Monitoring

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CHAPTER 11 Monitoring

Ronald S. Litman, David E. Cohen, Robert J. Sclabassi, Patrick Callahan, Franklyn P. Cladis and Etsuro K. Motoyama

Monitoring

Most studies that have determined the rate of cardiac arrests resulting from anesthesia have found a threefold to fivefold greater risk among children than adults (Graff et al., 1964; Keenan and Boyan, 1985). In children younger than 1 year, the incidence increases to 9.2 to 17 per 10,000 anesthesias, or 10 times the adult incidence (Olsson and Hallen, 1988; Cohen et al., 1990). Factors contributing to cardiac arrests in anesthetized children are likely to be related to the cardiovascular or respiratory system (Salem et al., 1975). Flick and others (2007) have reviewed cardiac arrest data at the Mayo Clinic. The incidences of cardiac arrest and mortality during noncardiac procedures were 2.9:10,000 and 1.6:10,000, respectively. However, the incidence of cardiac arrest in children undergoing cardiac operations was 127:10,000. The incidence of other serious complications is also greater for infants than for adults in the operating room (Tiret et al., 1988) and in the postanesthesia care unit (PACU) (Cohen et al., 1990). These data indicate that children are a high-risk population and should be monitored with particular attention to cardiovascular and respiratory variables.

Guidelines for the intraoperative monitoring of patients under anesthesia have been published by the American Society of Anesthesiologists (ASA) (2005) (see Box 10-1). These standards mandate the continuous presence of an anesthesiologist or a nurse anesthetist throughout the conduct of anesthesia and require continuous monitoring of oxygenation, electrocardiographic status, and adequacy of ventilation and circulation. The minimum standard for monitoring oxygenation includes an oxygen analyzer in the anesthesia breathing circuit, sufficient illumination to evaluate the patient’s color, and a quantitative method such as pulse oximetry, except under extenuating circumstances. Tracheal intubation must be verified by physical examination and the qualitative detection of carbon dioxide in the exhaled gas. Regardless of whether endotracheal intubation has been performed, continuous capnography is required unless it is invalidated by the nature of the patient, procedure, or equipment. Furthermore, quantitative monitoring of the volume of expired gas is strongly encouraged. The ASA also recommends monitoring of ventilation using observation of chest excursion and the reservoir breathing bag, as well as auscultation of breath sounds. When ventilation is controlled by a mechanical ventilator, there should be in continuous use a device that is capable of detecting disconnection of components of the breathing system, and the device must give an audible signal when its alarm threshold is exceeded.

ASA monitoring standards for circulation mandate that every patient receiving anesthesia have continuous electrocardiography (ECG), and determination of arterial blood pressure and heart rate at least every 5 minutes. In addition, every patient’s circulatory function should be evaluated continually, using at least one of the following methods: palpation of a pulse, auscultation of heart sounds, monitoring of an intraarterial pressure tracing, ultrasound peripheral pulse monitoring, and pulse plethysmography or oximetry. Finally, a method by which temperature can be measured should be readily available, and the patient’s temperature should be monitored when clinically significant changes in body temperature are intended, anticipated, or suspected.

Many of these provisions have been extended to the PACU. In standards adopted by the ASA in 1988 and updated in 2009, PACU monitoring should emphasize oxygenation, ventilation, circulation, and temperature assessment, with specific capability for quantitative determination of systemic oxygenation by pulse oximetry or its equivalent. Equipment should be readily available to enable the practitioner to meet these standards in all pediatric patients. The anesthesiologist is the ultimate monitor.

Physical Examination

Observation

The anesthesiologist can gain a tremendous amount of information from observation alone. Anesthetic depth can be inferred from the rate and pattern of respiration, and airway obstruction can be detected by chest wall retractions or “seesaw” paradoxical motion. The skin and mucous membranes should be continually assessed to confirm adequate oxygenation, because a pulse oximeter reading can significantly lag behind other indices of hypoxemia when placed on an extremity (Reynolds et al., 1993), or it may not detect a pulse at all during intense vasoconstriction. In rare circumstances, pulse oximetry falsely indicates normal saturations during hypoxic conditions (Costarino et al., 1987).

Capillary refill can provide valuable information about the intravascular volume and cardiac output of a euthermic patient. A child with cool, mottled, poorly perfused extremities should be examined closely for additional evidence of hypovolemia or reduced cardiac output even if the systemic arterial pressure remains normal. Progression of this mottled appearance onto the trunk indicates the extreme vasoconstriction that may herald imminent cardiovascular collapse.

