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.
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