Monitoring anaesthetics in exotics

Pre-surgical patient preparation

Pre-surgical patient preparation includes the gathering and organisation of all monitoring equipment, heat sources, emergency drugs and other supplies prior to induction of anaesthesia. Vascular support (intravenous (IV) or intraosseous (IO) catheterisation) is recommended for all patients undergoing anaesthesia, but especially for surgeries expected to last beyond 20 minutes, and for ill or unstable patients. Catheterisation is straight forward in rabbits and ferrets, and moderately challenging in larger guinea pigs. In smaller patients where vascular support is desired, IO catheterisation can be utilised using the humerus, femur or tibia in mammals and reptiles with legs, and distal ulna or proximal tibiotarsus in birds (Figure 1) (Gunkel and Lafortune, 2005)). Catheterisation is described in detail elsewhere.

Patient support

For uncomplicated surgical procedures, the standard mammalian fluid rate of 10 ml/kg/hour has been used successfully in exotic mammals and birds. Surgical fluid rates for reptiles have not been well described; some experts recommend 3–5 ml/kg/hour. Fluids are delivered via a paediatric infusion pump, syringe pump (Figure 2), or by intermittent hand injection where the hourly fluid rate is calculated and boluses are administered every 10–15 minutes.

Thermal support is critical. In studies of mammals and selected birds, a reduction in core body temperature occurs within 20 minutes of induction of anaesthesia; temperature drops are associated with cardiovascular instability, poor recovery and decreased patient survival. Common methods of thermal support include warm water blankets, forced air heaters, electric heating pads and radiant heat (van Zeeland et al, 2012). Some exotic companion mammals are especially susceptible to overheating, in particular the chinchilla; thus core body temperature should be monitored continuously with a flexible temperature probe inserted carefully into the rectum.

All exotic anaesthetic patients ideally should be intubated, which is commonly performed in birds, reptiles, exotic carnivores and rabbits. Intubation in psittacines and most other birds and reptiles is particularly easy due to the rostral location of the glottis at the base of the tongue. In cases of tracheal obstruction or abnormalities of the oral cavity, direct intubation of the airsac can be considered. Intubation of rabbits requires significant practice and can be accomplished by blind, endoscope guided and otoscope-guided techniques. Intubation of guinea pigs and smaller rodents is accomplished with the aid of an endoscope and is significantly more challenging. However, with practice, many exotic anaesthetists are regularly intubating patients such as rabbits, guinea pigs, chinchillas, prairie dogs and even larger rats. Another option for rabbits is the v-gel, a supraglottic airway device that does not require intubation. (v-gel, Docsinnovent, London) (O'Dwwyer et al, 2013).

The largest disadvantage of intubation of the more challenging patients is time required for successful intubation. Another disadvantage of intubation is that in small birds, reptiles and mammals, there is an increased risk of mucus occlusion of very small endotracheal tubes. In the author's experience, the risk of occlusion appears to increase with tube sizes below 2.5 mm. Some practitioners will not intubate birds smaller than 100 g for this reason. Many reptiles do not spontaneously breathe under anaesthesia; therefore, intubation of even very small patients is required.

Monitoring the exotic patient

Full continuous attention must be given to the anaesthetised exotic patient, as changes may be subtle and abnormalities may progress rapidly. A second assistant should be available to open packs and obtain additional equipment for the surgeon so attention is not diverted. The patient must not be obstructed from view by drapes, gauze or other equipment. Transparent drapes are ideal for this purpose and allow the anaesthetist full view of the patient. If during the course of the procedure view of the patient is compromised, the situation must be remedied immediately.

Monitoring parameters and equipment are summarised in Table 1. Obtain baseline measurements for all parameters soon after pre-medication for anaesthesia, or as soon after induction of anaesthesia as possible. Be mindful that in some cases, monitoring devices do not give measurements that are accurate, or otherwise reflect true values. Therefore, monitoring trends becomes extremely important throughout the procedure.

Respiratory rate and character are monitored via direct visualisation. As these are often amongst the first parameters to change, and change may be rapid, they should be monitored continually. Changes include an increase or decrease in respiratory rate and/or respiratory effort.