Auscultation

Continuous auscultation of heart and lung sounds by means of a precordial stethoscope is useful during all phases of general anesthesia, as well as during transport of the child between hospital locations. A precordial stethoscope allows the anesthesiologist to immediately detect changes in the rate and character of heart and breath sounds, and it often gives the first warning of a physiologic alteration (e.g., right main bronchial intubation, wheezing). Crisp heart tones are produced by the flow of blood through a briskly contracting heart. Myocardial depression initially results in a muffled and then in a distant quality to the heart tones. Careful auscultation may reveal arrhythmias or murmurs such as the mill-wheel murmur that results from a venous air embolus.

When selecting and placing a precordial or an esophageal stethoscope, one should consider the nature of the planned surgery, the proposed anesthetic, and any underlying patient condition that may affect auscultation. Breath sounds and heart tones are best heard when a precordial stethoscope is positioned near the left sternal border between the second and fourth interspaces (above the nipple line). An esophageal stethoscope is reserved for patients whose anesthetic management includes endotracheal intubation and in whom a precordial stethoscope either provides inadequate information or violates the surgical field. The proper method for accurate placement of the esophageal stethoscope is to listen while simultaneously advancing the device and placing it at the level where the heart and lung sounds are maximal. In small infants, unintentional placement of the esophageal stethoscope into the stomach can easily occur.

Esophageal stethoscopes are contraindicated in patients with esophageal atresia or in those who have a disease process involving the proximal portion of the esophagus. They confer a rigid feel to the esophagus, which might be mistaken for the trachea (Schwartz and Downes, 1977). As a result, the esophageal stethoscope is relatively contraindicated in neck dissections where the trachea is a critical landmark, such as a tracheostomy.

Electrocardiography

In pediatric anesthesia, ECG is most useful for tracking the heart rate and diagnosing intraoperative rate-related arrhythmias, of which the two most common are bradycardia and supraventricular tachycardia. ECG is much less prone to movement-related artifact than the original pulse oximeter, although new pulse oximeter devices have eliminated most motion artifacts. In small infants, hypoxemia-related bradycardia may occur before the pulse oximeter reveals oxyhemoglobin desaturation. Conversely, resolution of hypoxemia is heralded by the transition from bradycardia to normal sinus rhythm. Premature ventricular contractions are commonly observed when halothane is used as the general anesthetic agent, especially during periods of hypercapnia or catecholamine release. The precordial stethoscope as a single monitor provides a much better indication of cardiac contractility and thus the overall hemodynamic status.

Electrolyte abnormalities may also be uncovered through the use of ECG. Hyperkalemia produces the characteristically prominent T waves. Hypocalcemia, which may occur during rapid administration of citrated blood products, prolongs the QT interval. Because ischemic changes in normal pediatric patients are rare, and because lead II provides a good view of atrial activity for arrhythmia diagnosis, the latter is recommended for the routine intraoperative electrocardiographic monitoring of pediatric patients.

In children, the normal heart rate varies with age (Table 11-1). The normal heart rate of the newborn ranges from 120 to 160 beats per minute, although lower rates (e.g., 70) are frequently observed during sleep, and higher rates (>200) are common during anxiety or pain. Heart rates tend to decrease with age and in parallel with decreases in oxygen consumption. In addition, many children have a noticeable variation in heart rate with respiration (i.e., sinus arrhythmia).

TABLE 11-1 Normal Resting Heart Rates of Infants and Children

Age	Heart Rate (beats/min)
Age	Mean	Range (±2 SD)
0 to 24 hr	119	94 to 145
1 to 7 days	133	100 to 175
8 to 30 days	163	115 to 190
1 to 3 mo	152	124 to 190
3 to 12 mo	140	111 to 179
1 to 3 yr	126	98 to 163
3 to 5 yr	98	65 to 132
5 to 8 yr	96	70 to 115
8 to 16 yr	77	55 to 105

Modified from Liebman J, Plonsey R, Gillette PC, editors: Pediatric electrocardiography, Baltimore, Md, 1982, Williams & Wilkins.

Systemic Arterial Pressure

Noninvasive Measurement

Blood pressure is easily measured noninvasively in children and small infants using oscillotonometry. In children, oscillometric measurements of systolic arterial pressure (Bruner et al., 1981; Friesen and Lichtor, 1981) and mean arterial pressure (Kimble et al., 1981) usually correlate well with the Riva Rocci mercury column method, as well as with direct arterial pressure measurement, but oscillometric measurements tend to underestimate the diastolic component. During routine uncomplicated cases, measurement of blood pressure should be performed every 3 to 5 minutes while the child is anesthetized—determinations that are too frequent can result in limb ischemia. The blood pressure cuff is most commonly placed on the upper arm but can be placed on the forearm, thigh, or calf. There is inconsistent correlation of measurements obtained between the upper and lower limbs.

