Linear correlation of endotracheal tube cuff pressure and volume

Endotracheal tube cuff (ETTc) inflation by standard methods may result in excessive ETTc pressure. Previous studies have indicated that methods of cuff inflation most frequently used to inflate ETTcs include palpation of the tension in the pilot balloon or injection of a predetermined volume of air to inflate the pilot balloon. If a logarithmic relationship exists between ETTc volume and ETTc pressure, small volumes of additional air will result in dramatic pressure increases after a volume threshold is reached. Our goal was to determine whether the relationship between ETTc volume and ETTc pressure is linear or non-linear.

In this Institutional Animal Care and Use Committee-approved study, we recorded ETTc volume and pressure in four anesthetized and mechanically-ventilated canines ranging between 30-40 pounds (mean 34.7lb, SD 3.8lb) that were endotracheally intubated with a 7.0 mm ETT. The varying cuff pressures associated with a distribution of 28 progressively increasing volumes of air in the ETTc were recorded. Spearman correlation was performed to determine if a linear or non-linear relationship existed between these variables.

The Spearman rho coefficient of correlation between ETTc volume and ETTc pressure was 0.969, or approximately 97%, suggesting near-perfect linear relationship between ETTc volume and ETTc pressure over the range of volumes and pressures tested.

Over the range of volumes and pressures tested a linear relationship between volume and pressure results in no precipitous increase in slope of the pressure:volume curve as volume increases.