Static compliance of the respiratory system in healthy infants

We recorded static deflation pressure-volume (PV) curves from near TLC to FRC in 49 healthy, sedated, spontaneously breathing infants of 1 to 104 wk of age. Respiratory activity was transiently inhibited by inflating the respiratory system several times to a volume at an airway pressure of 30 cm H2O (V30). Passive deflation from V30 to FRC was then interrupted by multiple brief occlusions at the airway opening, in order to measure static recoil pressures. The expired volume from V30 to FRC was defined as V30E. Compliance of the respiratory system (Crs) was calculated as the slope of the linear portion of the PV curve from 5 to 15 cm H2O. Crs and V30E increased with increasing body length (p < 0.001). After adjustment for body length, males had greater Crs values than did females (p < 0.01). V30E was smaller in female infants (p < 0.05) and in infants whose mothers smoked during pregnancy (p < 0.04). Specific compliance (Crs/V30E) declined with increasing age (p < 0.01), but there were no differences related to sex or maternal smoking. We conclude that static deflation PV curves can be recorded in the age range from 1 to 104 wk, and that maternal smoking may produce hypoplastic lungs.     

Comparison of normal infants and infants with cystic fibrosis using forced expiratory flows breathing air and heliox

SUMMARY. The detection of early airway disease in infants with cystic fibrosis (CF) may lead to earlier intervention and an improved prognosis. We hypothesized that the ratio of maximal expiratory flows while breathing a mixture of helium and oxygen (heliox) and air, referred to as density dependence (DD), would identify early airway disease in infants with CF who have normal lung function. We also hypothesized that these infants with CF might be better differentiated from normal infants when the flows breathing heliox are compared instead of room air flows. We evaluated 10 infants with CF and 21 infants without CF and with normal lung function, defined as a forced vital capacity (FVC) and forced expiratory flows between 25-75% of expired volume (FEF(25-75)) of greater than 70% predicted (z-score > -2.0). Full forced expiratory maneuvers by the rapid thoracic compression technique were obtained while breathing room air and then heliox. Flow at 50% and 75% of expired volume (FEF(50), FEF(75)), FEF(25-75), and FVC were calculated from the flow volume curve with patients and control subjects breathing each gas mixture. The ratio of heliox to air flow at FEF(50) and FEF(75) was calculated (DD(50), DD(75)), and the point where the two flow-volume curves crossed (V(iso) V') was also measured. DD parameters did not distinguish the infants with CF from the infants without CF; length-adjusted FEF(50) breathing air was significantly lower in the infants with CF compared to the infants without CF (P < 0.05). Length-adjusted flows breathing heliox did not distinguish the two groups. We conclude that the lower FEF(50) value may reflect early airway obstruction in healthy infants with CF, and that measurements obtained with the less dense gas mixture did not improve detection of airway disease in this age group.     

Variability of respiratory system compliance in mechanically ventilated infants.

We measured respiratory flow, volume by integrated flow, and airway pressure in four mechanically ventilated infants. Compliance of the respiratory system (Crs) was measured by the multiple occlusion method. Airway occlusion and release were achieved by a balloon in an occlusion valve between the endotracheal tube and a pneumotachometer, all connected in series with the respirator circuit. Values of Crs varied greatly due to changes in mean airway pressure (MAP). The Crs increased with the elevation of MAP on some occasions and decreased on others. Values of Crs also varied corresponding to different occlusion pressures for individual levels of MAP. Thus Crs changed continuously, even within a single respiratory cycle. The observed variability in Crs was explained by the following mechanism: a pressure-volume (P-V) loop of tidal ventilation moves its position within the pressure-volume diagram and changes shape due to variations of the ventilator settings. In some infants the lungs were ventilated in the range of the linear portion of the P-V diagram while in others they may have been ventilated on the flattened portion. Some patients had convex expiratory P-V curves even with low pressure swings during tidal ventilation.     

