The Virtual Hospital: ElectricPE: The Diagnosis of Pulmonary Embolism: Obesity and New Atrial Fibrillation

ElectricPE: The Diagnosis of Pulmonary Embolism: Case Study 1

Obesity and New Atrial Fibrillation

Jeffrey R. Galvin, M.D. and James J. Choi, B.S.
Peer Review Status: Internally Reviewed

This case is a 72 year old, obese white female who was admitted to the hospital in acute renal failure related to cryoglobulinemia. One month after her admission she developed sudden onset of chest pain during the night. She described the pain as substernal pressure that was incompletely relieved by nitroglycerin. Her EKG showed new atrial fibrillation. Her arterial blood gases were as follows: PH 7.34 Pco2 36 Po2 122 On 2 liters of oxygen. She gained 3 kilograms over the last 3 weeks which the clinical team related to excess fluid intake.

A PA and lateral chest x-ray were obtained within hours of her developing chest pain. The PA showed that the heart was enlarged but unchanged from an exam obtained one week earlier. However, she had developed multiple linear densities in both lower lobes. This was interpreted as multiple areas of atelectasis and the possibility of pulmonary embolism was raised.

The perfusion scan obtained on the same day as the chest x-ray demonstrated a rounded perfusion defect in the posterior segment of the left upper lobe. In addition, there are large but non-segmental areas of minimal inhomogeneity in the lower lobes.

The ventilation scan shows homogenous distribution of radiotracer throughout both lungs. There is minimal retention of radiotracer in the left lower lobe on the washout phase.

The final reading of the V/Q scan suggested that the risk of pulmonary embolus was low.

Based on the reading of the Ventilation Perfusion scan it was decided to dismiss the diagnosis of pulmonary embolus. The patient was diuresed and alive one week later.

Case Assessmnt
This woman who is at bed rest presents with:

Sudden onset of shortness of breath
New atrial fibrillation
Rrespiratory alkalosis

Based on the low probability reading of her ventilation perfusion scan the clinical team decided that no further work up was needed. The problem with that line of reasoning relates to the poor reliability of the low probability category. If you look at the V/Q scan in the current case you can see many areas of minimal inhomogeneity in the perfusion scan. You may even disagree with the reading. This is not uncommon with indistinct or small abnormalities. Based on information from the PIOPED study the agreement between readers as to what constitutes a low probability scan is poor.

As you will note in this case, there are multiple areas of linear atelectasis on the chest x-ray that had minimal effect on the ventilation scan. This is not a surprise as Eugene Robin pointed out 15 years ago, that the spatial resolution of ventilation scan is rather poor and that there is an inherent "mismatch" between the spatial resolution of the perfusion scan and ventilation. The perfusion scan has better spatial resolution and as a consequence the concept of the ventilation perfusion mismatch has greater utility with large defects.

This is a patient at risk who develops classic symptoms and signs of pulmonary embolus. I believe that further investigation was warranted. Either a non-invasive assessment of her legs looking for deep venous thrombosis or a pulmonary angiogram would have been justified.

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