Bonnie D. Wright, DVM, DACVAA
Tenants cross all species. Variations can be learned to less typical patients. Importantly, a fusion between non-objective (experience, training, touch, instinct) and objective(machines)monitors is vital to good outcomes and understanding. Monitors can be extremely valuable, but they can also distract, add time, and add complications. Judicious use of monitoring doesn’t always mean using every monitor available. In very small patients (like most of the pet-exotic population) shorter anesthesia times beget improved recoveries, as it can be difficult to maintain homeostasis as well in tiny patients.
Critical for oxygenation, carbon dioxide and acid/base balance, as well as stability of anesthetic plane when using inhalants. Assessment tends to be real-time and rapid, in most cases (see need for speedNFS).
During very short anesthetic periods, respiratory monitoring takes center stage.
1. Rate and volume - physiology rules and knowledge of this allows extrapolation between species.
a. Rate x tidal volume (TV) = Minute ventilation (MV).
i. As rate increases, dead space (OS) volume becomes more important.
ii. Example: 10 breaths per minute at 10 ml/kg/breath is a MV of 100 ml/kg/m.
Increasing rate to 100 breaths per minute, and reducing TV to 1 ml/kg is still a MV of 100 ml/kg/m, but nearly no gas exchange would take place, as most of the gas movement would be within the DS of the patient (which varies widely among exotic species).
iii. Whether using endotracheal tube, supraglottic airway, or mask, limiting excess DS in the circuit becomes increasingly important as size of patient decreases. This includes dead space imposed by monitoring equipment (see capnography) or elbows (convenient, perhaps - but can be large contributors to DS).
b. Very rough tidal volume estimate is 10 ml/kg, across species.
i. Subjective assessment of TV is difficult in very small patients (airflow patterns within patient, and also within breathing circuit).
ii. Non-rebreathing circuits have high oxygen flow-rates which tend to obliterate the small breath size when observing a rebreathing bag (and some Mapleson styles don’t have a rebreathing bag).
iii. Using pediatric components in a circle system may make breath size easier to see, but this is still subjective, and bags less than 1/2 L are hard to find. A 1 kg patient (big for most exotics) would only have about a 10 ml TV, so only a 2% change in rebreathing bag (10 ml/500 ml bag volume).
c. Wright’s respirometer can record TV and MV on a circle system, but not a non-rebreathing system due to the continuous flow.
2. Respiratory gas measurement.
a. Capnography: The partial pressure of CO2 in the alveoli is in equilibrium with the partial pressure of CO2 in the blood, or PaCO2. Therefore, measuring the partial pressure of CO2 in the patient’s end-tidal gas (when alveolar gas is expired) gives an estimate of PaCO2.
i. Capnography has the advantages of being simple to introduce (NFS) and real-time, continuous. Accurate measurement of ETCO2 requires a normal respiratory pattern, indicated by a flat “plateau” on the capnograph waveform.
ii. In addition to exhaled carbon dioxide, with wave-form analysis information can be gathered re: intubation status (very useful in difficult to visualize species), cardiac performance (all or none), circuit integrity, and quality of CPR.
iii. Equipment considerations are very important in small patients, such as main-stream vs. side-stream, size of adapters, and type of circuit being used. In general, with use of a non-rebreathing circuit, the information gathered from a capnograph is faulty, and the DS is significant, so in these situation, it is probably best to forgo, or just spot-check the readings periodically.
b. Arterial or venous blood gas: Single time-point and time consuming, so seldom useful in very small patients, and very short procedures. For advanced procedures and longer times, this becomes extremely important in assessing other respiratory monitoring information (allowing dead-space and V/Q mismatch extrapolation, confirmation of oxygenation, and assessment of acid/base sequela).
c. Not really a respiratory gas monitor, pulse oximetry is a rough end-point of ventilation (ventilation combined with cardiac performance, normal oxygen carrying capacity, and peripheral perfusion).
i. Rapid and real-time (NFS).
ii. Error-prone and requires significant ‘fiddling.’
iii. Oxygenation issues are most likely during transitions, not mid-inhalant.
a) >30% inspired O2 precludes hypoxia in normal situations.
b) Know normal (accuracy tied to steep portion of Oxyhemoglobin curve).
c) Perfusion linked to: cardiac performance, body temperature, positioning, pharmacology of drugs (vasoconstrictors).
d) Reptilia have a much lower oxygen consumption rate compared to mammals and birds.
e) Ventilation (IPPV) isn’t always thought of as means of monitoring (rather seen under ‘support’). However, in reptiles, provision of ventilation commonly replaces the “wait and see” approach to respiratory monitoring frequently utilized in mammals.