The effects of varying inflation and deflation pressures on the maximal expiratory deflation flow-volume relationship in anesthetized rhesus monkeys

Deflation flow-volume curve analysis is a pulmonary function test sensitive to small airways dysfunction that is suitable for use in infants and children who are intubated. This test relies upon deflation flow-volume (DFV) curve analysis, which is a technique to obtain maximal expiratory flow-volume curves (MEFV) by forced deflation of the lungs in infants who are intubated. The method mimics the voluntary forced flow-volume curves that adults and older children undertake. We studied 10 anesthetized male Rhesus monkeys of the same weight as human infants but developmentally equivalent to older children. We reviewed the effects on forced deflation vital capacity (DVC) and flows at various subdivisions of vital capacity (PEF, MEF50, MEF25, MEF10) of systematically varying the required inspiratory and deflation pressure during the course of 56 consecutive deflation maneuvers. Inflation pressures of +40 and +50 cm H2O caused a marked but transient bradycardia along with a (probably spurious) short-lasting fall to 89% mean arterial oxygen saturation (SaO2). Increasing positive and negative pressures increased DVC and expiratory flows. The highest mean DVC was 75.6 +/- 1.3 ml/kg, PEF was 128.0 +/- 3.5, MEF50 was 85.9 +/- 2.2, MEF25 was 74.3 +/- 1.9, and MEF10 was 38.5 +/- 2.9 ml/kg/s, all obtained at the pressure gradient of 90 cm H2O (+50/-40 cm H2O) at the start of the deflation maneuver. At this gradient, the intraindividual coefficients of variation were: DVC = 0.8%, PEF = 3.1%, MEF50 = 2.2%, MEF25 = 2.1%, MEF10 = 5.4%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lung emptying in patients with acute respiratory distress syndrome: effects of positive end-expiratory pressure.

The pattern of lung emptying was studied in 10 mechanically-ventilated patients with acute respiratory distress syndrome. At four levels of positive end-expiratory pressure (PEEP) (0, 5, 10 and 15 cmH2O) tracheal (Ptr) and airway pressures (Paw), flow (V') and volume (V) were continuously recorded. Tidal volume was set between 0.5-0.6 L and V'/V curves during passive expiration were obtained. Expired volume was divided into five equal volume slices and the time constant (taue) and effective deflation compliance (Crs(eff)) of each slice was calculated by regression analysis of V/V' and postocclusion V/Ptr relationships, respectively. In each slice, the presence or absence of flow limitation was examined by comparing V'/V curves with and without decreasing Paw. For a given slice, total expiratory resistance (Rtot) (consisting of the respiratory system (Rrs), endotracheal tube (Rtube) and ventilator circuit (Rvent)) was calculated as the taue/Crs(eff) ratio. In the absence of flow limitation Rrs was obtained by subtracting Rtube and Rvent from Rtot, while in the presence of flow limitation Rrs equaled Rtot. The taue of the pure respiratory system (taue(rs)) was calculated as the product of Rrs and Crs(eff). At zero PEEP, taue(rs) increased significantly towards the end of expiration (52+/-31%) due to a significant increase in Rrs (46+/-36%). Application of PEEP significantly decreased Rrs at the end of expiration and resulted in a faster and relatively constant rate of lung emptying. In conclusion, without positive end-expiratory pressure the respiratory system in patients with acute respiratory distress syndrome deflates with a rate that progressively decreases, due to a considerable increase in expiratory resistance at low lung volumes. Application of positive end-expiratory pressure decreases the expiratory resistance, probably by preventing airway closure, and as a result modifies the pattern of lung emptying.     