The width of the blood pressure cuff should cover approximately two thirds of the total length of the upper arm (or other extremity portion to which it is applied). A cuff that is too small or too narrow incompletely occludes the artery, resulting in the premature return of detectable flow and hence falsely increasing the pressure measurement (Park et al., 1976; Kimble et al., 1981). The error can be as great as 30 mm Hg. A cuff that is too wide can dampen the arterial wave and result in a falsely low pressure, but the magnitude of this error is small (Kimble et al., 1981). Blood pressure increases gradually throughout childhood (Figs. 11-1 and 11-2) and depends on the height of the child: taller children demonstrate a higher blood pressure (Table 11-2). Blood pressure ranges in premature infants have been defined (Table 11-3) and vary depending on the health status of the infant and mother.

FIGURE 11-1 Age-specific percentiles of blood pressure measurements in boys, from birth to 12 months of age. Values for girls are slightly lower.

(From National Heart, Lung, and Blood Institute: Report of the Second Task Force on Blood Pressure Control in Children, Bethesda, Md, 1987, The Institute. Reproduced by permission of Pediatrics 79:1, copyright 1987.)

FIGURE 11-2 Age-specific percentiles for blood pressure measurements in boys, 1 to 13 years of age. Values for girls are slightly lower.

TABLE 11-2 Systemic Arterial Pressure

TABLE 11-3 Blood Pressure Ranges in Healthy Premature Infants*

Direct Measurement

Direct measurement of blood pressure via an arterial catheter is indicated when there is a need for precise beat-to-beat blood pressure monitoring or for frequent determination of arterial blood gas values. This patient population may include children who are expected to develop unstable hemodynamics or those undergoing a surgical procedure that could result in profound hemodynamic alterations related to blood loss (i.e., total loss >50% estimated blood volume [EBV], or acute loss >10% EBV), fluid shifts (i.e., third space losses >50% EBV), deliberate hypotension, or nonpulsatile blood flow (e.g., cardiopulmonary bypass [CPB]). The respiratory indications for direct arterial monitoring include significant abnormalities in gas exchange caused by either preexisting disease or the procedure (e.g., thoracotomy). Rarely, direct arterial monitoring is necessary because of the inability to measure systemic arterial pressure by any indirect technique.

There are no absolute contraindications to placing an arterial catheter, but a risk-benefit analysis should be performed in patients with a hypercoagulable state or bleeding disorder. The radial artery is a favored site for arterial cannulation because the vessel is superficial and easily accessible. Other anatomic sites frequently used are the ulnar, dorsalis pedis, posterior tibial, and femoral arteries. The axillary artery has gained favor because of increased collateral blood flow compared with the brachial or femoral artery (Lawless and Orr, 1989; Cantwell et al., 1990; Greenwald et al., 1990; Piotrowski and Kawczynski, 1995). In general, the brachial artery should be avoided because of the risk for median nerve damage and poor collateral flow around the elbow. Umbilical vessels are an alternative site for cannulating the aorta and inferior vena cava in neonates. In determining a site, one needs to consider the history of that vessel (i.e., whether it has been cannulated before), its collateral flow, the experience of the person inserting the catheter, and special physiologic issues (e.g., whether it arises on an aortic root proximal to the ductus arteriosus) or surgical issues (e.g., whether it arises from a vessel likely to be clamped or sacrificed during the procedure). Cannulation of vessels with good collateral flow, such as the arch vessels of the wrist or foot, may reduce the risk for ischemic tissue damage distal to the catheter.

As the largest superficial vessel, the femoral artery can be cannulated most predictably in situations when intense peripheral vasoconstriction may accompany low cardiac output and blood pressure. In less dire circumstances, the selection of a vessel may reflect a variety of anatomic and physiologic characteristics exhibited by certain vessels. The pedal vessels exhibit pressure wave amplification that results in pressure determinations exceeding aortic values by as much as 30% (Park et al., 1983).

After palpation and localization of the artery with the nondominant hand, one can cannulate the selected artery either by inserting the catheter directly into the artery using a catheter-over-needle device or by using the Seldinger technique. The Seldinger technique involves entering the vessel with a needle, placing a guidewire through the needle after the vessel is entered, removing the needle, and then placing the catheter over the wire into the vessel. A 22-gauge catheter is appropriate for peripheral artery cannulation in infants and children younger than 5 years, whereas a 20-gauge catheter may be substituted in older children. Aseptic technique should always be followed when placing an arterial line. When cannulating a peripheral artery, it is helpful to immobilize the extremity with a board.

