Q&A: Investigating for Suspected Respiratory Failure

Respiratory failure is commonly defined as respiratory dysfunction resulting in abnormalities of oxygenation and/or carbon dioxide (CO2) elimination and is classified as either hypoxemic (type I) or hypercapnic (type II), or a combination of both. These distinctions are clinically important and have diagnostic and therapeutic implications.

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This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Breathlessness article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.

In this article:
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

MCQ exam: clinical scenario

A 55 year old man who, in the past, had regularly seen his general practitioner for his hypertension suddenly stopped attending clinic. Three years later, he is admitted for shortness of breath relieved by rest, orthopnea and paroxysmal nocturnal dyspnea. Which of the following is not considered a routine investigation in patients with suspected respiratory failure?

a). Chest x-ray
b). Arterial blood gas analysis
c). Right-sided heart catheterization
d). Electrocardiography
e). A chemistry panel

MCQ exam: answer

The correct answer is C.

MCQ exam: explanation

Respiratory failure may be associated with a variety of clinical manifestations. However, these are nonspecific, and very significant respiratory failure may be present without dramatic signs or symptoms. This emphasizes the importance of measuring arterial blood gases in all patients who are seriously ill or in whom respiratory failure is suspected. Chest x-ray is essential. Echocardiography is not routinely done in all patients with respiratory failure. However, it is a useful test when a cardiac cause of acute respiratory failure is suspected. Pulmonary functions tests, if feasible, may be helpful. Electrocardiography should be performed to evaluate the possibility of a cardiovascular cause of respiratory failure; it also may detect dysrhythmias resulting from severe hypoxemia or acidosis. Right-sided heart catheterization is controversial.

LABORATORY STUDIES
Once respiratory failure is suspected on clinical grounds, arterial blood gas analysis should be performed to confirm the diagnosis and to assist in the distinction between acute and chronic forms. This helps assess the severity of respiratory failure and helps guide management.

A complete blood count (CBC) may indicate anemia, which can contribute to tissue hypoxia, whereas polycythemia may indicate chronic hypoxemic respiratory failure.

A chemistry panel may be helpful in the evaluation and management of a patient in respiratory failure. Abnormalities in renal and hepatic function may either provide clues to the etiology of respiratory failure or alert the clinician to complications associated with respiratory failure. Abnormalities in electrolytes such as potassium, magnesium, and phosphate may aggravate respiratory failure and other organ function.

Measuring serum creatine kinase with fractionation and troponin I helps exclude recent myocardial infarction in a patient with respiratory failure. An elevated creatine kinase level with a normal troponin I level may indicate myositis, which occasionally can cause respiratory failure.

In chronic hypercapnic respiratory failure, serum levels of thyroid-stimulating hormone (TSH) should be measured to evaluate the possibility of hypothyroidism, a potentially reversible cause of respiratory failure.

Reference(s)
1). ACP Hospitalist. Revisiting respiratory failure. Available online: http://www.acphospitalist.org/archives/2013/10/coding.htm