Sensitivity of spirometric measurements to detect airway obstruction in infants

We evaluated the ability of forced expiratory flow volume curves from raised lung volumes to assess airway function among infants with differing severities of respiratory symptoms. Group 1 (n = 33) had previous respiratory symptoms but were currently asymptomatic; group 2 (n = 36) was symptomatic at the time of evaluation. As a control group, we used our previously published sample of 155 healthy infants. Flow volume curves were quantified by FVC, FEF50, FEF75, FEF25-75, FEV0.5, and FEV0.5/FVC, which were expressed as Z scores. All variables except FVC had Z scores that were significantly less than zero and distinguished groups 1 and 2 with progressively lower Z scores. The mean Z scores of the flow variables (FEF50%, FEF75%, and FEF25-75%) were more negative than the Z scores for the timed expired volumes (FEV0.5 or FEV0.5/FVC) for both groups. In general, measures of flow identified a greater number of infants with abnormal lung function than measures of timed volume; FEF50 had the highest performance in detecting abnormal lung function. In summary, forced expiratory maneuvers obtained by the raised volume rapid compression technique can discriminate among groups of infants with differing severity of respiratory symptoms, and measures of forced expiratory flows were better than timed expiratory volume in detecting abnormal airway function.     

Shape of forced expiratory flow-volume curves in infants

An inflatable cuff was used to generate partial forced expiratory flow-volume (FEFV) curves in 36 infants with and without obstructive airway disease. Curves were recorded in each infant over a range of compression pressures as high as and exceeding the pressure required for the maximal partial FEFV curve. The maximal curves were quantitated and compared with passive compliance and conductance of the respiratory system and absolute lung volume measured by whole-body plethysmography. In some infants, the transmission of pressure between cuff and pleural space was determined. Partial FEFV curve shapes generated with a standardized compression pressure calculated from the transmission of pressure data to give an increase in pleural pressure at FRC of 10 cm H2O were compared between infants. For these standardized compressions, infants with convex curves tended to have better respiratory function than did those with concave curves. The combination of a concave curve and flow limitation during tidal expiration was associated with the worst function. Two parameters, the ratio of forced maximal expiratory flow (measured from the maximal partial FEFV curve) to tidal expiratory flow (measured from the expiratory flow-volume curve of tidal breathing) at midtidal volume (Vm1d(forced/tidal] and the minimal compression pressure required to generate maximal expiratory flow at FRC (Pmin), satisfactorily quantified respiratory function without the need for size correction with absolute lung volume. In addition, Vm1d(forced/tidal) provides an index of expiratory flow reserve. We conclude that useful information can be provided from the shape of a partial FEFV curve in an infant, provided that curves are generated by a standardized compression pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Assessment of airway responsiveness in infants with cystic fibrosis.

We compared the responses of cystic fibrosis (CF) (N = 14) and normal (N = 14) infants with inhaled methacholine. Airway function was assessed by forced expiratory flows at functional residual capacity (Vmax FRC) generated by the rapid compression technique, and methacholine responsiveness was quantitated as (1) TC: the threshold concentration to decrease Vmax FRC by 2 SD from baseline; (2) PC50: the provocative concentration to decrease Vmax FRC by 30%; and (3) SPC30; the slope of the dose-response curve between TC and PC30. There were no significant differences in age between CF and normal infants (16 +/- 8 versus 17 +/- 5 months, p greater than 0.3); however, the CF infants were shorter (74 +/- 10 versus 81 +/- 5 cm, p less than 0.05), had lower absolute Vmax FRC (241 +/- 103 versus 374 +/- 113 ml/s, p less than 0.001), and tended to have lower percentage of predicted flow values (87 +/- 13 versus 111 +/- 34%, p less than 0.10). Comparison of the indices of airway responsiveness revealed no difference in logTC; however, the CF infants had smaller, more negative values for logPC30 (-0.76 +/- 0.52 versus -0.22 +/- 0.53, p less than 0.02) and steeper slopes to their dose-response curves (logSPC30, 2.42 +/- 0.45 versus 1.88 +/- 0.74, p less than 0.025). Indices of airway responsiveness correlated significantly with baseline Vmax FRC (% of predicted). After the influence of baseline flow upon airway responsiveness was accounted for by multiple linear regression analysis, there was a tendency for CF infants to be more responsive than control infants.(ABSTRACT TRUNCATED AT 250 WORDS)     

