Anesthesia Clinical Case Challenge
Sammy, a Chihuahua presented to Cummings Veterinary Medical Center for an endoscopy and gastrointestinal biopsy due to chronic diarrhea and wasting with an acute episode of lethargy, in appetency, and pallor of mucous membranes.
The dog, eight months old, weighed 1.16 kg with a body condition score of 2/9. He could only support his weight when standing for brief periods of time. His abdomen was tense and uncomfortable on palpation. Apart from these findings the dog was mentally appropriate and other physical parameters were within normal limits. Medications at home were sucralfate and metronidazole.
Values from lab work provided with the referral were PCV 35 (L), TP 4 (L), albumin 2.4 (L), platelets 673 (H), neutrophils 12549 (H), calcium 8.1 (L), and Mg 1.1 (L).
On presentation, further lab work revealed evidence of ongoing blood loss with PCV 18, TP 3, albumin 1.9 and globulin 1.3. Other values were blood glucose 218, lactate 1.1, platelets 733, calcium unchanged at 8.1, and creatinine 1.3. The anemia was non-regenerative, and blood type was DEA 1.1 negative.
Issues facing the anesthesia team
Due to the small size of the patient, it can be challenging to place a catheter in tiny veins, which require tiny volumes of anesthetic drugs making it difficult to access monitoring during anesthesia, along with heat loss during the procedure, and potential hypoglycemia.
Anemia was of concern, especially with evidence of continued blood loss, and permission was obtained if a blood transfusion was necessary, before proceeding with the endoscopy and biopsy. Since it is difficult to predict blood loss with certainty for biopsy procedures, it is best to be prepared. A low PCV results in poor oxygen carrying capacity in the face of low cardiac output that is often seen in anesthesia. When PCV < 20 anesthesia is discouraged for any procedure where blood loss is anticipated. For procedures without the expectation of blood loss, anesthesia can be tolerated if anemia is mild and chronic. For this endoscopy, we did not expect significant, if any blood loss, and even though PCV was below the acceptable lower limit, the patient required urgent care, including anesthesia to determine the cause of his symptoms.
Low total protein, with low oncotic pressure, means great care must be taken when injecting drugs due to decreased binding to plasma albumin. The dose required for the desired effect will be much lower than the calculated dose, so drugs must be injected carefully for effect. The low oncotic pressure, because of low protein concentration, also affects the ability to maintain fluids within the vasculature. Excessive volumes of fluids can rapidly cause edema, particularly of the lung fields. Intravenous fluids need to be infused carefully with an infusion pump or similar device that controls volume. If an infusion pump is not available, a burette with an hour’s worth of patient fluids will prevent over-hydration in case a fluid bolus is given inadvertently.
Profound heat loss can cause bradycardia and hypotension and also delay the liver metabolism of drugs. Efforts should be made to maintain body temp normothermic or at least above 97 degrees F.
Cachexia of this patient, with muscle wasting and lack of fat, affects the distribution of drugs, especially those that are fat-soluble. The Fragility of the tiny body means great care must be taken with restraint and careful padding under the patient is essential to support and protect bony points. During the procedure, we need to be careful that weighty materials such as a heavy heating pad or cables and cords from equipment do not press on the body to impede ventilation. Another major concern with cachexia is the risk of burns because of the thin hair coat and possibly over-zealous efforts to prevent heat loss because of small size. This requires close attention to the heating system used and to the protection of the skin.
Our management of this case
As for any anesthesia case, a careful evaluation of the risks is always imperative. Without considering all the issues presented and their ramifications, we cannot provide optimal anesthesia. After evaluating all the presenting factors, an ASA status of III was assigned, meaning that there is a moderate risk for this anesthesia.
The tiny size of this patient meant that a non-rebreathing circuit was required. Although the calculated flow rate of fresh gas for this size of a dog is 200 ml/kg/min (in this case 232 ml/min), this would be insufficient to remove the exhaled CO2. The minimum flow rate to use for this circuit is 500 ml/min. A disadvantage of this flow rate is that it flows very close to the capnograph in the circuit, diluting the exhaled CO2 and providing an unrealistically low value. The capnograph also consists of a large volume of the circuit about the tidal volume, and this increases dead space that increases inspired CO2.
The procedure is not considered to be painful, so pure mu opioids agonists are not indicated. The dog was mentally appropriate and alert, so butorphanol was chosen at 0.2 mg/kg IV for its sedative and mild analgesic properties, its short duration of action, and its minimal impact on cardiovascular and pulmonary function. This produced mild sedation. Acepromazine was eliminated as a choice because hypotension and hypothermia were to be avoided and the dog did not require heavy sedation. There is also no specific antagonist for acepromazine, and the duration of action is prolonged.
Propofol was chosen for induction although alfaxan would have also been a suitable choice. Maintenance was with isoflurane and breathing was spontaneous.
Anticholinergics are not necessarily indicated if resting heart rate is normal, and these drugs will increase the tone of the pyloric sphincter. The tight sphincter will likely delay passage of the endoscope into the duodenum making endoscopy more difficult and prolonged.
As with every anesthesia case calculations of emergency drugs for this patient were made before starting anesthesia. This allows for a rapid response if an emergency arises and acts as a reminder to be prepared mentally for an untoward event.
The endotracheal tube was 4.0 mm in diameter and cuffed to protect the airway during the endoscopy.
Monitoring presented some challenges in this small patient as there was little room for all the equipment, and access was difficult for assessing the anesthetic depth and palpating pulses. The endoscopy procedure meant that the tongue was not available for oximeter pulse placement. The toe, prepuce and vulva are alternate sites in dogs and, in this case, the pulse ox was placed on a toe. For blood pressure measurement, the oscillometric method is unreliable in such a small animal, so a Doppler was used throughout to provide values for systolic blood pressure. Other parameters constantly monitored were heart rate (from the pulse ox plus manual palpation of an artery), ECG, CO2, and temperature. SpO2 and temperature were recorded every 15 minutes, and other values were recorded every 5 minutes. An infusion pump was used for IV fluids that were provided at 5ml/kg/hour and the volume infused was recorded every 15 minutes.
Endoscopy involves inflation of the stomach, which can impact venous return and cause pain from distention. This can cause a vagal response if inflation is excessive or occurs rapidly, especially with bradycardia. Although we decided that an anticholinergic was not indicated as part of the premedication, a dose of atropine (0.04 mg/kg) was calculated and readily available during the procedure in case of an emergency.
This tiny patient tolerated anesthesia very well during the 50- minute endoscopy with stable vital signs and anesthetic depth. The blood pressure within 5 minutes of propofol induction was 78 mm Hg and was 80 and above for most of anesthesia. The lowest value of 70 was recorded twice, at which time the isoflurane was reduced to 1.25% from 1.5 %. The lowest value for body temperature was 97.4oF, and this had increased to 99.4oF within half an hour of the end of anesthesia. Recovery was uneventful.
At the end of the procedure, buprenorphine was administered to treat any discomfort from distension of the stomach. At that point, the clinician decided that blood would benefit the dog in recovery in case of some slight bleeding that might develop so 12 mls. of packed red cells were administered over the next four hours.
A successful outcome, in this case, hinged on careful assessment, thorough preparation, and close attention to the patient during anesthesia.