From the Guidelines
Pulmonary embolism (PE) typically causes characteristic arterial blood gas (ABG) abnormalities including hypoxemia, hypocapnia, and respiratory alkalosis, as indicated by the 2019 ESC guidelines for the diagnosis and management of acute pulmonary embolism 1. When a PE occurs, blood clots block pulmonary arteries, creating ventilation-perfusion mismatch where areas of the lung are ventilated but not perfused. This leads to decreased oxygen transfer into the bloodstream, resulting in hypoxemia, often with a low PaO2, and hypocapnia, with a low PaCO2, as well as respiratory alkalosis, with an elevated pH 1. Some key points to consider in the ABG analysis of PE patients include:
- Hypoxemia is frequent, but ≤40% of patients have normal arterial oxygen saturation (SaO2) and 20% have a normal alveolar–arterial oxygen gradient 1
- Hypocapnia is also often present, due to hyperventilation in response to hypoxemia and possibly from stimulation of pulmonary receptors
- An increased alveolar-arterial oxygen gradient (A-a gradient) is also commonly observed These ABG findings, while suggestive of PE, are not specific and must be interpreted alongside clinical presentation and imaging studies, such as chest X-ray and electrocardiographic changes indicative of RV strain, as well as knowledge of the predisposing factors for VTE 1. In massive PE, patients may develop respiratory failure with hypercapnia and respiratory acidosis, which can be life-threatening and requires immediate medical attention. ABG analysis helps guide oxygen therapy and assess the severity of the PE but should not delay definitive diagnostic testing like CT pulmonary angiography or treatment with anticoagulation when PE is strongly suspected, in order to minimize morbidity, mortality, and improve quality of life.
From the Research
PE ABG Abnormality
- The diagnostic value of arterial blood gas (ABG) measurement in suspected pulmonary embolism (PE) has been evaluated in several studies 2.
- According to a study published in the American Journal of Respiratory and Critical Care Medicine, ABG data alone or in combination with other clinical data are not useful in the assessment of suspected PE 2.
- The study found that none of the ABG data or prediction rules had sufficient negative predictive value, specificity, or likelihood ratios to be useful in the management of patients with suspected PE 2.
- Other studies have focused on the treatment of PE, including the use of low-molecular-weight heparin (LMWH) and unfractionated heparin (UFH) 3, 4, 5, 6.
- A study published in the British Journal of Clinical Pharmacology discussed the initial treatment of haemodynamically stable patients with PE, including the use of LMWH and direct oral anticoagulant drugs (DOACs) 3.
- Another study published in The New England Journal of Medicine compared the use of LMWH with UFH in patients with symptomatic pulmonary embolism 4.
- A review published in Prescrire International discussed the initial treatment of deep venous thrombosis and PE, including the use of LMWH, UFH, and thrombolytic agents 5.
- A prospective, observational multicenter trial published in Clinical and Applied Thrombosis/Hemostasis found that LMWH treatment can be used safely in patients with PE after thrombolytic therapy 6.