Cardiac rate is measured via a stethoscope, ultrasonic Doppler, electrocardiogram (ECG) or other monitor. The stethoscope does not allow hands-free monitoring, and is therefore not preferred. However a stethoscope must be available at all times in case other cardiac monitors are dislodged or fail. Changes include increases or decreases in cardiac rate or intensity. In a carefully placed Doppler that has not been disrupted, decreases in blood pressure may result in decreased sound volume or intensity.

Indirect systolic blood pressure is routinely measured in ferrets, rabbits, larger guinea pigs, and larger birds, but requires some degree of practice. Pressure readings are increasingly difficult to obtain as patient size decreases, and are very challenging to obtain in reptiles. Methods include the use of a sphygnomamometer with Doppler or an oscillometric monitor (Figure 3). Of the two, the Doppler method is more reliable and more likely to give results in smaller patients. Studies indicate that indirect blood pressure measurements may not provide numbers that reflect the patient's actual blood pressure (Schoemaker and Bosman, 2009; Johnston et al, 2011). While this may be true, measuring blood pressure trends is extremely valuable. Obtain a blood pressure measurement as soon after administration of pre-aneasthetics or after induction of anaesthesia as possible. Obtain measurements every 5 minutes and watch for trends. Decreases in previously steady blood pressure measurements may represent a true change. Mucus membrane colour and capillary refill time are useful parameters in mammals, and can be observed using the everted vent in larger birds. These parameters are not practical to measure in reptiles. As with blood pressure, observe both parameters at the onset of anaesthesia, and compare throughout the procedure.

Body temperature is best measured with small flexible constant read out temperature probes. Some can be adapted for all but the smallest exotic patient. Probes are placed rectally in mammals, in the oesophagus of reptiles, and in the crop of birds. A common cause of temperature drop is displacement of the temperature probe due to peristalsis. Check the position of the probe prior to beginning aggressive rewarming procedures.

Oxygen saturation of haemoglobin is measured with a pulse oximeter (Figure 3). Many monitors can be used in exotic patients, although creativity must be employed when selecting a site for probe placement. Both rectal and clamp style probes have been used. Clamps may give readings when placed on ears, limbs, tongues or even the scrotum. Again, numbers may not be as important as trends in measurements over the length of the anaesthetic procedure.

End-tidal CO2 (ETCO2) is not commonly measured in small exotic pets. Probes are attached to the endotracheal tube, which means they are not useful for any patient that cannot be intubated. Due to the small size of the endotracheal tube in most exotic patients, special ‘side-stream’ models are most useful. The author has found these parameters difficult to measure in exotic animals.

Monitor failure

Every anaesthetist of every patient (exotic or not) should have a back up plan for monitoring device failure. One example might be a power failure. In the author's experience monitoring devices designed for canine/feline patients tend to be more problematic when used in small exotics, and patient size makes monitoring challenging; examples include surgeon movement of the small patient, and difficulty placing devices on small limbs/thorax, and such.

When monitoring equipment fails (especially cardiac monitors), the anaesthetist may attempt to briefly replace it. If a cardiac monitor is not restored quickly (e.g. within 20–30 seconds), a default method should be selected (stethoscope), and an assistant called in to continue to work with the non-functioning monitor. Prolonged attempts to manipulate monitoring equipment diverts the attention of the anaesthetist from the patient.

Anaesthetic emergencies

The goal of monitoring is to detect abnormalities in the early stage, prior to the need for drastic intervention. In the author's experience, the first parameter to change in birds and exotic mammals is the respiratory rate and/or character; for example, a slight decrease in respiratory rate or thoracic excursion. Since most reptiles do not spontaneously breathe under anaesthesia and are therefore continuously ventilated, detection of respiratory change is impractical. As in other species, blood pressure in birds and exotic mammals will often drop prior to the detection of other more severe abnormalities such as bradycardia. However, blood pressure measurement can be challenging in these species. (It should be noted that in the author's experience, blood pressure evaluation in reptiles is even more challenging and typically unrewarding). When blood pressure measurements begin to decrease, reduce general anaesthesia slightly, and consider administration of bolus fluids. Simultaneous decreases in respiratory rate and cardiac rate are addressed with manual ventilation, reduction of the amount of general anaesthesia, and when prolonged or severe, administration of drugs such as doxapram, atropine and vasopressin. Consider reversal of injectable anaesthetic agents that were administered such as medetomidine or dexmedetomidine.