A Doppler flow transducer is occasionally useful to locate an artery that is difficult to palpate. Surgical cutdown may be the preferred option when percutaneous placement is likely to be difficult or has failed. Indwelling arterial catheters are associated with several possible complications. Proximal emboli, distal ischemia, arterial thrombosis, and infection are common to all sites. Thrombosis of the radial artery is generally temporary, although it is more likely to persist after a cutdown (Miyasaka et al., 1976). Although small flush volumes (0.3 mL) in radial arterial catheters can be detected in the aortic arch vessels, cerebral infarcts have not been reported (Edmonds et al., 1980). The tip of an umbilical artery catheter should be placed in either a high (above the diaphragm) or a low (below L3) position to avoid direct flushing into the renal arteries. Despite these precautions, as many as 10% of neonates exhibit hypertension as a late complication attributed to umbilical artery catheterization (Bauer et al., 1975; Plumer et al., 1976; Horgan et al., 1987). Minor complications of umbilical artery monitoring include vasospasm of the lower extremity vessels, which are more common with low tip placement. Major complications (e.g., necrotizing enterocolitis, renal artery thrombosis) occur independent of location (Mokrohisky et al., 1978; Umbilical Artery Catheter Trial Study Group, 1992). The rarity of clinical complications is remarkable given that the incidence of aortic thrombosis on removal of umbilical artery catheters approaches 95% in some series (Neal et al., 1972), although most series define the incidence at 12% to 31% of neonates (Symansky and Fox, 1972; Horgan et al., 1987; Seibert et al., 1987).

Systolic Pressure Variation

Systolic pressure variation is a noninvasive way to determine volume status and fluid responsiveness. It is defined as the difference between the maximal and minimal values of systolic blood pressure during a positive pressure breath. Initially during a positive pressure breath, there is a transient increase in systolic blood pressure (delta up) followed within four or five beats by a decrease in systolic blood pressure (delta down). Increases in intrathoracic pressure during positive pressure ventilation cause a decrease in systolic blood pressure because of decreased preload to the right ventricle, increased afterload to the right ventricle, and decreased afterload to the left ventricle. This decrease is greater during hypovolemia. Systolic pressure changes in response to respiratory variation have been used to determine hypovolemia (Greilich and Johnston, 2007).

The difference between the maximal systolic blood pressure and minimal systolic blood pressure during a respiratory cycle can help predict volume status. In fact, when this value is divided by the mean of the two systolic pressure values, it provides a percentage of respiratory change in arterial pulse pressure. The equation for this calculation is as follows:

where ΔPP% is respiratory change in pulse pressure (mm Hg).

Michard and others (1999) demonstrated a strong relationship in adult ventilated patients between pulse pressure changes and cardiac output. Patients with pulse pressure changes (ΔPP) that are greater than 10% may be fluid responsive and benefit from the administration of intravenous fluids.

Changes in the pulse oximetry waveform (plethysmographic waveform amplitude) have been shown to predict fluid responsiveness (Pizov et al., 2010). Bedside use of this variable is challenging. The “pleth variability index” (Masimo Corp., Irvine, CA) automatically calculates the waveform amplitude variation and may predict fluid responsiveness noninvasively (Cannesson et al., 2008a, 2008b).

Central Venous Pressure

There are four relative indications for central venous catheterization: inadequate peripheral venous access, central venous pressure monitoring, infusion of hyperosmolar or sclerosing substances, and a planned operative procedure with a high risk for hemodynamically significant venous air embolism. There is no absolute indication for central venous pressure monitoring in pediatrics. Unlike direct systemic arterial pressures, central venous pressure itself rarely provides the sole basis for therapeutic action. It does, however, provide useful information that, taken together with other data, help to form a management plan. The procedures for which this monitoring deserves consideration include large estimated blood loss or fluid shifts (>50% EBV), deliberate hypotension, cardiac surgery with CPB, situations in which the usual signs of hypovolemia are likely to be misleading (e.g., renal failure, congestive heart failure), and procedures with expected moderate blood loss or fluid shifts. The normal values for central venous pressure in children are similar to those in adults (mean, 2 to 6 mm Hg).

Every insertion site that has been used in adults can be used in children. Access to the central circulation can be achieved from the internal and external jugular, subclavian, basilar, umbilical, and femoral veins. The site selected depends on the experience of the operator and the indication for the catheter. If venous access is the only requirement, one might elect to use visible veins (e.g., basilar, external jugular) or those with a lower risk for complications (e.g., femoral). Situations that require true intrathoracic central venous placement also require placement of the catheter into the internal jugular vein or subclavian vein. The umbilical vein can be used in neonates for volume resuscitation, but the high frequency with which these catheters enter the branch portal veins introduces a significant risk for permanent liver injury if sclerosing or hyperosmolar solutions are infused. Because a catheter tip can erode through the wall of the right atrium, care must be taken to avoid intracardiac tip placement. The catheter should be advanced only until the orifice lies in the intrathoracic great vessels, and its position should be confirmed radiographically.

