Which monitoring devices are recommended for AAA surgery?
Standard monitors are placed for all patients presenting for aortic surgery. ECG with the ability to monitor limb and precordial leads (II and V5 at the minimum) is desir-able to detect myocardial ischemia and dysrhythmias. Foley catheterization is also prudent in order to assess volume status and to provide an early indication of renal malperfusion. Because of the increased incidence of myocardial ischemia, coagulopathy, and wound infection, patients should remain normothermic perioperatively.
Anesthesia for aortic surgery is frequently complicated by sudden blood pressure lability. Shifts in intravascular volume, effects of anesthetic agents, and/or surgical manipulations are the major causes. Sudden hemorrhage may induce severe hypovolemia at almost any time. Dramatic reductions in preload may be due to blood loss from intercostal artery back-bleeding, aortic disruptions, extensive anastomotic suture leaks, evaporative losses, and third-spaced fluids. Large-bore intravenous access is extremely important in aortic surgery. Aortic rupture may occur at any time and the ability to rapidly infuse intra-venous fluids, blood, or blood products is necessary. One or two large-bore peripheral intravenous (IV) lines are recommended along with some form of central venous access. A rapid infusion system with a heat exchanger should be immediately available. Blood salvaging techniques for autotransfusion should be used. The easiest technique for blood salvaging is a centrifugal device, which scavenges and washes erythrocytes. The disadvantages of this tech-nique are the delays related to filling the centrifugal bowl, processing the blood, as well as the loss of plasma volume, proteins, coagulation factors, and platelets. Normovolemic hemodilution may theoretically decrease the need for banked blood, but has been severely criticized.
This surgical population also includes many patients with occlusive coronary artery disease, in whom hemody-namic aberrations may induce myocardial ischemia by adversely affecting the myocardial oxygen supply and demand balance. The use of aggressive perioperative β blockade may decrease postoperative cardiac complica-tions. A safe and reliable method of measuring acute changes in the blood pressure is required during aortic surgery. Intra-arterial monitoring accomplishes this goal by providing a continuous, beat-to-beat indication of the arterial pressure and waveform. Furthermore, an indwelling arterial catheter enables frequent sampling of arterial blood for laboratory analyses.
Central vein cannulation is routinely performed during aortic surgery. It allows for measurement of cardiac filling pressures (using a central venous catheter or a pulmonary artery catheter), provides a reliable route for drug delivery, and offers a site for rapid fluid administration. Ideally, central venous access can be accomplished with a large-bore cannula (“introducer”) in the right internal jugular vein or left subclavian vein.
Central venous pressures (CVPs) do not give direct indications of left heart filling pressures, but they may estimate left-sided filling pressures in patients with good left ventricular function. CVP has been shown to correlate with left-sided filling pressures during a change in volume status in patients with coronary artery disease (CAD) and ejection fractions (EF) >0.4. Other studies have not shown a consistent relationship between the CVP and the pulmonary capillary wedge pressure (PCWP).
Pulmonary artery catheterization (PAC) should be strongly considered in patients with decreased ventricular function, pulmonary hypertension, severe valvular disease, or advanced systemic organ dysfunction. Monitoring right-sided pressures as the sole indication of volume status is probably not sufficient because right heart pressures do not reflect left heart preload. PACs may also be used to determine afterload, cardiac output by thermodilution, and oxygen delivery by measuring pulmonary artery oxygen saturation. Although PACs are used frequently in AAA surgery, no study has demonstrated better outcomes when they are employed.
Transesophageal echocardiography (TEE) is a very use-ful monitoring modality during aortic surgery. Ventricular dysfunction and regional wall motion abnormalities may be diagnosed. TEE produces images of the heart and great vessels, affording such information such as regional wall motion abnormalities, indirect measurements of stroke volume and EF, valvular abnormalities, and aortic and pericardial pathology. Ventricular function and intravascu-lar volume status are probably the best indications for using TEE during these procedures. Personnel without proper training or credentials should exercise caution in using TEE in this modality.