The European Respiratory Society has just published the first standards for DLNO testing. This is a signal that DLNO is moving from a research setting into routine clinical testing. Although it is unlikely that most PFT labs will immediately jump into DLNO testing, the standard is still interesting because of an extensive discussion of DLNO, DLCO, DMCO and Vc measurements and physiology. The DLNO standards (and their supplementary material) are open-access and can be downloaded from the European Respiratory Journal.
DLNO is performed in the same manner as a single-breath DLCO and it is specifically recommended that DLCO and DLNO tests be performed simultaneously. There are however, specific test system requirements based both on the properties of NO and on the two types of NO analyzers:
- Nitric Oxide reacts with oxygen to form NO2 and at the levels used for DLNO testing (40-60 ppm) does so at a rate of approximately 1.2 ppm per minute. DLNO test gas is therefore usually stored as 400-1200 ppm NO in N2 and mixed into the DLCO test gas mixture (0.3% CO, 21% O2) ≤2 min before the DLCO/DLNO test. This would seem to require that the DLCO/DLNO test gas mixture to be held in a reservoir of some kind and to preclude the use of a demand valve but this was not specifically discussed. Because of uncertainties that occur when mixing the DLCO/DLNO gas mixture and in how long the mixture may be held in the reservoir the inspired NO concentration must also be measured immediately before the DLCO/DLNO test is performed.
- The type of NO gas analyzer will determine how the expiratory gas concentrations are measured. Chemiluminescent analyzers usually have a response time on the order of ≤70 msec, and for these reasons can be used to perform a real-time analysis of exhaled air. Chemiluminescent analyzers are expensive however, and can add significantly to the cost of a test system. Electrochemical cells are significantly less expensive but have a response time on the order of 10 seconds and are therefore suitable only to test systems that mechanically collect an alveolar sample.
Oxygen transport between the lungs and the body depends on numerous complex factors. Ventilation and the alveolar-capillary surface area are of course important but a critical component is hemoglobin. Oxygen is poorly soluble in water (which is what blood is mostly made of) and the transportation of oxygen throughout the body would not happen without hemoglobin’s ability to absorb and release oxygen on demand. Although it is possible to measure the diffusing capacity of oxygen (DLO2) the process is technically difficult and not at all suited to routine clinical testing.
There are a number of gases that are able to diffuse across the alveolar-capillary membrane and can be used in a variety of physiological measurements but in order for a particular gas to act as a substitute for oxygen it must be able to interact with hemoglobin. Carbon monoxide (CO) has an affinity for hemoglobin approximately 220 times greater than oxygen and was the first gas used to measure diffusing capacity (DLCO). DLCO has been a routine test for well over 50 years and has been measured by single-breath, steady-state and rebreathing techniques.
Nitric Oxide (NO) has an affinity for hemoglobin about 400 times greater than carbon monoxide (it is generally an irreversible process since the end product is methemoglobin whereas hemoglobin’s binding with CO is more reversible) and for this reason it can also be used to measure diffusing capacity. DLNO can also be measured by single-breath, steady-state and rebreathing techniques. Because of its high affinity and the speed at which the binding of NO to hemoglobin occurs numerous researchers have assumed that DLNO is equivalent to DMNO (the membrane component of diffusing capacity). This is not really true, but it can be a useful fiction and in order to understand why it’s necessary to look at the basic physiology of diffusing capacity tests.
Roughton and Forster’s seminal 1957 paper showed that diffusion is the sum of two resistances. I’ve discussed this previously but specifically:
DMCO = membrane component
θCO = the rate at which CO binds to hemoglobin
Vc = pulmonary capillary blood volume
The first resistance (1/DMCO) is the resistance to the diffusion of CO through the alveolar-capillary membrane and blood plasma to the surface of the stagnant plasma boundary layer around a red blood cell. The second resistance refers to the diffusion rate of CO through the plasma boundary layer, then the wall and interior of the red blood cell and finally the rate of reaction with hemoglobin.