Recognize a complication: Prevent an anesthetic crisis (Proceedings)

Summary

• Patients at risk for hypotension include old age, hypovolemia, hypoproteinemia, acute or chronic cardiac disease, large abdominal mass, pancreatitis, or sepsis and endotoxemia.

• Factors associated with potentially life-threatening hypotension during anesthesia include a change in body position, hemorrhage, deep anesthesia, injection of antibiotics and some anesthetics, and anaphylaxis from mast cell tumors.

• Hypoventilation during anesthesia and hypoxemia during recovery from anesthesia should be anticipated in old patients, and with opioid use, obesity and abdominal distension, thoracic or pulmonary damage.

Effective management of an anesthetic complication is dependent on early recognition of an abnormality and rapid implementation of a treatment plan. Early recognition is facilitated by a preanesthetic evaluation that identifies potential risk factors and by consistent monitoring during anesthesia and in the recovery period. Treatment is most effective when decisive action is taken as soon as a deviation from normal is noted. Previously constructed plans for different scenarios should be available for interns and technicians to utilize without delay. Emergency drugs should be located in a designated area.

Heads Up From Preanesthetic Evaluation

Monitoring The Cardiovascular System

Normal values for systolic/diastolic and (mean) pressures quoted by the Veterinary Blood Pressure Society 2002 for awake healthy animals are for dogs 133/75 (94) mm Hg and for cats 124/85 (98) mm Hg. Whereas the systolic and diastolic pressures vary from publication to publication, mean arterial blood pressure (MAP) values are often 90-100 mm Hg. Hypotension is generally defined as a MAP of less than 65 mm Hg. MAP of 55 is seriously life threatening. Non-invasive method of blood pressure measurement using oscillometry provides a digital display of systolic, diastolic, and mean pressures. The mean pressure usually correlates well with the mean value obtained by direct arterial catheterization. The non-invasive Doppler shifted ultrasound method (Parks, Aloha, Oregon) provides systolic pressure. The diastolic pressure is difficult to hear in some patients. Experiments have shown that in cats and small dogs the first sound heard with the Doppler method is closer to the mean pressure. To improve accuracy of measurement, the width of the cuff used for both methods should be equal to 40% (40-60%) of the circumference of the limb or tail, and the inflatable cuff part should be placed directly on the medial side of the leg (not over a joint) or the ventral side of the tail. Although noninvasive methods of blood pressure measurement are not entirely accurate, these methods accurately predict hypotension 80% of the time. Consequently, a low pressure obtained by such means is an indicator for treatment.

Other measurements of cardiovascular function must be assessed alongside the BP. Heart rate may or may not be useful as hypotension may exist when heart rate is within normal limits, especially during anesthesia, because baroreceptor response is blunted or abolished by anesthetic agents. Capillary refill time (CRT) should be 1 sec and prolonged refill indicates decreased cardiac output. Hypotension can exist with pale or pink mucous membranes. Pink membranes usually indicate vasodilation, and this is frequently the cause of hypotension during anesthesia with isoflurane or sevoflurane. A patient that has a MAP over 64 mm Hg, pink gums and CRT of 1 sec probably has an acceptable cardiac output. Some clinical studies have included cardiac output measurement and generated the conclusion that cardiac output can increase or decrease in response to anesthetic or surgical manipulation but at the same time mean arterial pressure is unchanged. In absence of cardiac output measurement, we must rely on observation of changes in mucous membrane color and CRT with BP measurement to provide sufficient information to assess changes in cardiovascular function. Interpretation of membrane color after medetomidine administration is complicated by vasoconstriction.

Hypotension

Hypotension is defined as a mean arterial blood pressure (MAP) of less than 65 mm Hg. The urgency for intervention may be determined by observation of mucous membrane color and capillary refill time (CRT). Blood pressure in the low 60's with pink membranes and rapid CRT is less cause for concern than pale membranes and slow CRT. Other signs that immediate management is indicated are patient's eyes that are central in the socket with dilated pupils, no bleeding from a skin incision, dark blood at the operative site, and, when the abdomen is open, pale not pink intestine. A sudden decrease in strength of a peripheral pulse should be recognized as a change in cardiovascular status. A decrease in end-tidal CO2 on the capnograph may also occur in response to decreased peripheral perfusion.

Most effective treatment of decreased cardiovascular function is to directly manage the cause of the complication. Most causes of hypotension fall into the categories of decreased cardiac function or decreased blood flow return to the heart.

