Do not use a Normal Arterial Blood Gas to rule out a Pulmonary Embolism

Do not use a Normal Arterial Blood Gas to rule out a Pulmonary Embolism

David N. Hager MD

Venous thromboembolic disease affects 200,000 persons in the United States annually. Among affected persons, almost half suffer symptomatic pulmonary embolism (PE). Without treatment, 30% of patients will die within 1 year. However, despite its high incidence, PE is a difficult diagnosis to make. Clinical presentations vary and the symptoms are often nonspecific. Though an abnormal arterial blood gas can be informative, normal indices of oxygenation and ventilation do not rule out PE.

Risk Factors

The major risk factors for PE are the same as those for venous thrombosis. Broadly defined, these include a prior thromboembolic event, neoplastic disease, immobility, recent surgery or trauma, estrogen replacement therapy (especially in the context of tobacco use), and a family history of hypercoaguability such as activated protein C resistance, hyperhomocysteinemia, proteins C and S deficiency, antithrombin III deficiency, and factor V Leiden.

Signs and Symptoms

PE most commonly presents as a triad of dyspnea (70% to 80%), pleurisy, and tachypnea (RR >20). Among those patients enrolled and proven to have PE in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study, 97% exhibited one of these three symptoms. A less common presentation is hemo-dynamic compromise, which may be severe enough to cause syncope. The elderly often present with symptoms consistent with unresolving pneumonia or heart failure despite appropriate therapy. On clinical exam an enhanced second heart sound, tachycardia, and jugular venous distention may be appreciated. A normal chest radiograph is consistent with PE, as are other findings such as pleural effusion, Westermark sign (a focal loss of peripheral vascular markings), and even focal infiltrates. Tachycardia and nonspecific ST-T wave changes are the most common abnormal finding on an electrocardiogram (ECG). The frequently discussed S1Q3T3 pattern is present in less than 12% of patients with PE.

Hypoxia (PaO2 <80 mm Hg), hypocapnia (PaCO2 <35 mm Hg), and an elevated alveolar-arterial oxygen difference (P[A–a]O2 >20mm Hg) are the most common arterial blood gas abnormalities in patients with PE. However, in a group of subjects suspected of having PE, these indices did not effectively discriminate between those ultimately proven to have PE and those who did not. Further, among individuals without prior lung disease, these indices will often fall within normal limits even in the presence of PE. For these reasons, arterial blood gas data contribute little to the diagnosis or exclusion of PE.

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Jul 1, 2016 | Posted by in ANESTHESIA | Comments Off on Do not use a Normal Arterial Blood Gas to rule out a Pulmonary Embolism
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