Catheters of various sizes (2.5 to 10 French), lengths, and composition are available for pediatric applications (Cook Critical Care, Bloomington, IN, and other companies). Selection is based on the size of the patient (Andropoulos et al., 2001) and the purpose of the catheter. The composition of the catheter depends on its intended use. Teflon is fairly resistant to thrombus formation, but concerns about perforation by catheters have prompted the development of softer catheter materials, especially for long-term use (e.g., Silastic and polyurethane). The catheters are generally inserted via the Seldinger technique, using landmarks that are similar to those used in adults.

There are no absolute contraindications to placing a central venous catheter, but each site has potential risks. All sites share the common complications of infection (site cellulitis, bacteremia), venous thrombosis with potential emboli, air embolism, catheter malfunction (occlusion, dislodgment, or fractures), dysrhythmias (when the catheter tip is in the heart), and bleeding. Universal precautions and sterile technique should be used when placing a central venous catheter. The risks involved in cannulating the internal jugular vein include carotid artery puncture, Horner’s syndrome, pneumothorax, and injury to the thoracic duct when the left internal jugular vein is cannulated. The high approach to the internal jugular vein, at the midpoint of the sternocleidomastoid muscle, results in comparable success with fewer complications than lower approaches (Coté et al., 1979). Two-dimensional ultrasound scanning improves localization of the internal jugular vein and increases the success rate of central venous cannulation in adults and children (Verghese et al., 2002; Hind et al., 2003). Using this device, Alderson and others (1993) reported an 18% prevalence of anatomic variations in children younger than 6 years that would preclude or significantly hinder the successful cannulation of the internal jugular vein using anatomic landmarks alone. In addition, Hong and colleagues (2010) reported that rotating the head away from the neutral position increases the degree of carotid artery and internal jugular vein overlap, and decreases the incidence of lateral positioning of the internal jugular vein to the carotid artery.

Mixed Venous Oxygenation and Monitoring

Mixed venous oxygenation is defined as the oxygenation saturation or content of venous blood from the pulmonary artery. The collection of a mixed venous sample requires a pulmonary artery catheter. The Fick equation for mixed venous oxygen saturation (Svo₂) depends on four variables: oxygen consumption (), cardiac output, hemoglobin, and arterial oxygen saturation.

Normal values for mixed venous saturation range from 65% to 75%. A decrease in Svo₂ occurs because of increased oxygen consumption (stress, pain, hyperthermia, shivering) or decreased oxygen delivery (anemia, decreased cardiac output, decreased Pao₂, decreased Sao₂). An increase in Svo₂ occurs because of a decrease in (hypothermia, anesthesia) or an increase in oxygen delivery (increased hemoglobin, increased cardiac output, increased Pao₂, increased Sao₂). Mixed venous oxygenation is used to assess the balance between oxygen delivery and oxygen consumption for patients in the operating room and intensive care unit. It is a global index of tissue oxygenation.

A significant limitation of mixed venous saturation is that it requires the collection of blood from the pulmonary artery. This is a particular limitation in the smaller pediatric patients. Blood from the superior vena cava (central venous saturation [Scvo₂]) and right atrium have been investigated as possible surrogate markers for mixed venous oxygen saturation, with mixed results.

Dueck and coworkers (2005) found a significant variation between central venous saturations and mixed venous saturations in the same patient, so individual values of Scvo₂ cannot be substituted for Svo₂ values. However, although the absolute values do not correlate, there is a correlation between the trends in Scvo₂ values and in Svo₂ values. Perez and colleagues (2009), in a retrospective pediatric study, identified a correlation between right atrial and mixed venous oxygen saturations. Scvo₂ is used clinically in pediatric patients. Continuous monitoring of Scvo₂ can be performed in neonates, infants, and children.

In infants, continuous monitoring of Scvo₂ can be achieved with a fiberoptic probe. One type of fiberoptic probe is designed as a percutaneous catheter from CeVOX (Pulsion Medical Systems AG, Munich, Germany). This 2-F probe is 31 cm in length and measures central venous oxygen saturations using spectrophotometry. Muller and coworkers (2007) described placing the catheter percutaneously through a 16-gauge single-lumen catheter in the femoral or subclavian vein. There were only three patients in this study, so accurate correlation with central venous blood samples cannot be determined.

