A NEW METHOD TO ANALYZE LUNG COMPLIANCE WHEN PRESSURE-VOLUME RELATIONSHIP IS NONLINEAR

To the Editor :

It was with great interest that we read the recent paper by Nikischin and colleagues (1) describing a newly developed algorithm for the analysis of nonlinear P/V relationship (APVNL). In the study the method of Mead and Whittenberger (2) was used for obtaining total resistance of the lung. This method assumes that the pressure necessary to overcome the elastance of the lung during inspiration and expiration at the same level of lung volume are equal. So that values of Ci and Ce calculated by the APVNL method should be equal at the same level of lung volume. It is mathematically shown as below using the equations that appeared in the paper by Nikischin and colleagues (1).

This calculation shows that Ci and Ce are equal at the same level of lung inflation, because Fexp is negative when Fins is positive and corresponding values are all the same.

Table 3 in the paper shows the results of Ci values and Ce values obtained by APVNL method at several levels of lung inflation. Ci values and Ce values at the same levels of lung inflation shown in the table are different from each other. This seems quite peculiar: Even though statistical analyses were done, the results from the same data sets should be the same. We wonder if some calculation error had occurred in the study. We are not favorably inclined to the idea of obtaining Ci and Ce separately in this kind of study using the Mead and Whittenberger method.

If the points of V/(Pexp  R · Fexp) or V/(Pins  R · Fexp) relationship are plotted on a volume diagram, it is easier to see how linear or upward convex the P-V relationships are, and to identify pulmonary overdistention.

Department of Pediatrics, Tokushukai Medical Center and Fukuoka University, Fukuoka, Japan

1. Nikischin, W., T. Gerhardt, R. Everett, and E. Bancalari. 1998. A new method to analyze lung compliance when pressure-volume relationship is nonlinear. Am. J. Respir. Crit. Care Med. 158: 1052-1060 [Abstract/Free Full Text].

2. Mead, J., and J. L. Whittenberger. 1953. Physical properties of human lungs measured during spontaneous respiration. J. Appl. Physiol. 5: 779-796 .

From the Authors:

We thank Drs. Muramatsu and Yukitake for their insightful letter.

In our study we used the method of Mead and Whittenberger to calculate total pulmonary resistance (R_t) and assumed that inspiratory and expiratory resistance were equal to R_t. We were aware of this assumption influencing the accuracy of the compliance measurements and discussed this in the text. We were not aware, however, that this assumption also meant that any difference between inspiratory (C_i) and expiratory (C_e) compliance disappeared and that both should be equal.

In our paper C_i and C_e values given in Tables 2 and 3 are similar but not identical. The small difference between C_i and C_e is explained by the sequence of steps our program took to calculate C. R-values were calculated first from the P/V loop at increasing steps of volume. C-values were then calculated from the scalar trace at increasing steps of volume, inserting the previously calculated R values into the equation of motion, C_i = V/(Pins  R · Fins), C_e = V/ (Pexp  R · Fexp). As it turns out, the calculations of R and C were not done at precisely the same volumes so that pressures and flows used to calculate R were not identical to the ones used to calculate C_i and C_e. We will use this insight to improve our computer program.

We agree, therefore, with Drs. Muramatsu and Yukitake that it is not meaningful to calculate C_i and C_e separately using our algorithm because the results reflect total compliance (C_t) regardless of whether the calculation is done during inspiration or expiration. These C_t values, as our paper shows, give a close estimate of true compliance, and the algorithm used is still the most reliable approach for identifying nonlinear P/V relationships.

The commentators' suggestion to plot V for Pins  R · Fins or V for Pexp  R · Fexp to obtain a P/V diagram is appreciated. This approach is essentially not different from our approach of utilizing the same measurements to obtain C (C = V/P at the different points of the breath) and showing how C changes with inflation.

One additional comment. The equation provided by Drs. Muramatsu and Yukitake in their letter contains an error that is probably just an oversight:

C_i and C_e are not equal to V/(Pexp Fins  Pins Fexp), but C_i = [V(Fins +  Fexp )]/(Pexp Fins  Pins Fexp) = C_e.

Division of Neonatology, Department of Pediatrics, University of Miami School of Medicine, Miami, Florida