Four monitors are used for virtually every pediatric anesthesia case: pulse oximetry, capnography, electrocardiogram (ECG), and blood pressure measurement. These monitors are components of the Basic Monitoring Standards of the American Society of Anesthesiologists. Temperature monitoring is also performed except in relatively short cases (i.e., <30 minutes duration) where hypothermia or hyperthermia is unlikely, and fluid and blood loss is minimal (e.g., myringotomy and tube insertion). Train-of-four (TOF) nerve monitoring should be used when a neuromuscular blocker is administered. In the following sections, we will review in more detail the use of monitors in pediatric patients.
Pulse Oximetry
Pulse oximetry estimates the oxyhemoglobin saturation by measuring the absorption of two different wavelengths of light, which is dependent on the amount of blood in the tissue and the relative amounts of oxygenated and deoxygenated hemoglobin. The oxygen saturation is then computed by comparing the light absorption ratio to a standard curve of arterial saturations that was determined by experiments on volunteers with arterial blood sampling.
The clinical usefulness of pulse oximetry is to alert the anesthesiologist to impending or actual hypoxemia before the onset of visible cyanosis. However, during rapid decreases in oxygen saturation, the oximetry value usually lags behind the true oxyhemoglobin saturation, such that the recognition of hypoxemia may be delayed. The position of the pulse oximeter probe will affect the rapidity of these changes. Central probe locations (e.g., buccal) will have less of a delay than an upper extremity, which will have less of a delay than a lower extremity. Conversely, the reestablishment of normoxemia may be associated with a persistent low pulse oximetry value for 30 seconds or more.
The precision and accuracy of pulse oximetry measurements in children can be a challenge because of factors such as movement artifact, poor tissue perfusion, low temperature, abnormal hemoglobin, tissue pigmentation, probe site and artificial light. Most oximetry devices have a precision of ±2% when the true oxyhemoglobin saturation is greater than 90%. In the range of oxygen desaturation below 80%, pulse oximetry tends to overestimate the true saturation compared with arterial sampling. The decrement in pulse oximeter precision at lower saturation values differs between manufacturers. Fetal hemoglobin, which is found in neonates and young infants, does not affect the accuracy of the pulse oximeter.
There are no outcome studies that demonstrate a proven benefit from the use of pulse oximetry. However, a single-blinded study (anesthesiologists were not shown the value on the pulse oximeter that was monitored by the investigators) showed that the absence of pulse oximetry resulted in three times as many episodes of hypoxemia (defined as less than 85% saturation) and delayed recognition of hypoxemia compared with having access to the pulse oximeter values. In that study, most of the hypoxemic episodes occurred in children younger than 2 years of age.
Alhough pulse oximetry is associated with a relatively high rate of false positive alarms (low saturation readings that are not accurate), all alarms must be taken seriously until proven otherwise. When the oximeter displays a low saturation value, the anesthesiologist should immediately turn attention to the adequacy of ventilation by simultaneously evaluating air entry, quality of the capnograph tracing (see below), and the quality of the pulse oximeter signal. Because surgical personnel may unknowingly compress the oximeter probe by leaning on the patient’s foot, we prefer to put the oximeter probe on the upper extremity, which can be positioned away from the surgical field. The audible tone on the oximeter should never be turned off or drowned out by loud music, since many anesthesia practitioners listen continuously to the device and respond to a decrease in pitch.
Another use of pulse oximetry is for determination of the heart rate value. The pulse oximeter-derived heart rate can be helpful in certain situations that cause artifact of the electrocardiogram (e.g., electrocautery). Furthermore, concordance between the heart rates on the ECG and the pulse oximeter is one method of verifying that the device is providing accurate saturation information.
The oxygen reserve index (ORi) is a new technology that may prove useful in detecting hypoxemia before it occurs. ORi is a pulse oximeter-based dimensionless index that ranges from 0 to 1 as Pa o 2 increases from approximately 80 to 200 mm Hg. A pilot study in 25 healthy children showed ORi detected impending desaturation a median of 31.5 sec before noticeable changes in SpO 2 occurred.
