Examining axial diffusion of nitric oxide in the lungs using heliox and breath hold

Exhaled nitric oxide (NO) is highly dependent on exhalation flow; thus exchange dynamics of NO have been described by multicompartment models and a series of flow-independent parameters that describe airway and alveolar exchange. Because the flow-independent NO airway parameters characterize features of the airway tissue (e.g., wall concentration), they should also be independent of the physical properties of the insufflating gas. We measured the total mass of NO exhaled ( AI,II) from the airways after five different breath-hold times (5–30 s) in healthy adults (21–38 yr, n = 9) using air and heliox as the insufflating gas, and then modeled AI,II as a function of breath-hold time to determine airway NO exchange parameters. Increasing breath-hold time results in an increase in AI,II for both air and heliox, but AI,II is reduced by a mean (SD) of 31% (SD 6) ( P < 0.04) in the presence of heliox, independent of breath-hold time. However, mean (SD) values (air, heliox) for the airway wall diffusing capacity [3.70 (SD 4.18), 3.56 pl·s−1·ppb−1 (SD 3.20)], the airway wall concentration [1,439 (SD 487), 1,503 ppb (SD 644>)], and the maximum airway wall flux [4,156 (SD 2,502), 4,412 pl/s (SD 2,906)] using a single-path trumpet-shaped airway model that considers axial diffusion were independent of the insufflating gas ( P > 0.55). We conclude that a single-path trumpet model that considers axial diffusion captures the essential features of airway wall NO exchange and confirm earlier reports that the airway wall concentration in healthy adults exceeds 1 ppm and thus approaches physiological concentrations capable of modulating smooth muscle tone.

Probing the impact of axial diffusion on nitric oxide exchange dynamics with heliox

Exhaled nitric oxide (NO) is a potential noninvasive index of lung inflammation and is thought to arise from the alveolar and airway regions of the lungs. A two-compartment model has been used to describe NO exchange; however, the model neglects axial diffusion of NO in the gas phase, and recent theoretical studies suggest that this may introduce significant error. We used heliox (80% helium, 20% oxygen) as the insufflating gas to probe the impact of axial diffusion (molecular diffusivity of NO is increased 2.3-fold relative to air) in healthy adults (21–38 yr old, n = 9). Heliox decreased the plateau concentration of exhaled NO by 45% (exhalation flow rate of 50 ml/s). In addition, the total mass of NO exhaled in phase I and II after a 20-s breath hold was reduced by 36%. A single-path trumpet model that considers axial diffusion predicts a 50% increase in the maximum airway flux of NO and a near-zero alveolar concentration (CaNO) and source. Furthermore, when NO elimination is plotted vs. constant exhalation flow rate (range 50–500 ml/s), the slope has been previously interpreted as a nonzero CaNO (range 1–5 ppb); however, the trumpet model predicts a positive slope of 0.4–2.1 ppb despite a zero CaNO because of a diminishing impact of axial diffusion as flow rate increases. We conclude that axial diffusion leads to a significant backdiffusion of NO from the airways to the alveolar region that significantly impacts the partitioning of airway and alveolar contributions to exhaled NO.

Gas Exchange in Abdominal Cavity during Laparoscopy

Gas exchange occurring in the abdominal cavity during laparoscopy, using carbon dioxide as the insufflating gas, was investigated in 25 female patients being ventilated with 66.6% nitrous oxide and 33.3% oxygen. The gas remaining in the abdomen at the end of the procedure was collected and measurements were made using an infrared spectrometer, a paramagnetic analyser and a mass spectrometer. The mean duration of the laparoscopy was 9.5 minutes and the mean volume of carbon dioxide delivered was 6.8 litres. Nitrous oxide concentration in the abdomen was found to increase significantly with the duration of the procedure, varying from 1.4% to 12.8% with a mean of 4.3% (s.d.±2.4). Oxygen concentration measured from 0.1 to 1.8% with a mean of 0.7% (s.d.±0.4). Nitrogen concentration varied from zero to 1.8%, having a mean concentration of 0.8% (s.d.±0.5). Carbon dioxide content was from 85.7 to 99.6% with a mean concentration of 94.2% (s.d.±3.1).