Advances in Pulmonary Laboratory Testing: Exhaled NO Measurement
Although the use of NEP and the plotting of the tidal FV loops are fairly easily applied techniques that advance our understanding of VE limitation in various populations without the need for more invasive studies (ie’ esophageal and gastric balloons). other factors that may contribute significantly to exercise limitation involving the pulmonary system clearly cannot be negated (ie’ the work and cost of breathing’ pulmonary hemodynamics’ pulmonary influences on cardiac function’ varying perceptions of dyspnea’ and gas-exchange abnormalities).
Exhaled NO Measurement
NO is a highly reactive molecule formed by the enzyme NO synthase (NOS) from the precursor amino acid L-arginine. It was first described as an endogenous endothelium-derived nitrovasodilator that acts by directly activating guanylate cyclase’ leading to increased cytosolic guanylate monophosphate by forming a reversible adduct with the heme moiety of the enzyme. Similarly’ it may be readily inactivated by hemoglobin or cytochromes; thus’ NO released into the blood stream is rapidly scavenged. It is a colorless gas that also can be carried in solution in plasma or in lipid membranes. In gaseous form’ NO reacts directly with oxygen to form N2O’ but in the absence of oxygen it can be quite stable. Its half-life in biological media is also quite short. ventolin inhalers
The roles of NO in physiologic processes are still being elucidated. It acts as a neurotransmitter in many tissues’ including the nonadrenergic noncho-linergic nervous system that innervates the pulmonary airway. There are at least four isoforms of the enzyme NOS: three constitutive forms and one inducible form. The constitutive forms require calcium for activation and produce a steady background low level of NO’ whereas the inducible form of NOS is not calcium dependent and plays a role in adaptive processes such as inflammation. In the lung’ the constitutive enzymes are found in endothelial cells, epithelial cells, smooth muscle cells, and inflammatory cells.