Are arterial blood gases necessary in the evaluation of acutely dyspneic patients?

The recent study by Burri and colleagues examined the usefulness of arterial blood gases (ABG) in the diagnosis and prognosis of 530 dyspneic patients with ABG drawn upon presentation to the emergency department [1]. The study was a retrospective analysis of prospectively collected data performed at a single center. They concluded that ABG cannot be used to distinguish between pulmonary and other causes of dyspnea.Prior to the introduction of automated blood gas analyzers, blood PO2 and PCO2 could be obtained only by laborious and often inaccurate laboratory methods, such as gas tonometry and Van Slyke manometric extraction of plasma total carbon dioxide. Improvements in electrode technology came with the development of the Astrup pH electrode [2], the Stow/Severinghaus-type PCO2 electrode [3,4] and the polarographic oxygen Clark electrode [5]. Although pulse oximetry has largely superseded the use of arterial PO2 (PaO2), the ease by which ABG can now be measured has made this test de rigueur in the workup of dyspneic patients.We agree with Burri and colleagues in that PaO2 values serve mainly to triage and to guide treatment, rather than to differentiate among the causes of dyspnea. Except for anxiety hyperventilation, it is difficult to rely solely on ABG to identify a specific clinical syndrome. This is not new information. The poor predictive value of PaO2 in diagnosing patients with pulmonary embolism is well known. In the PIOPED I study of patients with angiographically proven pulmonary embolism and no prior cardiopulmonary disease, 26% had PaO2 >80 mmHg [6]. This figure was 38% in the PIOPED II study (n = 42) [7,8]. The probability of diagnosing an acute pulmonary embolism based on changes in PaO2 did not achieve statistical significance. Further, no combination of PO2 and PCO2 values could reliably exclude pulmonary embolism.Burri and colleagues also report that arterial pH was a significant predictor of short-term and long-term outcome. Multiple physiologic