The Pediasat system (Edwards Life Sciences) has been described in infants and children having orthopedic, craniofacial, and cardiac surgery. It comes in four sizes (4.5 F, 5 cm; 4.5 F, 8 cm; 5.5 F, 8 cm; and 5.5 F, 15 cm) and provides continuous readings of central venous oxygen saturation. Liakapolous and colleagues (2007) and Ranucci and colleagues (2008) demonstrated good correlation between Scvo₂ values from the Pediasat system when compared with co-oximetry values obtained from blood samples drawn from the distal port.

Artifact may limit the clinical usefulness of Scvo₂ catheters for intraoperative care. Manipulation of the catheter by the surgeon when the chest is open or direct light from the surgeon’s headlight may alter the readings of the catheter.

Pulmonary Artery Catheters

Since its introduction in 1970, indications for the use of the flow-directed balloon-tipped pulmonary artery (Swan-Ganz) catheter in pediatric patients have been slow to evolve. Although the validity and value of the data that these catheters generate remain controversial in pediatrics, the technical difficulties and complications associated with their use are significant. Pulmonary artery pressure measurement can help guide therapy in children with elevated or volatile pulmonary vascular resistance, but the interpretation of the flow data they generate is hindered by several factors. First, the desired cardiac output varies according to age, disease state, and other elements of management that alter metabolic demand in complex ways, thereby introducing significant uncertainty in assigning a target value. Second, the prevalence of intracardiac communications that permit shunting of blood causes discrepancies in pulmonary and systemic blood flow that may vary continuously and are difficult to quantify. Finally, despite several studies demonstrating reasonable accuracy when thermodilution is compared with other methods of flow determination, such as the Fick equation (Freed and Keane, 1978) and dye dilution (Colgan and Stewart, 1977), the precision of these determinations in small infants is low and has a 25% intersample variability. In patients with congenital heart malformations, for example, measurement errors are introduced by shunting and complex anatomy, and the risks of improper placement of the flow-directed pulmonary artery catheter are increased. Alternatively, directly placed pulmonary artery catheters can provide the necessary information regarding pulmonary vascular resistance and residual left-to-right shunts, and left atrial catheters reflect filling and diastolic function of the left ventricle after cardiac surgery.

In some situations, pulmonary artery catheters provide useful information. In children who have severe coexisting pulmonary and circulatory failure, pulmonary artery catheters can help to quantify the hemodynamic impact of extreme respiratory support measures and guide complex fluid and pharmacologic regimens. They may also be useful in patients with underlying pulmonary hypertension or poorly compensated left ventricular dysfunction who undergo acute surgical stress (e.g., arteriovenous malformation clipping or aortic cross-clamping). Given the uncertainty about optimal systemic flow in a given child, mixed venous oxygen saturation may serve as a better indication of global perfusion. In the absence of left-to-right shunts, this sample is best obtained from the pulmonary artery.

Pulmonary artery catheters can be difficult to insert, especially in infants or in children with low cardiac output. They may be placed in any vein used for access to the central venous system, but the most reliable veins are the right internal jugular and the femoral. In infants and children smaller than 15 kg, it is technically difficult to place an introducer sheath in the neck vessels; the femoral veins are preferable. Multilumen catheters capable of thermodilution are available in two sizes, 5 and 7 F, with four options for the right atrium–to–pulmonary artery interluminal distance. Catheter recommendations are based on age (Table 11-4). The proper placement of these catheters can take a long time, and thus the assistance of fluoroscopy is recommended for infants and children less than 30 kg and for larger children who have a low cardiac output.

TABLE 11-4 Guidelines for Multilumen Pulmonary Artery Catheters for Infants and Children

Age (yr)	Catheter Size (F)	CVP to Pulmonary Artery Port Distance (cm)
Newborn to 3	5	10
3 to 8	5	15
8 to 14	7	20
>14	7	30

The risks of balloon-tipped pulmonary artery catheters are numerous and include the risks of central venous catheter placement discussed previously, as well as the complications seen in adult patients with pulmonary artery catheters: infection, air emboli, thrombus, pulmonary artery rupture, acute right bundle branch block, and intracardiac knots. Other complications are more common with children: misleading information, paradoxical systemic emboli, disruption of an intracardiac repair, and high-grade right ventricular outflow tract obstruction because of the relatively large balloon diameter. The presence of intracardiac and extracardiac malformations may result in an aberrant catheter course, leading to incorrect data and an increased risk for systemic emboli.

