Tests During An Asthma Attack

Blood gas measurements, chest x-ray, and electrocardiogram may be done during a severe asthma attack, along with lung function tests, such as spirometry or peak flow monitoring.1 These tests are done to:

  • Evaluate how severe the asthma attack is.
  • Check if you are in or near respiratory failure.
  • Evaluate other conditions that could make the attack worse.

Respiratory failure occurs when one or both of the following things happen:2

  • Not enough oxygen passes from your lungs into your blood.
  • Your body cannot get rid of extra carbon dioxide in your blood.

Signs of respiratory failure are drowsiness and confusion, absence of wheezing sounds, and slower heart rate. Your fingernails and lips may turn grayish or blue.

Pulse oximetry

Pulse oximetry measures how much oxygen your blood is carrying.3 The result it provides is called “oxygen saturation” (abbreviated SaO2 or O2 sat). Normal SaO2 is 95% or higher at sea level.1 The pulse oximeter is a small clip that is placed on your finger, toe, or earlobe. This device passes a beam of light through your blood to measure the amount of oxygen it is carrying. It also measures your heart rate. This test is painless.

Pulse oximetry is often used for infants and young children who are having an asthma attack.4 Young children cannot do lung function tests. Instead, their SaO2 indicates how severe the asthma attack is and whether the child needs to be hospitalized.1 Pulse oximetry is also used for adults who are in severe distress, have a very low FEV1, or cannot do a lung function test.1 Your FEV1 is the amount of air you can blast out of your lungs in the first second of exhaling, as measured with spirometry.

It is important to note that SaO2 can be within the normal range (95% to 100%) during an acute attack. Airway narrowing during an asthma attack means less air reaches the end of the airways, where little sacs called alveoli transfer oxygen into the blood. However, unless the alveoli are completely blocked, they continue to work normally, passing whatever oxygen is available to the blood. An SaO2 reading below 90% could indicate that respiratory failure is near.

Arterial blood gas

A blood test is used to measure arterial blood gases. A sample of blood will be taken from your arm or wrist. This test measures the amount of oxygen and carbon dioxide in your blood, as well as the pH (acidity) of your blood. The partial pressure of carbon dioxide (abbreviated PCO2) can indicate if you are in respiratory failure.1 It can also be used to indicate if oxygen therapy is working.

Chest x-ray

People with asthma do not usually need a chest x-ray. However, during an asthma attack, a chest x-ray might be used to check for co-existing diseases such as congestive heart failure, collapsed lung, or pneumonia.4 Occasionally, a chest x-ray might be used apart from asthma attacks to rule out other conditions that mimic asthma.

Electrocardiogram (ECG or EKG)

An electrocardiogram is sometimes done during an asthma attack for people who are older than 50 and people who have co-existing asthma and heart disease or COPD (chronic obstructive pulmonary disease).4

Written by: Sarah O'Brien | Last Reviewed: May 2016.
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