DLCO
DLCO is measured using the following techniques:
Pulmonary function tests are performed to assess lung function. Spirometry is the most common and widely used lung function test, followed by diffusing capacity of the lungs for carbon monoxide (DLCO). It is also known as the transfer factor. DLCO is a measurement to assess the lungs’ ability to transfer gas from inspired air to the bloodstream.
The respiratory membrane forms the diffusing barrier. It separates air within the alveoli from blood flowing in the pulmonary capillaries. It consists of the following layers:
The patient is then asked to hold his breath for 10 seconds at total lung capacity (TLC). Subsequently, the patient exhales out completely, and exhaled gas is collected for analysis after excluding the initial amount of gas from dead space. The collected gas is analyzed for CO and tracer concentrations. Total lung volume, initial and final CO concentration, and breath-holding time are used to calculate DLCO. The recommended timing method used is the Jones and Meade method, which measures breath holding time at thirty percent of inspiratory time up to half of the sampling time. Usually, an average of two or more attempts is considered for DLCO calculation in the single breath-holding technique.
Another method to calculate DLCO is the intrabreath method, which is calculated during exhalation. The gas that exits during the initial phase of exhalation has less time to diffuse from alveoli to capillaries and will have a higher concentration of CO compared to the gas during later stages of exhalation. The difference between various exhaled gas samples can be used to calculate DLCO.
The rebreathing technique is less frequently used, usually during exercise studies. However, a study carried out by Liu et al. showed the rebreathing technique to be more consistent with respiratory system physiology and could be a better test in measuring diffusing capacity than the intrabreath method.[5] Another study conducted by Suzuki et al. showed that the intrabreath method is a reliable alternative to the single breath method to measure diffusion capacity.