply of oxygen to the tissues is calculated as being the product of the arterial oxygen content (CaO2) and cardiac output. CaO2 depends on the Hb concentration, the SaO2, and the PaO2 and is calculated as follows:
CaO2 = ([Hb] x SaO2 x 1.34) + (0.003 x PaO2)
It is clear that the hemoglobin concentration plays an essential role in the arterial O2 concentration. Thus, in anemic animals without pulmonary disease, the SpO2 is normal and falsely reassuring, despite a low arterial oxygen concentration (linked to the low Hb levels), thus compromising the oxygen supply to tissues.
Finally, pulse oximetry gives erroneous results in the event of qualitative hemoglobin anomalies. The commonly used probes only emit two wavelengths, making it impossible to differentiate between non-functional hemoglobins (carboxyhemoglobin, methemoglobin, sulfhemoglobin, and carboxysulfhemoglobin) and normal hemoglobin (6).
Capnography is the measurement and graphic representation of instantaneous carbon dioxide concentrations during a respiratory cycle (6). Showing the results as a graph provides more information than capnometry alone and it is therefore preferable to choose a monitor that displays the CO2 concentrations graphically. Capnometry is the measurement of the partial pressure of carbon dioxide (CO2) present in the inspired and expired gases (7), with the most commonly used value being the CO2 concentration at the end of expiration, also known as End-Tidal CO2 (ETCO2).
There are currently several methods for measuring the partial pressure of CO2 which can be used in emergency and intensive care situations: mass spectrometry, infrared spectrophotometry, Raman spectrometry, and photoacoustic spectrometry. The most widely used is infrared spectrophotometry; this technique relies on the physical principle that gases consisting of molecules of more than two individual atoms have their own specific absorption spectrum in infrared light, which therefore represents their “identity card” (7).
Technically, capnography devices have a measurement cell either in the machine itself or somewhere along the circuit. In the first instance, the machine is known as a “sidestream” capnometer, where a sample of gas is aspirated by a small tube placed as close as possible to the patient’s airways (Figure 1). With the second option, known as a “mainstream” capnometer, the reading cell is integrated into the patient’s respiratory circuit, usually between the endotracheal tube and the anesthetic circuit or ventilator (8) (Figure 5).
Reading a normal capnogram
To interpret anomalies on the capnograph trace, it is important to know what a normal trace looks like. A normal capnogram can be separated into 4 phases (Figure 6) as follows:
One inspiratory phase
• Phase 0 corresponds to inspiration. There is a sudden drop in the curve when gases without CO2 start to enter the upper airways; the baseline then reads zero throughout inspiration.
Three expiratory phases
• Phase I corresponds to the start of expiration and thus to the anatomical dead space. No CO2 should be measured during this short phase.
• Phase II corresponds to a mix of gas from the dead spaces and alveoli, provoking a rapid increase in the amount of expired CO2.
• Phase III, or alveolar plateau, corresponds to the emptying of the gas from the alveoli. The maximal concentration achieved at the end of this plateau, shown as a red dot on Figure 6, is the maximal concentration of CO2 at the end of expiration, or ETCO2, and reflects the alveolar CO2 concentration.
Since CO2 is a highly diffusible gas, this value is a reliable and non-invasive approximation of the arterial partial pressure of CO2 (PaCO2) in healthy animals. Changes in the shape of this trace provide a wealth of information and enable the clinician to make an early diagnosis of cardiovascular and respiratory disorders, even before the oxygen and CO2 start to fall in the bloodstream.
Interpreting abnormal capnograms
Analysis of the shape of the capnogram curve and the ETCO2 values provides essential information regarding the patient’s cardiorespiratory function. Modifications can occur in individual phases or in the overall trend of the graph (8). In the majority of cases, the changes affect the alveolar plateau (phase III), the ETCO2 value, or the inspiratory phase (Phase 0).
Diagrammatic representations of the most common modifications and their interpretations are given in Figures 7-12, and other examples are available at www.capnography.com.
Capnography is a simple and non-invasive method for estimating PaCO2, thus avoiding the need for repeated sampling for arterial blood gas analysis. It has thus become an important part of anesthetic monitoring or for animals under mechanical ventilation in intensive care. The PaCO2 estimation provides information about CO2 production, pulmonary perfusion, alveolar ventilation, respiratory movements, and the elimination of CO2 by the ventilator (9). At-risk situations for theanimal are also detected rapidly, such as obstruction or displacement of the endotracheal tube, respiratory or cardiac arrest, or re-breathing of CO2 in the circuit. For animals under mechanical ventilation, ETCO2 monitoring enables the detection of alterations in ventilator parameters (particularly the respiratory rate).
The gradient (a-ET) CO2 between the PaCO2 (measured on blood gases) and the ETCO2 (measured by capnography) is a good estimation of the alveolar dead space (corresponding to the alveoli that are ventilated but not perfused) (8,9). Under physiological conditions, theETCO2 is 2-5 mmHg lower than the PaCO2. This normal gradient is due to the disparity in the ventilation/perfusion V/Q) ratio in the healthy lung. An increase in the (a-ET) CO2 gradient is indicative of an increase in alveolar dead space secondary to an overly long anesthetic circuit, hypoventilation, obstructive pulmonary disease, reduced cardiac output, pulmonary thromboembolism, or major pulmonary atelectasis (8).
Capnography is also extremely helpful during cardiopulmonary resuscitation. The Reassessment Campaign on Veterinary Resuscitation (RECOVER) emphasizes the importance of capnography in the early detection of cardiovascular deficiency, especially in anesthetized and ventilated animals (Figure 10) (10).
The ETCO2 is a useful index of pulmonary perfusion and cardiac output in intubated and ventilated animals receiving constant ventilation. Combined with clinical findings, capnography can help with the early detection of cardiorespiratory arrest in these patients (Figure 10) and also enables the detection of accidental esophageal intubation (Figure 7). Capnography is also a reliable and effective indicator for cardiopulmonary resuscitation and has prognostic value; resuscitation is more likely to be successful in patients with a higher ETCO2 value (10) and the routine use of capnography is advisable.
There is no doubt that pulse oximetry and capnography have major roles for effective monitoring of companion animals nowadays. A good understanding of the indications and limitations of these two techniques will enable practitioners to reliably monitor their patients, and thus reduce the risks of morbidity and mortality in animals admitted to intensive care.
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This article was previously published in 2014.