Significance of the changes in the respiratory system pressure-volume curve during acute lung injury in rats

The hypothesis that the changes in the respiratory system pressure- volume (PV) curve during pulmonary edema mainly reflect distal airway obstruction was investigated in rats. Normal rats had a well-defined upper inflection point (UIP) at low airway pressure. Airway occlusion by liquid instillation decreased compliance (Crs) and the volume (Vuip) of the UIP, and increased end-inspiratory pressure. The same changes were observed during the progression of edema produced by high volume ventilation (HV). Changes in Vuip and in Crs produced by HV were correlated with edema severity in normal rats or rats with lungs preinjured with alpha-naphthylthiourea. Vuip and Crs changes were proportional, reflecting compression of the PV curve on the volume axis and suggesting reduction of the amount of ventilatable lung at low airway pressure. In keeping with this explanation, the lower Vuip and Crs were before HV, the more severe HV-induced edema was in alpha-naphthylthiourea-injected rats. When edema was profuse, PV curves displayed a marked lower inflection point (LIP), the UIP at low pressure disappeared but another was seen at high volume above the LIP, and the correlation between Vuip changes and edema severity was lost. These observations may have clinical relevance in the context of the "open lung" strategy. Keywords: ventilator-induced lung injury; respiratory mechanics; acute respiratory distress syndrome     

Pulmonary dysfunction after primary closure of an abdominal wall defect and its improvement with bronchodilators.

To determine the extent of pulmonary dysfunction following primary closure of an abdominal wall defect, we obtained pulmonary function tests (PFT) in 11 newborn infants with gastroschisis and 6 with large omphaloceles admitted to a newborn ICU in a children's hospital. Patients were 1 to 30 days of age at the time of the PFT; all required endotracheal intubation and mechanical ventilation for operative procedures or for postoperative ventilatory support. Full-term infants (n = 21) undergoing minor surgical procedures provided comparative measurements. Flow-volume curves were obtained with manual inflation of the lungs followed by forced deflation using negative pressure, or by passive expiration, under sedation and pharmacologic paralysis. Deflation flow-volume curves gave measurements of forced vital capacity (FVC) and maximal expiratory flow at 25% of vital capacity from residual volume (MEF25). Modified passive mechanics technique gave passive expiratory curves that provided measurements of respiratory system compliance (Crs) and resistance (Rrs). Tests were done: within 48 h (period A), 3-7 days (period B), and 8-30 days after surgical repair (period C). Pulmonary function testing after nebulized 0.1% isoetharine (a bronchodilator), to test for bronchial reactivity, began midway during the study period in 15 patients. Preoperative and postoperative tests were obtained in 5 patients. Closure of an abdominal wall defect decreased FVC, Crs, and MEF25 by up to 50% of normal, reference values after surgery (P less than 0.05). FVC and MEF25 approached values of normal infants by 4 weeks, whereas Crs remained 50% lower.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of ventilatory system compliance in infants and young children

Ventilatory system compliance (Crs) was measured in 32 sedated sleeping infants and young children aged 3-54 months breathing spontaneously through a face mask. Airways were occluded during expiration at different lung volumes above FRC and the corresponding airway pressure was measured. A pressure-volume relationship was then calculated by the least square method. To ensure the presence of the Breuer-Hering inspiratory inhibitory reflex, the percent increase of occluded expiratory time relative to the preceding unoccluded breath was calculated in 28 subjects where several end inspiratory occlusions had been maintained up to the next inspiratory effort. This index ranged from 4 to 100% and correlated negatively with age (r = -0.50, P less than 0.01). Crs ranged from 4.5 to 21.8 ml/cm H2O and correlated best with height (H) (Crs = 5.36 X 10(-4) H2.27, r = 0.94).     

