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PULMONARY EMBOLISM ISN’T WELL-DIAGNOSED WITH ARTERIAL BLOOD GASES

Clinical bottom lines:

  1. Even in patients with no known previous pre-existing cardiac or pulmonary disease, arterial blood gas (ABG) analysis is not very helpful in the diagnosis/exclusion of pulmonary embolus.

  2. PaO2 ³ 100 mmHg on room air has LR of 0.38 of pulmonary embolus, but PaO2 of 90-99 has a LR of 1.

  3. The calculation of the (A-a) gradient only has moderate predictive ability and does not rule out PE.

Appraised by: HN Lee, January 19th 1993

The Evidence:
From the Prospective investigation of Pulmonary Embolism Diagnosis (PIOPED) study.
The Gold Standard: Pulmonary angiography, autopsy or negative follow-up.


The Study Setting: Multicentre study of 365 patients with no pre-existing cardiopulmonary disease. ABG's were done on Room Air (RA) in 88/117 patients with proven PE and in 202/248 patients without PE.

Diagnostic test

Result (ABG on RA)

Pulmonary Embolus

Present

Pulmonary Embolus

Absent

Likelihood Ratio of

Pulmonary Embolus present

Pa O2 < 60 mmHg

22

44

22/88 ¸ 44/202 = 1.14

Pa O2 ³ 90 mmHg

11

32

11/88 ¸ 32/202 = 0.79

Pa O2 ³ 100 mmHg

2

12

2/88 ¸ 12/202 = 0.38

P(A-a) O2 > 50 mmHg

22

40

22/88 ¸ 40/202 = 1.26

P(A-a) O2 £ 20 mmHg

12

44

12/88 ¸ 44/202 = 0.63

Total

88

202

  

Comments:

  1. 29/117 patients in the PE group and 46/248 patients in the non-PE group were excluded because no ABG's were available. This might have biased the results if all the PE group patients did not have ABG's done because the patients needed supplemental O2. However, even with this unlikely situation the results would not change dramatically.

  2. It has previously been shown [2] that patients with PE can have normal A-a gradients.

Citations:

    [1] Stein PD et al. Clinical, Laboratory and Electrocardiographic Findings in Patients with Acute PE and no pre-existing Cardiac or Pulmonary Disease. Chest 1991;100(3):596-603.

    [2] Overton DT. The A-a O2 grad.in patients with documented PE. Ann Intern Med 1988;148:1617-9.


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