(1) Reptilian control of respiration likely involves CO2 and pH, not oxygen.
(a) Episodic breathing pattern - in bursts.
(b) Pulmonary perfusion is also intermittent, and patterns coordinate with respiratory pattern.
(c) High oxygen may suppress ventilation in reptiles (various responses to CO2, as well…). Thus, recovery frequently involves IPPV with room air.
(d) Amphibians utilize cutaneous respiration and stop breathing when offered 100% (anesthetic gas provision/monitoring are challenging...).
(2) Mammals and birds- primary pattern based upon central respiratory pattern generator w/peripheral chemoreceptors; oxygen serves as a back-up when low.
(a) Birds: cross-current exchange:
(i) One of the few species that can have lower ETCO2 than PaCO2.
(ii) Birds are very easy to over-ventilate (also - air sac fragility)
As with ventilation, cardiovascular function is altered by anesthetic drugs, and so monitoring is a critical part of safe provision of anesthesia. Complex by nature, this system involves the entire body with regards to changes in vasomotor tone, as well as the heart itself, and fluid and electrolyte status of the organism.
1. Rate and rhythm may be measured simply (auscultation or amplification) or with monitors.
a. ECG shows electrical pattern through the heart. Each species has a common pattern that should be learned. This is real-time and simple to apply (NSF).
b. Electrical conduction through the heart does not necessarily result in mechanical activity (a normal ECG can be seen in a heart that is not beating).
c. Amplification can occur over a peripheral pulse, or over the heart directly (also consider use over eyes). This may provide subjective or objective information about quality of cardiac contraction.
2. Blood pressure is established in most mammals, and increases in importance with increasing size of patient (as does reliability of measurement).
a. Non-crocodilian reptile hearts are three-chambered, so direction of blood-flow is labile, and shunting is common. This can alter anesthetic (and other respiratory gas) uptake and elimination, and is not quantifiable, clinically.
b. Reptiles, birds, and mammals have conserved mechanisms for altering blood pressure (cholinergic, adrenergic and baro-receptors). However, reptiles have a large variation in “normal” that are also influenced by: temperature, activity and state of arousal. (Chelonian normal MAP if 15–30 mm Hg), while squamates may be allometrically scaled (larger snakes having higher BPs). In general, change from baseline is the suggested approach to blood pressure monitoring in reptiles.
c. Birds have higher blood pressure, lower heart rates, larger stroke volume, and cardiac output than mammals.
d. Types of blood pressure monitoring.
i. Amplification alone: Non-quantifiable, but can give subjective information about the quality of cardiac performance. Placement of crystal is mild to moderately time-consuming (skill level related).
ii. Amplification with cuff (Doppler) - limited by size and shape of limbs.
iii. Oscillometric (cuff limitations, but without need for crystal). Rapid application, but intermittent readings.
iv. Arterial cannulation - in general, most time consuming. Most appropriate with advanced procedures or longer duration procedures (specialist level).
Body temperature is a critically important consideration in ectomorphs, altering anesthetic drug uptake and distribution, cardiac and respiratory functions, and markedly influencing recovery. It is also important in small mammals and birds, but to a less critical degree. Exotic patients should always be managed at their ideal physiologic temperature, and external sources of heat should be modified to meet their environmental needs. Additionally, amphibians require provision of moisture to their skin for cutaneous forms of respiration to occur.
Anesthetic Depth and Analgesia
Often overlooked as topics during monitoring, these can be quite challenging in exotics. Across species, the importance of a multimodal approach has gained both traction and evidence-based research. In general, despite the difficulty in pain assessment (to be addressed in later lectures), the physiologic underpinnings are present for reptiles, birds, and small mammals to all feel pain. Not only is it, therefore, ethical to address pain, when it would be rational in better understood species, but these drugs often augment other anesthetics in a positive way.
1. Properties of the inhalant anesthetics.
a. MAC - definition based upon movement.
b. Autonomic reflexes - often utilized as a proxy for movement (2x MAC).
c. Toxicity varies by species and study but tends to fall at 4x MAC. (low therapeutic ratio compared to other drugs)
2. Properties of other drugs used in the peri-anesthetic period.
e. Alpha-two antagonists.
f. Local anesthetics.