Total respiratory system compliance in asymptomatic infants with cystic fibrosis

Total respiratory system compliance (Crs) was assessed by the weighted spirometer method in 11 asymptomatic infants (mean age, 11.1 months) with cystic fibrosis (CF) who had normal chest radiographs. In addition to Crs, functional residual capacity (FRC), respiratory rate (RR), and mixing index (MI) were measured. There was no significant difference in FRC between normal controls (n = 36) and CF infants (190 +/- 69 versus 186 +/- 63 ml; p less than 0.8), although the CF group had a higher RR (32 +/- 7 versus 37 +/- 7 BPM; p less than 0.05) and a lower MI (45 +/- 7 versus 40 +/- 8%; p less than 0.05), reflecting an abnormal distribution of ventilation. The lower Crs (9.0 +/- 3.4 versus 5.7 +/- 2.8 ml/cm H2O; p less than 0.01) and the lower specific compliance, Crs/FRC (0.049 +/- 0.013 versus 0.029 +/- 0.007 1/cm H2O; p less than 0.0001), in the CF group were the parameters that best distinguished the normal control and CF infants. We conclude that the measurement of Crs represents a noninvasive method for detecting early pulmonary function abnormalities in CF infants.     

Gas trapping in normal infants and in infants with cystic fibrosis

Two different methods for estimating trapped gas volume have been described in the literature. The purpose of this study was to use both of these methods to estimate and compare trapped gas volumes in normal infants and infants with cystic fibrosis (CF). Thirty normal infants and 29 infants with CF, ages 1 month to 3 years, were studied. Pulmonary function tests, including raised volume forced expiratory flows, plethysmographic functional residual capacity (FRC(pleth)), and fractional lung volumes, were measured. Then functional residual capacity was measured by nitrogen washout (FRC(nitrogen)). Following nitrogen washout, lungs were then inflated three times to 30 cm H(2)O, using 100% oxygen. This process was repeated until no further nitrogen could be washed from the lungs. The volume of trapped gas (tg) was calculated from the total additional amounts of nitrogen expired following lung inflations. The difference between FRC(pleth) and FRC(nitrogen) provided a second estimate of trapped gas volume (delta V). Mean tg and delta V values for normal infants were 2.5 +/- 3.5 ml and 15.6 +/- 30.4 ml, respectively. Mean tg and delta V values for infants with CF were 5.8 +/- 7.7 ml and 33.2 +/- 43.8 ml, respectively. Both tg and delta V did not differ significantly between normal infants and infants with CF. Measured following raised volume forced expiratory maneuvers, delta V and tg do not distinguish infants with CF from normal infants as well as do other currently available tests of infant lung function.     

Passive respiratory mechanics in newborns and children

When the Hering-Breuer reflex is used to relax the respiratory muscles, the passive compliance (Crs), resistance (Rrs), and time constant (Trs) of the respiratory system can be measured from the subsequent expiration. We used this method to assess 22 newborns with respiratory illness. Passive expirations were also recorded in 6 paralyzed, ventilated children. Using a simple slide valve, airway occlusion pressure was measured from a face mask or endotracheal tube, and expiratory flow was measured from a pneumotachygraph. In all subjects, there was a linear function of expiratory flow versus its integral, volume; by extrapolating the linear function to zero flow and zero volume (i.e., initial volume at time zero) Crs, Rrs, and Trs could be calculated; Crs was significantly reduced in mechanically ventilated versus spontaneously breathing infants, and Rrs was significantly higher in intubated versus extubated infants. During passive expiration in newborns, inspiration occurred at a volume above passive functional residual capacity. The passive expiratory flow-volume technique is simple, noninvasive, and appears to provide accurate measurements of respiratory mechanics in the newborn and in paralyzed children.     

