Dynamics of breathing in infants - CiteSeerX

Dynamics

of breathing

in infants

JACOPO P. MORTOLA, JOHN T. FISHER, BRUCE SMITH, GORDON FOX, AND SALLY WEEKS Department of Physiology, McGill University, and Department of Anaesthesia, Royal Victoria Hospital, Montreal, Quebec H3G 1 Y6, Canada

MORTOLA,JACOPO P., JOHN T. FISHER,BRUCESMITH,GORnegligible can be obtained by measuring the mouth presDON Fox, AND SALLY WEEKS. Dynamics of breathing in insure during an inspiratory effort against closed airways. fants. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. In this situation the contraction of the muscles is ap52(5): 1209~1215,1982. Passive compliance (C) has been measproximately isometric, and the pressure generated is a ured in 10 infants at lo-90 min after birth and in 10 infants at * good index of muscle activity (7, 29). It appears that to a few days of life by recording mouth pressure after airways overcome the elastic and flow-resistive properties of the occlusions at end inspiration. From the slope of the expiratory respiratory system, the inspiratory muscles have to genflow-volume curve, the passive time constant (7) and resistance erate a force that corresponds to a pressure substantially = T/C) have been also computed. Examination of the (R higher than that actually measured in terms of the changes of C with time and of the expiratory flow-volume pleural pressure swing; therefore, in dynamic conditions curves indicates that the end-expiratory volume is maintained above functional residual capacity at both ages, but significantly the system behaves as if its active compliance and resistmore so at a few days (7.6 ml) than at lo-90 min (3.5 ml). The ance were, respectively, lower and higher than the corpassive time constant (T = CmR) is shorter at the early age due responding passive values. This dynamic behavior of the to the smaller C. The active compliance (C’) and resistance (R’) respiratory system is the basis of its better stability to values have been estimated from the pressure generated by the loading than predictable from its passive properties (14). infant when the airways are occluded at end expiration. The If the reduction in lung compliance during active active time constant of the respiratory system (7’ = C’ .R’) is breathing is more important than the increase in resistless than 7, due to a smaller active compliance, particularly at ance as recently suggested (25), the time constant of the a few days. The active resistance is on the contrary similar to respiratory system can be less than that computed from R. The active stiffening of the respiratory system provides more the passive values of compliance and resistance. This stability of the infant’s respiratory system and a more ready volume response for any given change in pressure; its price, could represent an important aspect of the dynamics of however, is a higher work of breathing. At optimal breathing breathing in the newborn, in whom the high breathing rates, in fact, the active work is 127% (lo-90 min) to 183% (a frequency would require a very high rate of pressure few days) higher than that computed from the passive values. development to inflate the lungs if the time constant of The inspiratory flow wave tends to be squared at both ages the respiratory system were not adequately short. minimizing the energy losses due to friction. compliance; resistance; work of breathing; functional residual capacity

flow-volume

curve;

TO EXPAND THE LUNGS some pressure must be applied to the respiratory system. During active inspiration the pressure is the result of the contraction of the respiratory muscles, and the external work can be computed from the actual pressure (P) developed and the volume (V) inspired. The “active” work generated by the respiratory muscles is however likely to be higher than that calculated from the PV diagram. Part of the muscle force potentially available for inspiration will not produce pressure because it is lost during the shortening of the inspiratory muscles (force-length relationship) and their contraction with a finite velocity (force-velocity relationship). In infants, due to the relatively high chest wall compliance that often results in paradoxical rib cage movements (17), another important source of force loss is related to chest wall distortion. An estimate of the pressure that could be generated if the force losses due to the intrinsic properties of the inspiratory muscles were 0161-7567/82/0000-0000$01.25

Copyright

0 1982 the American

Physiological

In this study we have analyzed some aspects of the dynamics of breathing in infants at a few hours and a few days of life. Passive and active values of respiratory system compliance and resistance have been measured and from these data the corresponding work of breathing has been computed. METHODS

