Chapter 86

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1450 PART 5 ■ Anesthetic, Surgical, and Interventional Procedures: Considerations Figure 86-9. The site of mixing of fresh gas flow and expired gas is always proximal to the endotracheal tube connector. At an expired gas flow of 500 mL/min-1, the influence of fresh gas flow rate on the site of mixing and the degree of dilution is significantly greater. ANESTHETIC GAS ANALYSIS: Most available monitors aspirate gas volumes of approximately 150-200 mL/min which is con siderable if compared to the tidal volume of the neonatal child. The gas is also most frequently sampled close to the Y-piece of the ventilatory tubing. This invariably causes mixing of inspired and expired gases and also distorts the end-tidal CO2 tracing. The values derived in this setting are, thus, not accurate but trend changes are still detectable. INTRA-ARTERIAL BLOOD PRESSURE MONITORING: Maintenance of functional arterial lines throughout the operation is not an easy task during neonatal surgery. Pre-existing arterial lines must be checked before draping or a new line should be estab lished. The wrist should be splinted when using a radial approach. The insertion site should be protected from electrical interference (electrical cautery). Extension lines should be long enough when a femoral approach is planned, otherwise there is a risk that the catheter kinks or becomes dislodged (small gauge central venous lines are good options in this setting). The arterial line must be continuously flushed with a dilute heparin solution (1 U/mL) to prevent clotting. If flushing the system is necessary, it must be done over a short period of time to limit the injected volume; the distance from the wrist to the aortic arc is short and even small saline boluses may cause systemic and cerebral embolism. NEAR-INFRARED SPECTROSCOPY: This relatively new method to continuously assess cerebral oxygenation, displayed as cerebral tissue oxygenation index (cTOI), has gained widespread use in pediatric cardiac anesthesia and is able to give a fast but unspecific warning of cerebral ischemia. A sensor, similar in size compared to the bispectral (BIS)-monitor, is put on one or both sides of the forehead of the child (unilateral monitoring is usually sufficient in most instances) and is then attached to a relatively small moni tor. 104 The cTOI value has also been shown to correlated to central venous oxygenation (SvO 2 ) in neonates and infants. 105 This new modality of monitoring will definitely gain widespread use both in adults and children and will in the future most likely be considered standard of care for all cases of major surgery in the neonate. ANESTHETIC DEPTH MONITORING: BISPECTRAL INDEX MONITORING (BIS), SPECTRAL ENTROPY AND AUDITORY EVOKED POTENTIALS (AEP): This electroencephalography (EEG)-derived monitoring device has gain popularity in adult anesthesia as an indicator of anesthetic depth and safeguard against awareness. However, there is widespread skepticism with regards to the fact that this monitor in fact has anything to do with anesthetic depth, although there is a clear covariation between the BIS value and anesthetic depth in adults. Due to the differences in EEG patterns between adults and neonates and small children, the BIS monitor cannot be recom mended in these age groups. 106 A different concept of attempting to assess anesthetic depth is the so-called Spectral Entropy that is based on the EEG power spectrum. Although this method appears comparable to BIS in adults and older children spectral entropy has not been properly validated in neonates and small infants. 107 A third monitoring system in this category is Auditory Evoked Potentials (AEP). Despite measuring the response to an active auditory stimulus this method does not appear to have any advantage over the other two systems described above. 108 Thus, at present the various option for anesthetic depth monitoring that are widely used in adults cannot be recommended in the context of neonatal anesthesia. ANESTHETIC MANAGEMENT Pharmacokinetic Data for Commonly Used Drugs Because of the lack of interest and incentive for drug companies to register drugs for use in neonates and small infants, there has been a substantial knowledge gap regarding fundamental pharmaco - kinetic data for a large number of analgesic and anaesthetic drugs that are frequently used in the context of neonatal anaesthesia. However, a significant scientific effort from various research groups during the last few years have provided valuable informa - tion regarding pharmacokinetic data in this field, often using the approach of population pharmacokinetics, nonlinear mixedeffects modelling (NONMEM) and allometric scaling ( 3 / 4 power modeling). The provision of the new data provide a basis for more appropriate dosing as well as minimising the risk for accumulation and undesired side effects. A summary of pharmacokinetic data are given below in Table 86–9. Preoxygenation The median time necessary to achieve an end-tidal oxygen concentration of at least 90% using a tight fitting mask and 6 L/ min oxygen flow is 40 seconds (range: 20–50 sec) and a preoxy - genation period of 60 seconds is, thus, recommended up to 5 years of age. 115 However, even after a 2-minute period of preoxygenation by manual ventilation with 100% oxygen following anesthesia induction and muscle relaxation oxygen, desaturation (SpO 2 90%) occurs after about 80 to 90 seconds of apnea in neonates. 116 Induction Techniques Neonates undergoing surgery usually already have intravenous access with an infusion running. In this setting, intravenous induction obviously is the first option. Pharmacologic properties