Capnography
Infrared devices that detect exhaled carbon dioxide have been used since the 1970s. They not only measure the P ET CO 2 concentration (capnometry), but also display it graphically (capnography). Before 1998, capnography was considered an American Society of Anesthesiologists (ASA) standard monitor for confirming the correct placement and continuous presence of an endotracheal tube. These standards have been since amended and now indicate that capnography should be used to confirm adequate ventilation during the conduct of general anesthesia without an endotracheal tube (e.g., during laryngeal mask airway, face mask, or natural airway anesthesia). Specifically, these guidelines now state:
“Continual monitoring for the presence of expired carbon dioxide shall be performed unless invalidated by the nature of the patient, procedure, or equipment.… Continual end-tidal carbon dioxide analysis, in use from the time of endotracheal tube/laryngeal mask placement, until extubation/removal or initiating transfer to a postoperative care location, shall be performed using a quantitative method such as capnography, capnometry, or mass spectroscopy.”
As in adults, capnography in pediatric patients is used to confirm correct placement of the endotracheal tube and to continuously assess the adequacy of alveolar ventilation. Capnography also provides information about respiratory rate, breathing pattern, airway patency, effectiveness of cardiopulmonary resuscitation, amount of right-to-left intracardiac shunt, change in pulmonary blood flow, and indirectly, the degree of neuromuscular blockade.
The amount of CO 2 in expired gas is a function of the amount produced by the tissues, the alveolar ventilation and the pulmonary blood flow. In pediatric patients, an abnormal increase in P ET CO 2 most commonly signifies hypoventilation but may also indicate administration of sodium bicarbonate, release of a tourniquet, abdominal insufflation with CO 2 , or increased CO 2 production that occurs with temperature elevation or as an early sign of malignant hyperthermia. Conversely, an abnormally low P ET CO 2 may indicate hyperventilation, hypothermia, increased dead space, or a state of low pulmonary perfusion caused by hypotension, venous air embolism, or failure of a systemic to pulmonary artery shunt. An abrupt fall in P ET CO 2 can be caused by compression of the trachea and endotracheal tube during intrathoracic surgery or placement of a transesophageal echocardiography probe. Sudden absence of the capnograph tracing can indicate a breathing circuit disconnection, occlusion of the gas sampling line, accidental endotracheal placement of an orogastric tube, or complete loss of cardiac output, while the abnormal presence of inspired CO 2 signifies the presence of a faulty unidirectional valve, exhausted CO 2 absorbent, or when a semiopen circuit is being used, rebreathing secondary to insufficient fresh gas flow. Capnography and capnometry can be used to measure the efficacy of cardiopulmonary resuscitation (i.e., cardiac output generated by chest compressions).
Capnography use in infants and small children (<12 kg) often underestimates the true P ET CO 2 value because of the relatively large ratio of dead space to tidal volume. The closer the sampling port is to the endotracheal tube, the more accurate the P ET CO 2 .
Although mainstream capnography may provide the most accurate P ET CO 2 measurement, it adds bulk and dead space to the circuit and may correlate poorly to blood gas values in small infants. Therefore, side-stream capnography is most often employed for pediatric patients. Disadvantages of side-stream capnography in pediatric patients include kinking of the sampling line, slow response time, and with some devices, a relatively large sampling volume. Innovations in capnography (“Microstream technology,” Covidien, Minneapolis, MN) have enabled a sampling rate as low as 50 mL/min (as opposed to the conventional 150–250 mL/min) without compromising waveform integrity or accuracy of P ET CO 2 .
Disposable colorimetric CO 2 connectors are commonly used in emergency situations to verify correct endotracheal tube placement. The colorimetric connector changes color from purple to tan to yellow to indicate the presence of carbon dioxide. In one study of pediatric resuscitations, the colorimetric detector was used during cardiopulmonary resuscitation; only patients with a yellow reading had return of spontaneous circulation and survived to ICU admission.
Small infants often lack an alveolar plateau on the capnograph ( Fig. 15.1 ). This can result from a higher respiratory rate, an excessively high sampling flow for the volume of CO 2 produced, excessive dead space in the breathing circuit, or a leak around the endotracheal tube. In most cases, switching the capnograph tubing closer to the patient’s airway will reveal a more accurate result ( Figs. 15.2 and 15.3 ).
