Nitrogen washout test

The nitrogen washout test is helpful in trying to distinguish cardiac and respiratory causes of cyanosis. It works on the assumption that if there is right to left shunting in cyanotic heart disease, no amount of oxygenation in the pulmonary circulation will alter the desaturating effect of the shunt. However, if there is a pulmonary defect causing cyanosis this may be corrected by increasing the inspired oxygen.

The test is carried out by placing the infant in 100% oxygen for ten minutes. If the infant remains cyanotic after this period, the cyanosis is said to be secondary to cyanotic heart disease. This can be defined in blood gases as follows:

• arterial oxygen less than 20 kPa, cyanotic heart disease likely
• arterial oxygen less than 27 but more than 20, equivocal
• arterial oxygen more than 27 kPa, respiratory disease

Clearly there are exceptions to this rule - severe respiratory disease may result in persistant cyanosis even in 100% inspired oxygen. However, it remains a helpful test in the absence of imaging, and may avoid an inappropriate referal.

Note that most cardiologists accept that the nitrogen washout test is unlikely to close the duct, and thus is not contraindicated in duct dependant lesions. However, some clinicians prefer either not to use the test or to have prostocyclin E2 to hand to reopen a duct if needed.