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B.D. Wright1
1Veterinary Anesthesiologist — Integrative Pain Management Specialist,
Monitoring basics: 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.
Respiratory: 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 speed- NFS). 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 (DS) volume becomes more important
II. Example: 10 breaths per minute at 10 ml/kg/breath is a MV of 100 ml/kg/min
Increasing rate to 100 breaths per minute, and reducing TV to 1ml/kg is still a MV of 100 ml/kg/min, 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 10ml/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 (10ml/500ml 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 sequella)
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)
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