Causes of decreased cardiac function and venous return

Decreased cardiac function (contractility or abnormal rate): Cephalosporins and gentamicin given rapidly IV will drop MAP abruptly and should be given slowly over about 5 minutes. Similarly IV injection of opioid during inhalation anesthesia can cause an abrupt decrease in MAP. Treatment is symptomatic, that is, decreased vaporizer setting, IV bolus of balanced electrolyte solution, ± administration of dobutamine or dopamine.

Decreased venous return to the heart: A common cause of low blood pressure with an inhalation agent is a functional decrease in circulating blood volume resulting from vasodilation induced by the anesthetic agent. Volume expansion and infusion of dopamine ± ephedrine should be used if hypotension develops.

Systemic blood pressure can be decreased by any cause of increased abdominal pressure, particularly when the animal is in dorsal recumbency (caval compression). Common scenarios are ovariohysterectomy of a pregnant animal, splenic neoplasia, intussusception or intestinal mass.

Blood loss (15-20% of circulating blood volume) will significantly decrease cardiovascular function in the absence of adequate treatment. A loss of one-third to half of that volume will acutely decrease blood pressure if the loss occurs rapidly. Animals more susceptible to hypotension from blood loss are old, have cardiac disease, or have a low hematocrit or total protein before anesthesia. Blood volume in dogs is 86 ml/kg (8 ml/lb) and in cats is 56 ml/kg (5 ml/lb). Lactated Ringer's solution should be infused at 2.5 to three times the estimated blood loss. Serial measurements of PCV and TP will still be useful at this fluid infusion rate and can be used to assess the decrease in red cell volume. Splenic contraction that occurs when the animals regain consciousness will increase the PCV a few %. Treatment with more than crystalloid fluid may be required, including decreasing anesthetic delivery and use of vasoactive drugs. Blood volume expansion with hypertonic saline 2-4 ml/kg over 10 min, hetastarch 10-20 ml/kg over 30 min, plasma, or blood may be necessary.

A decrease in blood pressure frequently occurs when an anesthetized animal is rolled over during or at the end of anesthesia. The animal's pulse strength, rate, and CRT should be checked at this time as a precaution. Treatment varies from "wait a few minutes and see" to immediate reduction in anesthetic administration and infusion of dobutamine. Occasionally the drop in blood pressure progresses to cardiac arrest, consequently, the occurrence of hypotension arising from change in body position should never be taken lightly.

When and How To Give Dopamine, Dobutamine, Ephedrine:

Dopamine and dobutamine (Abbott Laboratories, North Chicago, IL 60064) are catecholamines that increase blood pressure by increasing myocardial contractility. Ephedrine (Taylor Pharmaceuticals, Decatur. IL 62522) causes some increase in contractility and causes venoconstriction and increased venous return. These agents are useful when hypotension is the result of anesthetic agent depression, but they are less effective in the presence of acidosis or decreased blood volume. Both dopamine and dobutamine may cause increases in heart rate and ventricular dysrhythmias. Dopamine is converted to nor-epinephrine, which may increase irritability of the heart. Thus dobutamine is a better choice in animals with cardiomyopathy or thoracic trauma and traumatic myocarditis. In contrast, dopamine may be the drug of choice in life-threatening situations requiring cardiopulmonary resuscitation (CPR) because the nor-epinephrine causes vasoconstriction, increased blood pressure and improved cerebral perfusion. Dopamine also actives dopamine receptors in the renal, splanchnic, and coronary circulations producing selective vasodilation and at infusion rates of less than 7 µg/kg/min will cause increased urine output and improved splanchnic perfusion. Dopamine and dobutamine must be made up as solutions for continuous infusion as they are metabolized quickly. Ephedrine is usually given as a bolus IV injection that may last about 30 minutes.

Dopamine and Dobutamine

Make 100 µg/ml solution in a 500 ml bag of 0.9% saline for infusion:

Dopamine stock solution 40 mg/ml; Dobutamine stock solution 12.5 mg/ml

Add 50 mg = 1.25 ml dopamine or 4 ml dobutamine

Infusion rate for dopamine or dobutamine is 3-7 µg/kg/min. Infusion begins at 5-7 µg/kg/min and is decreased to 3-5 µg/kg/min when MAP has increased to 70 mm Hg. When a rapid response is necessary the infusion rate can be increased to 10 µg/kg/min:

Calculate 5 µg/kg/min for a 10 kg (22 lb) dog using a pediatric administration set (60 drops/ml)

= 30 drops/min (or 1 drop every 2 secs)

Calculate 5 µg/kg/min for a 3 kg (6.6 lb) cat using a 60 drop/ml administration set

= 9 drops/min (or 1 drop every 7 secs)

Ephedrine stock solution is 50 mg/ml and should be diluted for accurate administration of 0.06 mg/kg.

