Hensley's Practical Approach to Cardiothoracic Anesthesia

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III. Cardiac Anesthesia

peripheral intravenous (IV) is inserted, it is reasonable to start IV fluids in the preoperative area at a maintenance rate. Many patients undergoing surgical myectomy are young and ner vous, and anxiolytic such as midazolam 1-2 mg IV should be offered. However, anxiolysis can downregulate catecholamine excess and provoke hypotension, exacerbating LVOT obstruc tion, so careful monitoring after premedication is recommended. Preinduction arterial line placement should be performed for patients with particularly severe LVOT gradients, severe pulmonary hypertension, and a history of arrhythmias. B. Intraoperative Management 1. General considerations Surgical myectomy for HCM is usually a relatively short procedure, with cardiopulmo nary bypass time < 45 minutes. Patients with HCM can exhibit swings in blood pressure in response to anesthetic agents, positioning, and surgical stimulation. Thus, emergency drugs, such as phenylephrine, vasopressin, and esmolol should be available to use. Goals of intraoperative management include maintenance of LV preload and afterload, avoidance of tachycardia, and suppression of myocardial hypercontractility as needed. Ultimately, whatever mix of anesthetics and analgesics is selected, it should be tailored to enhance preload and afterload and only result in a mild decrease in contractility. Inha lation anesthetics reduce myocardial contractility and potentially could reduce dynamic LVOT gradient, but these agents may also reduce peripheral vascular resistance, which, conversely, might increase the dynamic LVOT outflow gradient. The net effect would de pend on the relative balance between these two effects and would be influenced by the use of vasopressors to offset the vasodilatory effect of volatile agents. Preoperative fasting and relative hypovolemia, typically offset by the infusion of fluids, are among other factors that might affect the venous return and change cardiac chamber dimensions compared to preoperative measurements. These processes reduce cardiac output and might shift the equilibrium between “forward” cardiac output and SAM-related MR. Autologous periop erative blood collection in patients with HCM is not routinely performed as it can cause intravascular volume shifts, which are poorly tolerated. Commonly used narcotics such as fentanyl and sufentanil have vagotonic effects that will reduce heart rate, increase diastolic time, and improve LV filling. 2. Induction and maintenance of anesthesia A commonly used technique is balanced IV induction with fentanyl 1-2 μg/kg, ketamine 1 mg/kg, propofol 0.5-1 mg/kg, and rocuronium 1-1.2 mg/kg. A bolus of phenylephrine 50-100 μg/kg is usually administered with the induction drugs to prevent an acute drop in systemic vascular resistance and hypotension. After endotracheal intubation, anesthesia is maintained with 1 MAC (minimum alveolar concentration) of isoflurane. 3. Separation from cardiopulmonary bypass Typically, patients with HCM separate from cardiopulmonary bypass without difficulty. However, it is important to vent the LV of air before separation from bypass. Transient right ventricular dysfunction can occur from air bubbles entering right coronary artery located superiorly. Full cardiopulmonary bypass support or partial bypass with higher mean arte rial pressure can drive air out of coronary circulation and restore contractility. Inotropic support is usually not required. Once hemodynamic conditions have stabilized, TEE should be performed to confirm normalization of ventricular function. An augmentation of sys temic vascular resistance with boluses of phenylephrine and/or ephedrine helps to separate from bypass. Care should be taken to return blood from the cardiopulmonary bypass to provide adequate preload so that the assessment of residual gradients and MR is performed in the state as close as possible to the physiologic conditions with normal arterial and cen tral venous pressures. Post-bypass TEE should rule out potential iatrogenic complications (including ventricular septal defect and aortic valve injury), and the mitral valve should be assessed to determine whether SAM is still present as well as the severity of residual MR. TEE should also confirm the absence of turbulent flow in LVOT (Figure 16.3D), with care taken to assess both the LVOT and the mid-LV for the presence of obstruction. It is prudent to remeasure the gradient (or rather its absence) between the aorta and the LV to confirm

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