Page 398 - Feline Cardiology

418 Section O: Anesthesia in the Patient with Cardiac Disease

Nonpathologic Murmurs accuracy. The gold standard for blood pressure measure-
A common cause of heart murmurs in cats is dynamic ment during invasive or long-duration procedures is
right ventricular outflow tract obstruction. This is not direct measurement, if resources are available. Once the
considered a pathologic finding. An echocardiogram is cat is anesthetized, a 24-gauge catheter is inserted in the
needed to differentiate an innocent murmur caused by dorsal pedal or femoral artery and connected to a pres-
dynamic right ventricular outflow tract obstruction sure transducer and monitor. Arterial catheterization is
from other pathologic murmurs caused by structural or challenging in cats and should be attempted only by
functional cardiac disease. Similarly to other causes of experienced personnel. Advantages of direct blood pres-
nonpathologic murmurs, anesthesia can be conducted sure measurement include better accuracy, and continu-
as in any other cat without cardiac disease. ous measurement, which is particularly useful if
hemodynamic instability is expected. Disadvantages, in
addition to the technical difficulty, include risks for clot-
ANESTHETIZING THE FRACTIOUS CAT
ting, hemorrhage, poor perfusion of the area distal to
Fractious cats with suspected or known cardiac disease the catheter, and infection. Using a continuous flushing
are particularly challenging to anesthetize, because the device, catheterizing the dorsal pedal rather than the
information available is often limited, a thorough physi- femoral artery, limiting arterial catheterization to 6
cal examination is often not possible, and heavy seda- hours or less, and using an aseptic technique limit these
tion may be required for placement of an intravenous risks.
catheter. This increases the risk of sedation/anesthesia Ventilation can be spontaneous, unless marked
in these patients and should be discussed with the hypoventilation occurs or in cases in which the intraop-
owner. Three basic options are available: box induction erative onset of pulmonary edema interferes with oxy-
with an inhalant anesthetic, intramuscular administra- genation. In these cases, intermittent positive pressure
®
tion of a dissociative (Telazol or ketamine), or intra- ventilation with or without positive end-expiratory
muscular administration of an alpha-2 agonist. An pressure is usually indicated. Oxygenation should be
opioid/benzodiazepine combination can be tried, but assessed by blood gas analysis, or by the use of a pulse
will often not provide adequate sedation in these oximeter.
patients. Fluid administration and replacement of blood loss
Because hypertrophic cardiomyopathy is the most must be judicious to optimize preload without causing
common cardiac disease in cats, and it may not be pos- pulmonary edema. Typically, crystalloids are adminis-
sible to assess the severity of the disease without seda- tered at a slower rate than in the normal cat; however,
tion or anesthesia, sympathetic stimulation should be hypovolemia should be adequately corrected, and large
avoided as much as possible. Box induction with an volumes of fluid should be administered if necessary. A
inhalant anesthetic and dissociative agents cause sympa- typical fluid protocol for a cat with substantial heart
thetic stimulation, the former because of the stress asso- disease (structural abnormalities demonstrated on
ciated with the technique, and the latter due to a drug echocardiography or moderate cardiomegaly on tho-
effect. Therefore, alpha-2 agonists are preferred in this racic radiographs) is administration of lactated Ringer’s
situation. Medetomidine or dexmedetomidine are the solution at a rate of 3–5 ml/kg/h. Colloids should be
agents of choice, since xylazine has been reported to be used with extreme caution because they expand the cir-
culating volume for a longer period of time and may
arrhythmogenic. Higher doses (medetomidine 20–
Anesthesia 40 µg/kg; dexmedetomidine 10–20 µg/kg) than in non- result in plasma volume expansion greater than the
volume administered. This leads to an increased risk of
fractious patients may be required. If an injection cannot
be performed, box induction with an inhalant is the only
author typically avoids the use of colloids in cardiac
option, but the risk associated with the technique should iatrogenic congestive heart failure with these fluids. The
be understood and approved by the owner. patients, except for the treatment of anemia with blood
products. Monitoring of central venous pressure may
provide limited but sometimes useful guidance for fluid
SUPPORT AND MONITORING
administration, but does not reflect filling pressures on
A lead II ECG, temperature, and blood pressure should the left side of the heart or reliably predict development
be monitored during anesthesia. The ECG should ideally of pulmonary edema in cats with left heart disease.
be assessed during induction of anesthesia as well. Blood Central venous pressure monitoring may be useful in
pressure is routinely measured using noninvasive tech- following trends of overall fluid balance and is extremely
niques, preferably with Doppler (see Chapter 21). useful for monitoring cats with right heart disease (see
Oscillometric methods can be used but may have lower Chapter 24).