Update on Anesthetic Monitoring Devices
C.A. E. Mosley
INTRAOPERATIVE MONITORING AND SUPPORT
The anesthetist’s goals are to achieve a reasonable surgical plane of anesthesia while preventing anesthetic overdose by making careful anesthetic dose adjustments based on careful assessment of the individual patient’s requirements. These requirements are determined by careful patient monitoring. The anesthetist should monitor several reflexes and the response of the cardiopulmonary system to the dose of anesthetic being delivered. In more traditional veterinary species, these reflexes and the cardiopulmonary responses have been well described for the commonly used anesthetics. In addition, the anesthetist frequently uses ancillary automated monitoring equipment to more accurately assess changes in cardiopulmonary function. The accuracy of much of this equipment has been confirmed by methodical research. Unfortunately the use of ancillary monitoring equipment is relatively rare in reptiles and its accuracy is only beginning to be determined.
REFLEX MONITORING
Reflex monitoring is commonly used to assess the level of surgical anesthesia. Muscle relaxation in general should be complete, response to painful stimuli should be absent, and there should be no spontaneous patient movements at a surgical depth of anesthesia. Interestingly, when reptiles are induced with inhalant anesthetics, muscle relaxation starts at midbody and moves cranially, then caudally so that tail tone is lost last. These features can be used when assessing depth during induction and recovery. Palpebral reflex, corneal reflex, jaw tone, and cloacal sphincter tone should all become increasingly obtunded with increasing depth of anesthesia, but loss of the corneal reflex is suggestive of excessive anesthetic depth.
CARDIOVASCULAR MONITORING AND SUPPORTIVE CARE
Auscultation
Direct auscultation of cardiac function is a simple method of assessing heart rate and rhythm. External auscultation is best performed using a stethoscope with a small pediatric bell. However, interference from scales and the carapace and plastron of chelonians can make auscultation difficult. Dampened gauze can be used between the chest wall and the stethoscope bell to reduce noise from scales. In anesthetized patients, a small esophageal stethoscope works very well for direct auscultation of the heart. The stethoscope tubing should be advance in increments until the point of maximal sound intensity is reached. It is not uncommon for some reptiles to have heart rates of 20 beats per minute. If the esophageal stethoscope is not advanced slowly it is easy to pass the heart and place the stethoscope in the stomach. This may facilitate movement of stomach contents into the esophagus, which may lead to an esophagitis.
Doppler Flow Detection and Blood Pressure
An excellent alternative to direct auscultation is to use Doppler shift technology to detect blood flow in major vessels and the heart itself. There are several types of probes available, flat probes (adult and pediatric) and pencil probes. These probes are easily placed over the heart although the carotid arteries, the coccygeal artery (base of the tail), and femoral arteries can also be used. In chelonians the shell generally precludes use of the heart. Pediatric probes have greater sensitivity in detecting flow in small vessels and are preferred for use in reptiles. In addition to assessing blood flow through vessels, in some reptiles blood pressure can be estimated using a small cuff located proximal to the probe. The probe is generally placed over the coccygeal artery of the tail or a distal vessel in one of the limbs. The accuracy of this technique is highly questionable but the changes in estimated blood pressure may be useful for evaluating trends in blood pressure.
Electrocardiogram
The electrocardiogram (ECG) can be used to assess the electrical activity of the heart in reptiles and provides an assessment of heart rate and rhythm. It should be noted, however, that electrical activity could continue in the heart despite loss of muscular activity, which is known as pulseless electrical activity (PEA) or electromechanical dissociation. Therefore, it is best not to rely solely on an ECG for evaluation of cardiovascular function. The ECG of reptiles has been described and is similar to that of mammals, but with an SV wave preceding the P wave. The ECG leads on most reptiles are positioned similarly to the standard three lead configuration in mammals. However, some modification in lead placement will improve signal strength and ECG quality by increasing wave deflection size. The heart in many lizards is located in the pectoral girdle and the right and left forelimb leads are best placed in the cervical region. In snakes, the active leads are placed two heart-lengths cranial and caudal to the heart. The heart in snakes is located 20 to 25% of body length from the head and can often be identified by direct visualization of ventral scale movement caused by cardiac contractions. In chelonians, the forelimb leads are placed on the skin between the neck and the forelimbs. Stainless-steel suture loops or needles can be placed through the skin and attached to the leads to improve signal strength.
RESPIRATORY MONITORING AND SUPPORTIVE CARE
Direct Visualization
Direct visualization of respiratory movements can be extremely difficult to detect in many reptiles, particularly chelonians and very small species. Chest and body wall excursions, bag movement, and fogging of the endotracheal tube can be misleading and may not always correlate with adequate ventilation.
Ventilation
Reptiles have a much lower oxygen requirement than mammals and hence have developed a respiratory strategy that both meets their metabolic needs and is energetically efficient. Most reptiles are episodic breathers, meaning they take several breaths in series followed by a prolonged pause at the end of inspiration. Presumably this pattern allows the animal to remove carbon dioxide that entered the lungs during the last pause phase and replace it with oxygen to be used during the subsequent pause phase. When reptiles are anesthetized the respiratory drive appears to be dramatically blunted. The reasons for this are not entirely clear but may be related to the inspiration of 100% oxygen and the direct depressant effect of the anesthetics on the respiratory center.