Cardiac output can be estimated in children through indicator dilution (e.g., thermodilution or dye dilution) and noninvasive techniques. Doppler determinations of aortic blood velocity can be used to quantify systemic flow if the angle of the incident ultrasound beam and the cross-sectional area of the aorta are reliably determined (Alverson et al., 1982). Transthoracic and transesophageal evaluations of Doppler cardiac output in children have proved to be less promising (Notterman et al., 1989; Muhiudeen et al., 1991). Thoracic bioimpedance, a method that estimates stroke volume on the basis of changes in thoracic impedance, has been applied to children as small as 3.6 kg. Although some correlation exists between bioimpedance and indicator dilution methods, reproducibility is poor (O’Connell et al., 1991). Further details and the complexities encountered in the measurement of cardiac output in children are beyond the scope of this chapter but have been reviewed previously (Tibby and Murdoch, 2002).

A noninvasive cardiac output monitor has been developed that determines cardiac output via the Fick principle for rebreathed CO₂ (Respironics; Novametrix Medical Systems Inc., Wallingford, CT) (Capek and Roy, 1988). The noninvasive cardiac output monitor has been clinically validated in adults and is approved by the U.S. Food and Drug Administration (FDA) for use, but it requires tidal volumes of 200 mL or greater (Guzzi et al., 2003; Watt et al., 2004).

Transesophageal Echocardiography

The value of transesophageal echocardiography (TEE) for monitoring hemodynamics and to evaluate preoperative and postoperative cardiac anatomy has been appreciated from the time of its introduction to the operating room. In the infancy of TEE in the late 1970s, an M-mode transducer was passed into the esophagus, plotting the distance of structures from the transducer on the y-axis and time on the x-axis (Frazin et al., 1976). Although they provided valuable information, M-mode images were too limited alone. In 1982, Schlüter and colleagues (1982) described their experience using a transducer capable of two-dimensional images mounted on a gastroscope. The usefulness of the images obtained was readily apparent, and since that time technology has catapulted the field of TEE to the forefront of cardiovascular monitoring. Now, multiple companies (Phillips, Acuson/Siemens, General Electric) manufacture advanced TEE-specific probes capable of two- and three-dimensional imaging on top of the original M-mode. Doppler has also been incorporated, providing the examiner the ability to extrapolate vast amounts of information from their patients.

The probes use ultrasound waves generated by the vibration of piezoelectric crystals in the tip of the transducer. Theses waves have frequencies between 2.5 and 7.5 MHz. The wave’s ability to travel through tissues depends on the specific density of the tissues. Differences in impedance cause some of the energy generated by the crystals to be reflected and allow some to continue on to the next tissue plane until all the energy has dissipated. If the impedance is too great, as is the case with bone, all of the energy will be reflected. Images are then generated on the basis of the time from the impulse and the remaining energy found in the waves as they return to the transducer.

The quality of the image depends on a number of factors. The closer structures are to the transducer, the greater is the intensity of the image. Transducers with higher frequencies have less penetration than lower-frequency transducers; thus, if one is trying to visualize structures far away, a lower frequency provides better imaging. Images can also be adjusted with gain, so that unwanted echoes in the near field may be dampened and echoes in the far field enhanced.

Doppler technology uses low-intensity ultrasound reflected from moving columns of blood. The frequency of the returning echo is analyzed and estimations of direction and velocity are made. Accuracy of the estimation depends on the direction of flow relative to the direction of the beam. Parallel orientation provides the most accurate values, and increases in the angle of measurement falsely lower the predicted velocities. Measurements taken at angles greater than 20 degrees have a significant amount of error. Both pulsed-wave Doppler, if one is interested in determining velocity at a specific location, and continuous wave, to identify the highest velocity across an entire line of sight, are available for use during a TEE examination.

A modification of the Bernoulli equation allows an examiner to estimate pressure gradients using the measured velocities. Simplified, the change in pressure between two points is equal to four times the maximum velocity squared (Holen et al., 1977; Hatle et al., 1978). Using this principle, the stenosis of valves, severity of aortic coarctation, obstruction caused by muscle bundles or membranes, and a multitude of other clinical questions can be answered.

TEE has played a vital role in improving outcomes in pediatric patients with congenital heart disease. Typically, examinations are performed preoperatively to confirm anatomy, and postoperatively to assess repairs and evaluate function. One study looking at 865 consecutive examinations demonstrated alterations to surgical plans based on preoperative TEE examinations in 2% of patients. The same study found that 12% of patients had post-bypass examinations that led to surgical interventions. Many of these interventions saved patients from unnecessary revision operations and their associated morbidity. In addition to surgical interventions, medical management with drugs and fluids was affected in 20% of the examined patients (Bettex et al., 2003).

An earlier report found that pediatric patients leaving the operating room without residual defects seen on the echocardiographic examination had a risk for reoperation of 3%, versus 42% for those who were found to have residual defects (Ungerleider et al., 1989). This ability to detect problems early provides a significantly improved outcome and subsequently reduces costs, as the need for repeat operations is reduced (Randolph et al., 2002).