Occlusion of the endotracheal tube often presents as an increase in respiratory effort. Mechanically ventilating may relieve an occlusion. If mechanical ventilation does not improve respiratory effort, the patient should be extubated and maintained on a facemask; re-intubation is often too challenging once the surgical procedure has begun.

Restoration of normovolaemia

Optimal fluid therapy is critical for treatment of hypovolaemia associated with anaesthesia or blood loss. While little information exists on specific guidelines for treatment of hypovolaemia in exotic animal species, information can be extrapolated from work with other more traditional pet species (Lichtenberger and Lennox, 2012a; 2012b). The endpoint of fluid resuscitation is the restoration of normal blood pressure and normalisation of other vital parameters. The following strategy outlines the treatment of hypovolaemia in exotic companion mammals, and has been used with success in birds as well (Lichtenberger and Lennox, 2012a; 2012b).

The amount of general anaesthesia should be reduced. Administer rapid intravenous infusion of warmed isotonic crystalloids at 10 ml/kg, followed by colloids (Hetastarch, 6% (B Braun Medical Inc., Irvine, CA) at 5 ml/kg over 5–10 minutes. Boluses of isotonic crystalloids and colloids are continued until systolic Doppler blood pressure reads above 90 mmHg.

An alternative to initial treatment with crystalloids is the use of 7.2–7.5% hypertonic saline at 3 ml/kg as a slow bolus over 10–20 seconds. Hypertonic saline quickly draws fluids from other compartments into the intravascular space. The effect is maintained with follow-up administration of crystalloids as discussed above.

Respiratory arrest

Respiratory arrest requires the establishment of a patent airway, if not already in place. When direct intubation of an arresting mammal is not possible, consider forced mask ventilation. A tight fitting mask is placed over the mouth and nose, and the patient firmly ventilated. This technique will allow a reasonable amount of air to enter the airways, but may also cause accumulation of air in the stomach. Another option is emergency tracheal intubation using a large bore catheter. Use of a catheter for this purpose requires an adaptor from the catheter to the anaesthetic machine. Many endotracheal tube fittings can be used for this purpose. During resuscitation, another team member should begin establishing vascular access, if not already available.

Cardiac arrest

In cardiac arrest immediate basic life support principles should be initiated, including airway support as discussed above. Chest compressions are recommended for exotic companion mammals; however effectiveness is unknown in birds and reptiles. In mammals, begin chest compressions at 100–120 times per minute by placing one hand on each side of the chest at the widest portion. The duration of compression should take up half of the total compression-release cycle. Another staff member should attempt IO or IV access at this time if not already in place (Lichtenberger and Lennox, 2012a; 2012b).

Epinephrine and vasopressin can be administered intravenously, intraosseously or via an endotracheal tube (double dosage). Anecdotal reports of success are reported, but true efficacy is unknown, especially for birds and reptiles.

Haemorrhage

Treatment for acute severe blood loss includes blood transfusion or the use of colloids with oxygen carrying ability such as Oxyglobin, which is no longer available in the US. With the exception of the ferret, exotic patients are known to have, or should be assumed to have, distinct blood types. However, the likelihood of transfusion reaction after a single transfusion is unlikely. The risk of reaction must be weighed against the risk of withholding transfusion. It should be noted that the author has transfused a single sun conure with blood from three separate donors on three separate occasions without apparent ill effect.

Conclusion

Anaesthetic monitoring is an important part of any surgery; monitoring of exotic patients requires close attention and the ability to adapt to rapidly changing conditions, such as equipment failure. With experience, many veterinary nurses are becoming experts at exotic animal anaesthesia and monitoring, with increasing positive outcomes.