Imaging Studies

Echocardiography, either transthoracic or transesophageal, may demonstrate right atrial, right ventricular, or proximal pulmonary arterial emboli. If right ventricular dysfunction is present in combination with normal systemic hemodynamics the need for careful monitoring and consideration of more aggressive therapy is suggested.18 Transesophageal echocardiography is also useful in excluding thoracic aortic dissection as well as detecting massive PE in transit in hemodynamically unstable patients.18!9 The ability to perform these tests at the bedside is especially useful in critically ill patients.

other studies are used to detect DVT in the lower extremities, since the treatment of DVT and hemodynamically stable PE is the same. Noninvasive studies such as duplex ultrasonography (DUS) and impedance plethysmography (IPG) have largely replaced invasive contrast venography, although this last test remains the "gold standard." Using the criteria of vein compressibility, DUS is 95 to100 percent sensitive and more than 95 percent specific in diagnosing proximal DVT. Both IPG and DUS are insensitive in detecting DVT below the knee, but PE from this site is rare without proximal propagation. If DVT is strongly suspected and DUS or IPG are negative, serial exams are needed to detect extension of thrombus into the ileofemoral system.

V/Q scans of the lung often raise more questions than they answer. Although they are commonly used to diagnose or exclude PE, they have been shown to be reliable only at the extremes of interpretation—normal or high probability. All other results require further investigations to confirm or exclude PE, depending on the a priori probability.

one of the difficulties in interpreting V/Q scans is that they are reported as "probabilities" rather than normal or abnormal. This probability must then be judged based on the clinical suspicion for PE. Table 52-3 shows an interpretation of V/Q probabilities based on PIOPED data.™ This demonstrates that a high-probability scan ■ ■

(Fig 52-2/A and Fig 52-26) is only 80 percent accurate in diagnosing PE, while a low-probability result is only 20 percent accurate in excluding the diagnosis. Overall, 11 • Q

scans are 98 percent sensitive but only 10 percent specific in the diagnosis of PE.10 Infiltrate on CXR, preexisting cardiopulmonary disease, and previous PE all increase the rate of false-positive results.

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