To the Editor:—
I read with interest the report from Nyktari et al. on the interaction between the physical properties of halogenated vapors and pulmonary resistance.1Their observations have important implications on the choice of anesthetic agents in selected patient populations.
These results are consistent with modeling their experimental apparatus as a simple orifice. Flow through an orifice is directly proportional to the square root of the pressure gradient across the orifice and inversely proportional to the square root of the density of the gas. This is shown in the following equation
where Q is the volumetric flow, ΔP is the pressure gradient, and ρ is the density. As the authors calculated resistance from the pressure gradient needed to deliver a constant flow, equation 1can be modified by squaring both sides and rearranging terms into a form analogous to Ohm’s law (V = IR):
In equation 2, the term Qρ corresponds to the resistance; when the flow is held constant, as in the described experiment, the resistance will increase linearly with the density of the gas. Plotting the density (calculated as the weighted sum of the molecular weights divided by the molar gas volume at standard temperature and pressure) of a mixture of various minimal alveolar concentration (MAC)-multiples of desflurane, sevoflurane, and isoflurane diluted in 25% oxygen and 75% nitrogen produces a graph that is strikingly similar to the authors’ figure 4 (see fig. 1). Using only the mean values in the authors’ figure 4, there is a strong linear relationship between MAC-multiple and resistance (isoflurane, P = 0.060, r2= 0.88; sevoflurane, P = 0.067, r2= 0.87; desflurane, P = 0.002, r2= 0.995). These relationships would have been even more significant had all the data points available from their figure 3 been included in the regression. The linear relationship between MAC-multiple and resistance is masked in figure 4 by the authors’ decision to make the interval from baseline (MAC = 0) to 1 MAC the same as that between 1 and 1.5 MAC and between 1.5 and 2 MAC.
In addition to resistance for orifice flow, the critical velocity (the volumetric flow velocity at which flow transitions from laminar to turbulent) is also inversely proportional to density. Thus, low-potency agents may also increase the likelihood of airway turbulence at lower gas velocities, further increasing resistance.
These physical principles are the justification for the use of helium, a low-density gas, in patients with stenotic airways. As such, desflurane should probably be used with caution in patients with airway obstruction, even ignoring its propensity to aggravate airway reflexes. Despite the authors’ note that desflurane has been successfully used in spontaneously breathing patients, it should be recognized that desflurane will, through its effects on gas density, increase the work of breathing in patients with a native glottis (facemask or laryngeal mask airway), in which the vocal cords act as an orifice. The final decision as to whether these considerations are of clinical significance awaits further study, but the authors should be congratulated for bringing this issue to our attention.
Children’s Hospital Medical Center, Cincinnati, Ohio. email@example.com