Inspiratory Pressure Reserve and Tension Time Index

Rationale. Measurement of inspiratory pressure reserve, i.e., the ratio of the mean inspiratory pressure (Pi) during resting breathing over Pi,max at FRC, assesses the potential limitation of the inspiratory muscles to generate pressure at the end-expiratory level.

Measurements. inspiratory pressure reserve can be assessed in children by noninvasive mouth pressure measurements. in-spiratory pressure is calculated as 0.5 X a X Ti (where a is Pmo,0.1 X 10) (16). Inspiratory power for breathing at rest can be calculated as Pi X Vt/Ti X Ti/total inspiratory time (Ttot) (16). The critical inspiratory power (65), i.e., the inspiratory power above which fatigue occurs, can be calculated as Pi,max X Vt/Ti X Ti/Ttot. Critical Pi,max is assumed to be equal to 60% of Pi,max generated at FRC (65). A noninvasive pressure-time index for all the inspiratory muscles (PTImus) can be estimated as PTImus = Pi/Pi,max X Ti/Ttot (16, 55, 56).

Advantages. Assessment of inspiratory muscle function requires only noninvasive measurements. PTimus assesses all the inspiratory muscles.

Disadvantages. The validity of the estimation of Pi cannot be proved over the entire Ti. The rise in pressure during inspiration is approximated by a single power function of time assuming a linear applied pressure profile. if the inspiratory driving pressure increases as an exponential function of time, Pi is overestimated by extrapolating the Pmo,0.1 over the entire Ti. Therefore, all derived parameters including Pi may be affected by an overestimation. However, in healthy adults and in adults with chronic obstructive pulmonary disease, Pi has been shown to be significantly correlated with Pdi, and PTimus with the tension-time index of the diaphragm (TTIdi) (66). No comparison of PTImus and PTIdi is available in children.

Normal values. Table 3 shows that Pi/Pi,max at FRC decreases with age in children. TTimus decreases because of a decrease in Pi at rest and an increase in Pi,max at FRC, with no change in Ti/Ttot (15). However, no study has defined the critical PTimus above which fatigue occurs in children, as has been done in adults for PTidi using electromyographic criteria (67).

Clinical application. Such measurements are helpful in children with increased load of breathing to estimate the degree of reduction of their inspiratory pressure reserve, the inspira-tory power for breathing at rest, and the breathing strategy required to remain below the fatigue threshold of the respiratory muscles. It has been shown that the greater the Pi/Pi,max at FRC, the lower the Ti/Ttot in children with chronic obstructive pulmonary disease (16). The reduction in inspiratory pressure reserve limits the possibilities of adaptation of the pattern of breathing when an increase in the breathing load occurs, such as during exercise (68) or during sleep. The younger the child, the greater the risk (64). Such measurements should be performed in the follow-up of severe respiratory disorders, such as during the wait for lung transplant.

TABLE 3. NORMAL VALUES OF OCCLUSION PRESSURE, INSPIRATORY FORCE RESERVE, AND PRESSURE-TIME INDEX IN HEALTHY CHILDREN AND ADOLESCENTS

4 Male 3.60 Female 3.60 6 Male 2.80 Female 2.80 8 Male 2.34

Female 2.34

10 Male 2.04 Female 2.04 12 Male 1.82 Female 1.82

14 Male 1.65 Female 1.65 16 Male 1.52 Female 1.52

Definition of abbreviations: Pmo,0.1 = occlusion pressure; Pi = mean inspiratory pressure; Pi,max,FRC = maximal inspiratory pressure generated at functional residual capacity; PTImus = pressure-time index for all the inspiratory muscles.

* Pmo,0.1 (occlusion pressure) from Gaultier and coworkers (15); Pmo,0.1 is not significantly different between males and females.

* Pi,max,FRC (maximal inspiratory pressure generated at functional residual capacity) from Gaultier and Zinman (13).

* Ti/Ttot does no change with age in children (15), TTImus was calculated using a Ti/ Ttot equal to 0.45, i.e., the mean value obtained in children between 4 and 16 years of age.

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