Measurements have been obtained in a total of 15 infants, 10 of them at lo-90 min after birth and 10 at 1-5 days of age, with 5 infants in common to both age groups. The study was approved by an Ethics Committee of the Royal Victoria Hospital in Montreal, where the experiments were conducted, and informed written consent was obtained from all mothers. All infants were full term with vertex presentation. Ten of the infants were elective repeat cesarean sections with three of the mothers receiving general anesthesia (02 66% in NzO and halothane 0.5%) and seven epidural anesthesia (2% lidocaine hydrochloride-COz, 15-17 ml). The remaining five infants were vaginal deliveries, and all the mothers received epidural anesthesia. All the babies had an Apgar score of at least 7 and 8 at 1 and 5 min, respectively, and were judged Society

1209

1210 healthy at clinical examination during the 3-5 days that they remained in the hospital nursery. The recording apparatus consisted of a rubber mask with an inflatable border that was placed over the nose and mouth of the infant. A Fleisch pneumotachograph (no. 00, 1.7 ml dead space) and a port for sampling mouth pressure were connected to the mask. The output of the pneumotachograph was amplified using a Hewlett-Packard differential pressure transducer (HP 270)) and the pressure signal was amplified using a Statham differential pressure transducer (model PM5ETC). By recording simultaneously an oscillatory pressure wave, no appreciable lag time was observed between these two transducers up to frequencies of 150 cycles/min. The total dead space of the apparatus was 10.0 ml, and the flow resistance was 9.45 cmHz0 01-l 0s. The flow signal, its integral tidal volume, and mouth pressure were recorded on a Gould Brush pen recorder and a 4-channel Hewlett-Packard tape recorder for later play back at a paper speed of 25 mm/s. After the mask was placed on the face and the infant was resting quietly in the supine or lateral posture, recordings were made for 5-10 min, and then mouth occlusions were performed by manually occluding the outlet of the pneumotachograph. No measurements were considered if the infant had eyes open, gross body movements, or was crying. Although no attempts were made to define the state of arousal, it is likely the measurements were obtained during sleep. All the records were digitized manually with a graphic tablet (Summagraphics), and the data of the spirometric variables (tidal volume, respiratory rate, minute ventilation, inspiratory and expiratory time, and peak inspiratory flow) were stored on tape for statistical analysis and graphical representation by a minicomputer (HP 85). The two-tailed t test for paired or unpaired variables was performed to detect sign&a& differences. A statistical difference was defined by a P < 0.05. Respiratory system compliance. Respiratory system compliance was measured by occluding the airways during expiration at different lung volumes, including the end-expiratory level, and measuring the corresponding mouth positive pressure as previously suggested by Olinsky et al. (18). Data of pressure and volume were then plotted on an x-y graph, and the slope of the linear regression of the data points represents the compliance, whereas the intercept on the y-axis represents the difference between the end-expiratory volume and the passive resting position of the respiratory system (FRC). No other criteria than the reproducibility of the values (indicated by the high correlation coefficients of the linear regressions, 0.902 t 0.027 SD) were followed as an index of “relaxation”; however, this method yields values similar to those reported in paralyzed ventilated infants (18, 24). The compliance value obtained will be referred to as passive compliarice, C. Expiratory flow-volume loops. These loops were constructed by plotting the values of flow (x-axis) and volume (y-axis) measured every 0.02 s of the expiratory cycle. Twenty consecutive breaths were analyzed in each infant. Flow data were obtained by continuous digitizing of the expiratory flow signal. This input was fed into a

MORTOLA

ET

AL.

minicomputer (HP 85) that provided the integrated signal every 0.02 s. In this way any possible time lag between flow and volume was avoided. The passive time constant of the respiratory system was computed through the analysis of the expiration following the release of airway occlusion above end-expiratory level. Even though no attempts were made to verify the completeness of the expiratory relaxation, the small variations of the results and the fact that a linear relationship was apparent in the last portion of expiration suggested that the portion of the expiratory cycle considered was very close to the ralaxed condition. The slope of this portion of the flowvolume loop will be referred to as the passive expiratory time constant of the respiratory system, 7. From 7 and C the passive total expiratory resistance of the respiratory system R was obtained (R = 7/C). Active compliance and resistance. For these measurements at least three occlusions at end expiration were analyzed in each infant. The pressure wave of the inspiratory effort and the inspiratory flow wave of the three preceding control breaths were digitized. The data was fed into a minicomputer (HP 85) that integrated the flow signal every 0.02 s to obtain the inspired volume and constructed the pressure/volume (P/V, y-axis) versus flow/volume (V/V, x-axis) plots. According to the equation of motion of the respiratory system, P = l/C V + RV and therefore P/V = l/C + R *V/V; the interest of this representation is that the slope of the function represents the resistance and the intercept the compliance of the respiratory system (6, 25). Even though the relationship was in general very close to a linear function, indicating that the flow-resistive component related to turbulence was negligible, polynomial regression analysis was used to derive the compliance C and the first and second Rohrer’s constants K1 and Kz according to the equation P = l/C V + K$ + K&‘. Because the pressure P represents the total pressure available during isometric muscle contraction (7,25,29), the values of C, K1, and Kz will be referred to as “active” compliance, “active” laminar airflow resistance, and “active” resistance to turbulent flow, and will be labeled C’, K;, and K& respectively. Work of breathing. From the values of C and R and of C’, K; and K& the respective passive and active inspiratory work of breathing (W) was computed according to the formula l

