Invasive Monitoring Techniques

General Considerations

Invasive pressure monitoring should never be the initial step in resuscitation. When clinically indicated, arterial line or pulmonary artery catheter (PAC) placement may be considered after initial stabilization is completed. If possible, these procedures should be deferred until the patient reaches the more controlled environment of the intensive care unit, unless there will be a significant delay.

The two essential components of any pressure monitoring system are a properly placed and secured catheter and a functioning pressure transducer-monitor. Ideally, the transducer and line for pressure monitoring should be set up and ready for use prior to the patient's arrival in the emergency department. Examples of transducer systems are illustrated in Fig 18-1.

Zefron Arterial Line Anchor

FIG. 18-1. Arterial pressure monitoring systems. A. Systems for continuous flush with heparinized saline solution connected to a mechanical pressure transducer. B. System for manual flush. Either system can be used with an electronic pressure transducer, shown in B. The pressure done should be maintained at the level of the patient's heart. [From Beal JM (ed): Critical Care for Surgical Patients. New York, Macmillan, 1982. Used by permission.]

Arterial Cannulation

Arterial lines offer several advantages over monitoring blood pressure with an arm cuff. The arterial line provides continuous measurement of blood pressure and can be used for easy sequential sampling of blood gases. In the setting of marked vasoconstriction or hypotension, the arterial line usually gives more accurate pressure readings than a blood pressure cuff. The American College of Cardiology/American Heart Association practice guidelines for patients with acute myocardial infarction recommend intraarterial pressure monitoring for patients with severe hypotension or cardiogenic shock or who are receiving potent vasoactive infusions. 1 Other appropriate scenarios include hypertensive crisis, hypothermic cardiac arrest, and prolonged emergency department resuscitation.

The radial and femoral arteries are readily accessible for rapid cannulation. Percutaneous puncture is preferred. In hypotensive patients it may be easier to cannulate the femoral artery (because it is a larger vessel with constant landmarks) than the radial artery. Cutdown on the radial artery is an alternative in such patients. It is prudent to document the result of Allen's test for ulnar artery patency prior to radial arterial line placement, although its accuracy is questionable. 2 If the test result is positive, another site should be considered, but if that is not practical, then the clinician may have to proceed with radial artery cannulation. 3

Landmarks for radial and femoral artery cannulation are shown in Fig 18-2. The catheter (usually 20 gauge, 2 in long for radial cannulation and 18 gauge, 4 in long for femoral cannulation) can be introduced by direct puncture threaded over the needle or by Seldinger technique threaded over a guidewire. Freely flowing, pulsatile, bright-red blood indicates proper placement. With marked hypotension or hypoxia, arterial blood flow may be mistaken for venous (nonpulsatile, dark blood returned). In all cases, connection to the transducer should reveal an arterial waveform if the catheter is in the proper position. Failure to visualize a waveform can be due to venous placement, air in the line, a closed stopcock, or a malfunction in the transducer or monitor.

Femoral Vein Puncture Complication

FIG. 18-2. Anatomic landmarks for arterial line placement. A. Femoral triangle. Note that the femoral artery lies lateral to the vein and midway between the pubis and the iliac crest. B. Radial aspect of the wrist. Note that mild extension of the wrist aids in successful placement. [From Beal JM (ed): Critical Care for Surgical Patients. New York, Macmillan, 1982. Used by permission.]

The complication rate for either radial or femoral artery cannulation is about 7 percent. 4 These include local hematoma and hemorrhage; both can usually be controlled with a pressure dressing. Arterial occlusion, thrombosis, or embolization with distal ischemia may occur; they are associated with placement in smaller vessels or in atherosclerotic vessels, with prolonged catheterization, and with use of end arteries that supply areas with poor collateral circulation. Sepsis may result from local infection at the insertion site. Using the femoral or radial site can minimize these complications, along with proper attention to sterile technique and removal of the line as soon as feasible after the patient is stabilized.1