CHAPTER 86 ■ Management of the Neonate: Anesthetic Considerations 1451 TABLE 86-9. Pharmacokinetic Values for Some Analgesic and Anesthetic Drugs Commonly Used in Neonates and Infants Drug Clearance Volumes of Distributions Volumes of Distributions PK Model Used/Age Groups Ketorolac 109 Central volume Peripheral volume Two compartment/Infants R()isomer 7.5 mL min-1 1.2 L 0.83 L 6–18 mo S() isomer 45.3 mL min-1 3.2 L 0.22 L Midazolam 110 157 mL min-1 Central volume 3.8 L Peripheral volume Two compartment/ 30.2 L Infants 3–24 mo Morphine 111 71 L h-1 (70 kg)-1 Volume of distribution One compartment/ 136 L (70 kg)-1 0–3 yr Proparacetamol 112 55 L h-1 (70 kg)-1 Central volume Peripheral volume Three compartment/ 24 L (70 kg)-1 30 L (70 kg)-1 Premature babies–14 yr Propofol 113 442 mL min-1 Apparent volume of distribution Three comparment/ (70 kg)-1 at steady state 259 L (70 kg)-1 Neonates Tramadol 114 17.5 L h-1 Central volume 228 L (70 kg)-1 Peripheral volume Two compartment/ (70 kg)-1 0.6 L kg-1 0–6 yr of intravenous agents are fully evaluated in Chapter 14. Thiopental has withstood the test of time and is a good routine induction agent in hemodynamically stable patients. It should, however, be remembered that the terminal half-life of thiopental is very long in premature and newborn babies 117 and can result in prolonged emergence from anesthesia as well as an increased tendency for the development of postoperative apnea. Recently, the pharmaco - kinetic profile of propofol has been outlined in small children and infants 118 ; propofol might prove to be a useful alternative to thiopental also in the neonate. Based on clinical experience larger doses than normally recommended are often required to induce anesthesia with thiopental (4–8 mg/kg) or propofol (3.0–3.5 mg/kg) in neonates compared to adults. In hemodynamically unstable patients ketamine (1.5–3 mg/kg) is the drug of choice for intravenous induction. In patients with a compromised airway or if intravenous access appears difficult, an inhalational induction should be used. The newer volatile agent sevoflurane tends to replace halothane as the induction agent of choice in these situations due to its relative lack of hemodynamic depression. 119 In the setting of inhalational induction with sevoflurane the anesthesiologist should be aware of one unique characteristic of sevoflurane compared to the other volatile anesthetics. Contrary to the other volatile agents, the minimum alveolar concentration (MAC) value of sevoflurane is not reduced in preterm and newborn babies compared to older children (see Chapter 13). Thus, the MAC for sevoflurane in the neonate is approximately 3.2%, which is similar to the MAC values for 1 to 6 months old infants 119 (Figure 86–10). A drawback with sevoflurane is the slightly more pronounced depression of ventilation compared to halothane above 1.4 MAC, 120 but this is not believed to be of clinical importance. 121 Airway Management General Considerations Preservation and protection of a patent airway is fundamental to the practice of neonatal anesthesia. Due to practical problems and the vulnerability of the newborn infant, tracheal intubation should be used liberally to avoid unnecessary ventilatory problems. This is particularly true if the anesthesiologist in charge has a limited experience with neonates. A face mask anesthetic can be consi - dered for very brief procedures. However, even during short procedures ventilation should at least be assisted, since most anesthetics will cause a significant degree of respiratory depression in the neonate. If, despite a good mask seal, the anesthesiologist is experiencing difficulties with keeping a clear airway, the use of an oropharyngeal airway or the application of 3 to 4 cm H 2 O of continuous positive airway pressure are often helpful. If the situation is still not satisfactory at this point, the use of a laryngeal mask airway (LMA) can be used even in very small neonates. 122 Should problems still persist, tracheal intubation must be performed without any further delay. Laryngeal Mask Airway The number 1 size LMA can be used successfully even down to approximately 1.0 kg. 122,123 To have a good LMA fit in such small patients, injection of a muscle relaxant is helpful to allow adequate relaxation of the pharyngeal muscles during the insertion procedure. Due to the relatively large dead space of the LMA often associated with high end-tidal CO 2 concentrations, ventilation should at least intermittently be assisted in order to avoid hyper - capnea and atelectasis formation. To avoid gastric overdistension, Figure 86-10. The mean ( standard deviation) end-tidal concentration of sevoflurane in oxygen for each of the six age groups from neonates to older children up to 12 y of age. The data for MAC at 30 y of age were obtained from reference 9.
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Chapter 86

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