Routine treatment of hypotension

Recognition of Hypoventilation

Assessment of adequacy of ventilation can be difficult without specific monitors. Observation of a shallow thoracic excursion during inspiration may indicate hypoventilation. Animals with neurologic disease may have altered breathing patterns. Close observation may reveal diaphragmatic breathing without normal thoracic movement. Opioid administration frequently causes respiratory depression observed with a normal rate but shallow breathing, or panting ans shallow breathing. Inadequate ventilation (hypoventilation, hypercarbia, hypercapnia) is common in sick patients during anesthesia. Hypoventilation is likely to be present even in healthy anesthetized Bulldogs and Pugs, overweight dogs and cats, animals that are prone and hanging off the end of the table, when the operating table is put in a head down tilt, and when the abdomen open with a retractor for an exploratory laparotomy. Hypoventilation is probable when the animal has rapid shallow breathing or breathing at a respiratory rate of less than 6 breaths/min. Significant hypoventilation is present when an end tidal CO2 monitor (capnography) reads greater than 50 mm Hg or a blood gas analysis confirms a PaCO2 greater than 55 mm Hg. Hypercapnia can cause hypertension, tachycardia, or hypotension.

Capnography

Capnography measures carbon dioxide that is expelled from the animal's lungs (Vetrosonics Tidal Wave, Datex, Surgivet, Cardell). There are 2 types of capnographs: one that measures the CO2 in-line at the endotracheal tube adapter and one that continually aspirates gas from the anesthetic circuit and measures the CO2 at a distance. The in-line measuring monitor is necessary for use with non-rebreathing circuits and for the smallest dogs and cats. The peak CO2 value (alveolar concentration, ETCO2) is approximately 4-6 mm Hg less than arterial PCO2. The ETCO2 will be 30-35 mm Hg in dogs and cats with normal ventilation. An ETCO2 value of 50 mm Hg or above is significant hypoventilation. When the capnograph displays a high ETCO2, then arterial CO2 is high. A large difference between ETCO2 and PaCO2 may occur in animals that are breathing shallowly or have a small tidal volume, or that have significant lung collapse from pleural effusion or a mass in the chest or diaphragmatic rupture. Thus, when ETCO2 is normal or low, then arterial CO2 may be high, normal or low.

Fig. 1 Capnography waveform

Hypoxemia

Hypoxemia in awake animals may induce panting, open mouth breathing, facial grimace, cyanosis, tachycardia, or very little behavior change other than restlessness. During anesthesia, the hypoxemic animal may have mucous membranes that vary from muddy pink to cyanotic. Measurement of hemoglobin saturation (SpO2) using a pulse oximeter or arterial PO2 by blood gas analysis can confirm or deny hypoxemia. Hypoxemia is confirmed when hemoglobin saturation less than 90%; this corresponds to a PaO2 of ≤ 60 mm Hg. Pulse oximeter readings should be checked for accuracy by confirming the heart rate from another method and by waiting at least 30 seconds after application of the probe. The probe should be screened from light by covering it with wet gauze. In an anesthetized animal, a decrease in SpO2 to less than 90% when the value had been stable at 97% is suspicious but first the position of the probe should be moved to ensure that it was not a malfunction due to local compression of capillaries.

Troubleshooting the capnogram

Hypoxemia in anesthetized critically ill patients breathing oxygen can be due to severe lung collapse (such as pneumothorax or pleural effusion) or due to inadequate circulation. The endotracheal tube should be checked for correct placement (not in the esophagus or a tube that is too long and has entered one bronchus) and the circuit should be checked for leaks. Controlled ventilation should be instituted and the SpO2 monitored for an improvement. Pulmonary perfusion may be improved by infusion of dobutamine or dopamine, however, hypoxemia originating from another cause will not be effectively treated by administration of a vasoactive drug.

Measurement of SpO2 >90% does not guarantee adequate oxygen delivery to tissues, merely that the hemoglobin in the blood is carrying oxygen. Animals that are anemic or have major blood loss may have insufficient hemoglobin to carry enough oxygen to the tissues. The packed red cell volume should be maintained over 20% in anesthetized animals. Peripheral blood flow as determined by cardiac output, mean blood pressure, and capillary blood flow also determine how much oxygen the tissues receive.