Mechanical ventilation is appropriate for several practical reasons. First, reptiles subjected to gas anesthesia may breathe infrequently or sporadically enough to prevent attainment of a stable anesthetic depth and second, the respiratory depression can predispose the patient to hypoxia and hypercapnia. There are no studies that specifically address the effects of intermittent positive pressure ventilation (IPPV) on pulmonary blood flow and intracardiac shunting. However, if pulmonary vascular resistance is increased during IPPV, this could increase right-to-left intracardiac shunting impacting the uptake of inhalant anesthetics. In spontaneously breathing reptiles pulmonary perfusion and heart rate generally increase with ventilation. It remains unclear how IPPV affects the cardiovascular system and potentially anesthetic uptake in reptiles. Current recommendations for IPPV are 2 to 6 breaths per minute using tidal volumes ranging from 15–40 ml/kg with peak airway pressures less than 10 cm H2O. Manual IPPV is commonly performed but several automated small animal specific ventilators are now available that greatly facilitate accurate and safe ventilation in reptiles. These ventilators are also useful for a wide variety of other nontraditional pets including birds, rodents, rabbits, and ferrets.
Blood Gas and Acid-Base Analysis
Blood gas analysis in reptiles is subject to over-interpretation and misinterpretation. Numerous factors such as species, site of sampling, arterial versus venous blood, inspired oxygen concentration, thermoregulatory status, and the ventilatory status of the patient (spontaneous versus controlled) can all affect interpretation of blood gas values. Reptiles tend to be much more tolerant to alterations in pH, PCO2 and PO2 than mammals and thus normal values for mammals may not be applicable for all reptiles at all times. However, in general normal pH in reptiles tends to be similar to mammals provided comparisons are made at identical temperatures. Most reptiles, however, have body temperatures below that of most mammals and consequently normal pH tends to be higher. PCO2 values tend to be lower as do PO2 values. PO2 values are lower as a result of intracardiac shunting but also as a result of diffusion impairment to oxygen in the lung, intrapulmonary shunting, and ventilation-perfusion mismatching. The value of routine blood gas analysis in reptiles remains questionable.
Pulse Oximetry
Pulse oximetry is a noninvasive method used to assess functional hemoglobin saturation (SpO2). Under normal circumstances, this value correlates closely with arterial hemoglobin saturation (SaO2). Pulse oximetry uses a combination of plethysmography (detects pulsatile blood flow) and spectrophotometry (measures changes in light absorbencies) to determine arterial hemoglobin saturation and both are susceptible to measurement errors, artifact, and technical limitations. Pulse oximetry has been developed and refined to maximize accuracy when used on normal mammals, meaning it works best at normal mammalian heart rates, detects typical mammalian pulsatile blood flow, and is most accurate at hemoglobin saturations seen in mammals (> 90%). In reptiles, the characteristics of these parameters may be markedly different compared with mammals. Although pulse oximetry is a commonly used anesthetic monitor in reptile patients, the readings should be interpreted cautiously. A reflectance probe is most commonly used and placed in the esophagus or cloaca. The probe may need to be manipulated slightly to obtain an appropriate signal. The heart rate reported by the pulse oximeter should correlate with the heart rate determined using direct methods (auscultation). The efficacy of pulse oximetry has been assessed in the green iguana (Iguana iguana). One study reported good correlation between pulse oximetry and blood gas evaluations of hemoglobin saturation while the other found little correlation. The discrepancies between the studies may reflect methodological differences or the inherent deficiencies of pulse oximetry in reptiles. Further evaluation is required to comment on the value, utility, and correlation of pulse oximetry as a respiratory monitor in reptiles.
Capnometry
Capnometry measures the amount of end-tidal (exhaled) carbon dioxide during ventilation. The end-tidal level of carbon dioxide is generally less but is reflective of carbon dioxide in arterial blood and can serve as an indirect noninvasive method of assessing the adequacy ventilation. A more complete picture of carbon dioxide transfer can be obtained from a capnogram, similar to an ECG tracing. A capnogram provides a continuous waveform that reflects the changes in airway carbon dioxide levels throughout inspiration and expiration. In contrast, capnometry simply reports the maximum and minimum carbon dioxide levels associated with expiration and inspiration respectively; similar to the heart rate output from an ECG. Capnography is a useful monitoring tool in mammals with normal lungs. In reptiles, however, capnography has not been evaluated and the presence of right-to-left intracardiac shunts and dead space ventilation associated with the unique structure of many reptilian lungs makes this technology prone to inaccuracies. The end-tidal carbon dioxide will not necessarily reflect arterial carbon dioxide levels and the gradient between arterial and end-tidal carbon dioxide cannot be predicted.
THERMOREGULATION
Reptiles are exothermic animals and derive nearly all their body heat from the external environment. Normal thermoregulation in reptiles is a complex interaction between the animal’s internal environment and the external environment. Thermoregulation is regulated primarily through complex behavior patterns and alterations in the cardiovascular system. It is well recognized that most reptiles have a preferred body temperature range that is associated with maximal metabolic function. It is probably best to maintain animals in hospital care at the upper end of the species’ preferred body temperature range. This is accomplished intraoperatively by using circulating warm water blankets, warm water bottles, and forced warm air blowers. Suboptimal temperatures may be associated with prolonged drug effects and may impair the animal’s immune system and ability to heal. It has been demonstrated that ambient temperature and, more specifically, body temperature will affect inhaled and injectable anesthetic properties in vivo. Induction, anesthesia and recovery times are commonly prolonged while anesthetic dose is reduced when body temperature declines.
No hay comentarios:
Publicar un comentario