Adult-type pulmonary function tests in infants without respiratory disease

A new method that permits the measurement of adult-type maximal expiratory flow-volume curves and fractional lung volumes in sedated infants was recently described. The purpose of this study was to define the normal range for these new measures of pulmonary function in infants and young children. Measurements of forced expiratory flows and fractional lung volume were made on 35 occasions in 22 children (ages 3-120 weeks) without respiratory disease. Maximal expiratory flow-volume curves were measured by the raised lung volume, thoracoabdominal compression technique. Functional residual capacity (FRC) was measured plethysmographically. Measurements of total lung capacity (TLC), residual volume (RV), FRC, forced vital capacity (FVC), and forced expiratory flows at 25, 50, 75, 85, and between 25% and 75% of expired FVC (FEF(25), FEF(50), FEF(75), FEF(85), and FEF(25-75), respectively) all increased in relation to infant length (P<0.001). RV/TLC, FRC/TLC, and FEF(25-75)/FVC declined in relation to increasing length (P<0.001). The forced expiratory flow and fractional lung volume measurements using this method were similar to previously reported estimates using other methods. These estimates represent a reasonable reference standard for infants and young children with respiratory problems.     

Pulmonary function in newborns after repair of congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is associated with pulmonary hypoplasia that limits survival, but the nature and extent of pulmonary dysfunction in neonates with CDH have not been studied. We performed pulmonary function tests (PFTs) in eight intubated infants who survived neonatal repair of CDH (wt, 3.33 ± 0.15 kg; age, 20.1 ± 2.7 d; mean ± S.E.M.). PFTs obtained from six full-term infants (wt, 3.56 ± 0.10 kg; age, 25.0 ± 3.3 d) with no respiratory illness served as controls. The deflation flow-volume curve technique produced maximum expiratory flow-volume (MEFV) curves, giving reproducible measurements of forced vital capacity (FVC) and maximal expiratory flow at 25% of FVC (MEF₂₅). Respiratory system compliance (Crs) and resistance (Rrs) were obtained with a modified passive mechanics technique. In seven of eight infants PFTs were repeated after nebulized bronchodilator (0.1% isoetharine). In neonates surviving CDH repair, as compared to those with normal lung function, FVC was significantly reduced (20.78 ± 3.32 vs. 39.83 ± 3.30 mL · kg^?1, P < 0.05). MEF₂₅ was also markedly reduced (8.41 ± 1.46 vs. 32.32 ± 4.35 mL · kg^?1 · s^?1, P < 0.05), indicating lower airway obstruction. After administration of nebulized bronchodilator, PFTs showed significant increases from control values in both FVC (15.9%) and MEF₂₅ (200%) without changes in Crs and Rrs. These findings indicate that neonates with CDH have restrictive lung defects, reflecting hypoplasia. After surgical repair and mechanical ventilation airway reactivity develops, primarily in smaller airways, and this may complicate the postoperative course. Pediatr Pulmonol. 1991 ; 11 :49-55.     

Assessment of tidal breathing parameters in infants with cystic fibrosis.

Simple methods are needed to assess lung function in infants with cystic fibrosis (CF). This study determined the relationship between simple measurements obtained from tidal breathing with those from more complicated forced expiratory manoeuvres. Healthy infants and infants with CF were recruited from two maternity units and five specialist CF hospitals, respectively. Respiratory rate, tidal volume, minute ventilation and the tidal breathing ratio (TPTEF:TE) were measured in sedated infants and compared with forced expiratory volume in 0.4 seconds (FEV0.4) measured by the raised volume technique. Altogether, 95 healthy infants and 47 infants with CF of similar age, sex, ethnicity and proportion exposed to maternal smoking were recruited. There was no difference in TPTEF:TE and tidal volume between healthy infants and those with CF. Minute ventilation was significantly greater in infants with CF due to a mean (95% confidence interval) increase in respiratory rate of 5.8 (3.2-8.4) min(-1). Thirteen (28%) infants with CF had a respiratory rate elevated by >2 SD. However, no association between respiratory rate and FEV0.4 could be identified. Tidal breathing ratio was not useful in identifying diminished airway function in infants with cystic fibrosis. An elevated respiratory rate may be due in part to ventilation heterogeneity but is poorly predictive of diminished airway function measured by forced expiration.     

