foals.Anesthetic‐induced hypotension may reverse blood flow through the PDA and create pulmonary hypertension.
C Primary myocardial disease
Rare in horses.
Congestive heart failure (CHF) is associated with limited cardiac output, increased neurohumoral activity, sodium retention, edema in tissues and transudation of fluid into body cavities.Valvular disease is the most common cause of (CHF) in the horse.Horses with CHF are at an extremely high anesthetic risk.
D Secondary cardiovascular compromise
Common in horses presented for anesthesia.
Causes include circulatory shock (e.g. severe hemorrhage), sepsis (e.g. colic) and profound electrolyte imbalance (e.g. uroabdomen in foals).
Cardiovascular changes that occur in sepsis include:Decreased cardiac output resulting from a direct decrease in cardiac contractility and a decrease in preload due to splanchnic pooling and vascular leakage.Pulmonary hypertension (with subsequent hypoxemia).Complex alterations in systemic BP (initial hypertension followed by hypotension with loss of vascular tone).Drastic alterations in hematologic function, including hypercoagulability followed by hypocoagulability.
IX Anesthetic plan for horses with cardiovascular disease
A Patient preparation
All patients scheduled for anesthesia should have a thorough physical examination.
Because anesthetic drugs can drastically alter cardiovascular function, techniques to evaluate the cardiovascular system should be emphasized, especially in patients with primary cardiovascular disease or cardiovascular compromise secondary to other systemic disease (e.g. sepsis).Laboratory tests should include serum chemistry and a complete blood count.
Regardless of the cause of cardiovascular compromise, the patient must be stabilized prior to anesthesia. This includes:Restoration of circulating blood volume (use of whole blood if necessary).Intravenous fluids must be used judiciously in horses with heart failure.Restoration of electrolyte balance.Serum [Ca++] and [K+] are often decreased.Promotion of cardiovascular function (e.g. IV fluids, positive inotropes, and analgesics).
B Sedation and induction
Following stabilization, the patient should be sedated with low dosages of sedatives (e.g. alpha2 agonists, acepromazine).
Pre‐emptive analgesic drugs should be utilized (e.g. opioids, alpha2 agonists, NSAIDs) to decrease the horse's stress and to decrease the dosages of induction and maintenance drugs.
A balanced induction technique should be used (e.g. guaifenesin + ketamine) and low dosages of the drugs should be administered to effect.
Intubation and oxygen administration should occur as soon as possible.
C Maintenance
Inhalational anesthetics are generally used for maintenance but dosages should be kept as low as possible to minimize the hypotensive effects of the drugs.Balanced anesthesia (e.g. inhalational agent plus a ketamine and/or lidocaine infusion) should be considered.
Analgesia is imperative and can be supplied via systemic administration of drugs or by the use of local anesthetic blockade, or ideally, by using both techniques.
Monitoring is extremely important and should include arterial BP, ECG, and arterial blood gases.A cardioselective inotrope (e.g. dobutamine) is recommended for correction of hypotension.
Fluid therapy should include evaluation and support of PCV, total protein (TP), acid–base balance and electrolyte concentrations.
D Recovery
Is as critical as the other steps of anesthesia.
Patient support (including monitoring, fluid administration, oxygen administration and provision of analgesia, should be maintained, when possible, throughout the recovery period.
Evaluation of the Cardiovascular System
Daniel G. Kenney
Evaluation of the horse should include detailed evaluation of the medical and performance history. Of particular note for the cardiovascular system is whether there is an unexplained decrease in body condition, the presence of respiratory abnormalities, edema formation, and decreased exercise tolerance.
A thorough physical examination should be performed in a systematic fashion, including detailed assessment of the cardiovascular system.
I Cardiovascular evaluation
A Body condition
Poor body condition and decreased exercise tolerance may indicate cardiovascular disease.
B Edema
Presence of edema in ventral areas (abdomen, thorax, prepuce, udder, limbs) may be indicative of cardiovascular disease.
C Mucous membranes
Assess for hydration, color, and capillary refill time.
The oral mucous membranes are readily assessed in most horses.
Vulva membranes and sclera (observation only) may also be assessed.
Color:
The mucous membranes are visually assessed; normal membranes are pink.
Pale membranes (pale pink to whitish) may be due to poor cardiac output/poor perfusion and/or anemia.
Dark membranes (deep pink, red, brown, purple) may be due to poor perfusion, vascular congestion, septicemia, and/or toxemia.
Bluish membranes may indicate poor perfusion.
Yellow membranes occur with elevated bilirubin concentrations (icterus/jaundice). Horses with reduced feed intake often have a mild yellow component to the membrane color. Icterus also occurs to a more severe extent with hemolysis (intravascular or extravascular) or with liver disease.
Hemorrhagic membranes may present as “pinpoint” to “expansive” lesions (petechiae to ecchymosis). These may occur with thrombocytopenia, coagulopathies or microangiopathies.
Moistness:
Hydration is assessed by:Observation and by touching the membranes.Normal membranes are moist.Tacky to dry membranes can occur due to dehydration resulting from many disease processes. The effects of dehydration can be categorized into three groups:
Mild dehydration (~5% of body weight) – slight tacky membranes. Skin turgor is normal and eyes are not sunken.
Moderate dehydration (6–8% of body weight) – moderately tacky membranes and increased duration of skin tenting (decreased turgor). Eyes are not sunken.
Severe dehydration (10–12% of body weight) – markedly tacky membranes, decreased skin turgor (prolonged tenting), eyes are sunken.
Note: Caution should be used when assessing sunken eyes as this may also occur with a decrease in the retrobulbar fat pad due to weight