Pulmonary Artery Cannulation

A PAC is helpful in the measurement of critically ill patients with hemodynamic instability, particularly in the setting of acute myocardial infarction. Most important, the PAC can help to differentiate between shock due to intravascular volume depletion and that due to extensive left ventricular (LV) dysfunction. When the balloon tip of a PAC is properly wedged in a branch of the pulmonary artery, the pressure sensed by the catheter corresponds approximately to that in the left atrium. Left atrial pressure (which equals LV filling pressure) is an excellent indication of the adequacy of fluid resuscitation. If this pressure is low (<12 mmHg), additional fluid resuscitation is indicated. If this pressure is high (>20 mmHg), additional fluids are unlikely to improve cardiac performance; instead diuresis, afterload reduction, inotropic support, or vasopressors should be considered. Although the PAC can yield useful diagnostic information ( Table 18:1 and T.a,bl.e..1.8.:2), the distinction between the need for more fluids and the need for more LV support is its most useful application during resuscitation. 1 Volume assessment via central venous pressure monitoring is less reliable than PAC, especially in the presence of valvular or pulmonary disease.

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TABLE 1B-1 Hemodynamic Diagnosis of Shock States

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TABLE 18-2 Hemodynamic Subsets in Acute Myocardial Infarction

A standard PAC is shown in Fig 18-3. The catheter has two fluid paths, one that terminates at the tip (the distal port) and a second that opens 10 to 15 cm from the tip

(the proximal port). A third lumen connects to a 1.5-mL balloon located at the tip of the catheter. When inflated during insertion, the balloon surrounds the tip of the PAC to prevent it from causing injury to the heart or great vessels as well as to help float the catheter through the heart as it is advanced. A temperature sensor is located about 5 cm proximal to the catheter tip and can be used to measure cardiac output via the thermodilution technique.

FIG. 18-3. Pulmonary artery thermodilution catheter. [From Beal JM (ed): Critical Care for Surgical Patients. New York, Macmillan, 1982. Used by permission.]

The procedure for insertion is described in detail elsewhere 5 but briefly is as follows. After central venous access is secured (see Chap 17, "Vascular Access") the

PAC, which has already been attached to a monitor and a pressure transducer, is slowly advanced through the introducer sheath. Once the tip of the catheter enters the vein, the balloon is inflated. By observing the pressure waveform transmitted via the distal port, the operator can follow the progress of the catheter through the heart and, ultimately, into a branch of the pulmonary artery (Fig 18:4). Fluoroscopy is helpful to ensure quick and proper placement but rarely is available in the emergency department.

FIG. 18-4. Hemodynamic aspects of balloon catheter insertion into the pulmonary artery. [From Gottlieb AJ (ed): The Whole Internist Catalog. Philadelphia, Saunders, 1980. Used by permission.]

With the PAC in position, the clinician can occlude the branch pulmonary artery by inflating the balloon and wedging it in the artery. This gives the pulmonary artery occlusion pressure, or, as it is more commonly known, the pulmonary capillary wedge pressure, which best reflects the pressure in the pulmonary capillary bed and the left atrium. The clinician can also rapidly measure pulmonary artery pressure, cardiac output, and central venous pressure. These measurements, when combined with arterial pressure measurement, enable the clinician to calculate systemic vascular resistance. These parameters are useful in the diagnosis and treatment of various shock states and in guiding therapy in acute myocardial infarction ( Table 18:1 and IabJei18:2).

As with arterial cannulation, therapeutic procedures or definitive care should not be delayed solely to allow PAC placement. Complications include all the complications of central venous line placement (Chap, 17). In addition, cardiac dysrhythmias and right bundle branch block may occur as the catheter traverses the heart. Other potential complications include pulmonary embolism or infarction, knotting of the catheter, infection, and rupture of a small branch of the pulmonary artery.

The safety of PACs in the critical care environment has been questioned recently because of an association between its use and increased mortality rates. 6 Current consensus favors the continued use of PACs1 but underscores the need for definitive investigation.7 Thus, the appropriateness of PAC insertion in the emergency department setting for a given patient must be weighed carefully. The advent of newer techniques, such as transesophageal echocardiography, 89 transthoracic impedance cardiography,10 and magnetic resonance velocimetry11 may further reduce the role of PACs in the emergency department.

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