W insp = + (V;/C)

+ + K 1 7~~f V$ + Q K2 7~~f” V;T

proposed by Otis et al. (21)) which flow wave, or

assumes

a sinusoidal

W hP =$(V;/C)+aK1fV;+4Kzf2V; which assumes a squared flow wave, with VT and f being frequency, respectively. the tidal volume and respiratory The inspiratory work computed in this way represents the total work to the extent that expiration is passive. It should also be noted that a presumably small component of the inspiratory work is the result of the passive recoil of the chest wall during inspiration. RESULTS

Figure 1 shows for each infant l the regression lines of the volume and mouth pressure values obtained by oc-

RESPIRATORY

DYNAMICS lo-

IN 90

1211

INFANTS

min

1-5

days 3,

c 10

15

FIG. 1. Pressure-volume (P, cmHz0; abscissa; V, ml; ordinate) relationships of respiratory system at 10-90 min after birth (Left panel) and at a few days of life (right panel). Each line refers to a different infant, the number of which is indicated. Dashed Lines: mean relationships.

end exp. volume

10

15 P, cm

eluding the airways during expiration. The left panel refers to the measurements at lo-90 min and the right panel to the measurements at a few days of life. The slope of each line represents the passive compliance (C) of the respiratory system, and its value is reported in Table 1. The compliance C is significantly smaller at lo-90 min (2.12 ml/cm Hz0 t 0.62 SD) than at a few days of life (3.68 t 0.82). By extrapolating the mean function to the volume corresponding to zero pressure, the end-expiratory level is maintained above the passive resting position (FRC) for both age groups, but is significantly more so at a few days (7.57 ml t 3.07 SD) than at lo-90 min (3.46 t 2.5). Figure 2 shows two examples of expiratory flow-volume curves after reopening the airways after an occlusion at end inspiration. The linearity of the late portions of the relationship suggests that relaxation occurred, and its slope represents the passive time constant (7) of the respiratory system; 7 was significantly smaller at lo-90 min (0.137 s t 0.044 SD) than at a few days (0.208 t 0.089). From the values of 7 and C in each baby, the total passive expiratory resistance of the respiratory system (R) was computed. The higher mean value of R at lo-90 min (Table 1) is not significantly different from the fewday-old infants, indicating therefore that the smaller 7 of the younger group is mainly due to the lower C. Some examples of expiratory flow-volume curves during tidal breathing are shown in Fig. 3 for three infants. The dashed line is the corresponding passive “relaxed” function computed as previously described. The position of the curve relative to zero volume (FRC) has been adjusted on an individual basis from the values of the end-expiratory volume-FRC difference computed above. In these as well as the remaining infants, expiration seems to occur very close to the passive function; small displacements to the right (Fig. 3, bottom panel) are

Hz0

1. Passive and active compliance and resistance values in infants

TABLE

lo-90

Body wt at birth, kg C, ml/cmHzO Time constant, s Rexp, cmHa0. ml-’ es C’, ml/cmHaO Ki insp, cm H20ml-‘0s HzO=m12s” K inspy cm c/c, 5% Active FRC, ml

min (10)

3.49t0.43 2.121t0.618 0.137t0.044

0.0708t0.0240 1.871t0.543 0.0418~0.0150

0.00045t0.00034 got_15

3.465t2.500

l-5

days

(10)

P

3.41t0.49

NS