As evidence supporting the value of TEE in cardiac surgery has increased, recommendations for proper performance of the examination have matured. The American Society of Echocardiographers and the Society of Cardiovascular Anesthesiologists have developed guidelines for adult intraoperative TEE examinations to encourage complete examinations with standardized views and nomenclature so as to improve communication between various care providers (Shanewise et al., 1999). Discussion of the complete examinations is beyond the scope of this chapter, but some of the probe locations and angles are useful for obtaining basic information regarding a patient’s condition (Table 11-5). Similar guidelines have also been published to assist clinicians with the pediatric examination (Lai et al., 2006).

TABLE 11-5 Transesophageal Echocardiography Cross Sections

Location	Angle (degrees)	Structures Visualized
Transgastric	0-20	Left and right ventricles, and both atrioventricular valves
	80-100	Two-chamber view of left side
	90-120	Long axis of left side, including left ventricular outflow tract
Midesophageal	0-20	Standard four-chamber view
	30-60	Aortic valve on short axis, coronary arteries
	60-90	Right ventricular inflow and outflow tracts
	80-110	Bicaval view
	120-160	Long-axis view of aortic valve and left ventricular outflow tract
Upper esophageal	0	Aortic arch on long axis
Upper esophageal	90	Aortic arch on short axis

Modified from Shanewise JS, Cheung AT, Aronson S et al: ASE/SCA guidelines for performing a comprehensive intraoperative multiplane transesophageal echocardiography examination: recommendations of the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologists Task Force for Certification in Perioperative Transesophageal Echocardiography, J Am Soc Echocardiogr 12:884, 1999.

Although the benefits have been demonstrated, providers must also be aware of the complications associated with the TEE examination. Problems that may be encountered include damage to the oral cavity, esophagus, or stomach; compromised ventilation; inadvertent extubation; right main stem advancement; vascular compression; and arrhythmias. In light of these serious but rare events, caution should precede placement of the transesophageal probe (Stevenson, 1999). Contraindications to the TEE examination include an unrepaired tracheoesophageal fistula, esophageal web, and recent esophageal or gastric surgery. Failure to ascertain a history of such events may lead to significant damage or even perforation of the esophagus.

Another issue encountered in the pediatric population is the size of the patient. Adult-sized probes are generally appropriate for patients older than 8 years or heavier than 20 kg. Pediatric probes are available for patients less than 20 kg. Most clinicians refrain from placing transesophageal probes in patients weighing less than 3 kg. If echocardiographic images are important to a particular surgery and TEE is contraindicated or impossible, an epicardial probe can be passed to the operative field via a sterile sleeve to obtain necessary imaging.

Temperature

Temperature monitoring is vital during pediatric anesthesia, as children may exhibit hypothermia or hyperthermia, both of which can have profound physiologic consequences (see Chapter 6, Thermoregulation).

Urine Output

Urine output often reflects intravascular volume status and cardiac output. Proper assessment of urine output requires recognition of the physiologic mechanisms that exert an affect on urine flow in children. During the first week of life, the glomerular filtration rate and renal plasma flow are only 25% of normal adult values (Arant, 1978). The neonatal kidney is limited in its ability to concentrate the urine (Simpson and Stephenson, 1993). By the end of the first week of life, the kidneys begin to reach absorption thresholds for sodium and glucose that approach adult levels.

Normal newborns produce between 0.5 and 4 mL urine/kg per hour in the first 3 hours of life (Strauss et al., 1981). Urine flow, which initially ranges from 15 to 60 mL/kg per day, reaches as much as 120 mL/kg per day by the end of the first week of life, with 90% of neonates producing 0.5 to 5 mL/kg per hour (Douglas, 1972; Guignard, 1982). In the neonate who is less than 1 week old, urine flow alone is not a sensitive index of changes in cardiac output or intravascular volume. The limited capacity of the neonatal kidneys to compensate for diminished or excessive intravascular volume demands more precise management of blood and fluid replacement in these infants. Beyond the neonatal period, a urine flow of 0.5 to 1 mL/kg per hour usually indicates adequate renal perfusion and function.

Intraoperative monitoring of urine output is indicated in procedures in which large shifts in fluid, blood, or hemodynamics are anticipated, including blood loss greater than 20% of the EBV, third-space replacement exceeding 50% of the EBV, CPB, neurosurgery, deliberate hypotension, planned use of diuretics, or planned hemodilution. Silastic Foley catheters are available in sizes small enough (6 F) for full-term neonates. Alternatively, a small feeding tube can be used in premature infants and in those with a small urethra. In infants, urinary bladder catheters should be connected to a urinometer capable of measuring small volumes, or to a vented 10- to 20-mL syringe.