Respiratory system compliance assessed by the multiple occlusion and weighted spirometer method in non-intubated healthy newborns

In 28 healthy newborn infants (median age 3.5 days), we compared the weighted spirometer (WS) with the multiple occlusion (MO) method for measuring respiratory system compliance (Crs). The MO method was unsuccessful in 8 infants. On average the two methods gave comparable results for compliance (Crs,ws = 40.4 +/- 13.8 and Crs,MO = 45.2 +/- 10.4 mL.kPa-1) in the remaining 20 infants; however, within-individual differences were often considerable, so that the methods did not give interchangeable results. Individual pressure-volume curves almost always intercepted the volume axis below the functional residual capacity with the MO technique, compatible with dynamic elevation of end-expiratory lung volume (EEL) due to inspiratory muscle activity during expiration. A (small) negative volume intercept occurred in less than 50% of curves with the WS method; in these cases it probably reflects alinearity of the compliance curve, an alteration in laryngeal braking or in respiratory muscle control of EEL, or all of these. Both methods provide valuable means for the non-invasive determination of respiratory system compliance in newborn infants, the differences in Crs being small and of minimal physiological significance; however, for individual follow-up they should not be used interchangeably.     

Maximum flow ratios at mid-vital capacity in young healthy adults

Upper airway obstruction is usually diagnosed by visual examination of maximum expiratory and inspiratory flow-volume curves and by calculating a ratio of expiratory to inspiratory flow at 50 percent of vital capacity (mid-vital capacity flow ratio); however, reference values of this ratio have not been well established, and considerable variability exists. The purpose of this study was to examine the range of mid-vital capacity flow ratios in a group of healthy subjects and to determine if some of the variability is accounted for by different maximum inspiratory pressures. We measured maximum expiratory and inspiratory flows at 50 percent of vital capacity from the flow-volume curves, and maximum inspiratory pressures in a group of 60 healthy nonsmokers (30 men and 30 women) whose ages ranged from 21 to 40 years. We found that mid-vital capacity flow ratio (mean +/- SD) was 0.72 +/- 0.19 in men and 0.77 +/- 0.18 in women. The coefficient of variation of the mid-vital capacity flow ratio was 28 percent for men and 23 percent for women. The 95 percent confidence limits for the mid-vital capacity flow ratio were 0.65 to 0.79 for men and 0.70 to 0.84 for women. Maximum inspiratory pressures (mean +/- SD) were 129 +/- 30 cm H2O in men and 91 +/- 16 cm H2O in women, not significantly different from previous studies. Normalizing maximum inspiratory flow for maximum inspiratory pressure did not reduce the coefficient of variation, which became 29 percent in men and 30 percent in women. We conclude that the range of mid-vital capacity flow ratios is wide, and it cannot be reduced by standardizing it for maximum inspiratory pressures.     

Monitoring of nonlinear respiratory elastance using a multiple linear regression analysis.

The elastic pressure/volume (P/V) curve obtained by the multiple linear regression (MLR) technique using a new model, was compared with the quasi-static P/V points obtained by the rapid airway occlusion technique. Seven infants were studied during mechanical ventilation using a pressure controlled mode. The resistive pressure was subtracted from airway opening pressure, thus determining the elastance related pressure, which was then plotted against the volume to make an MLR-elastance curve. Quasi-static P/V curves of the rapid occlusion technique were constructed by plotting the different inspiratory and expiratory volumes against the corresponding values of the quasi-static airway pressure. The calculated MLR-elastance curves closely fit the experimental quasi-static P/V points obtained by the occlusion technique. There were, however, some discrepancies due to the viscoelastic behaviour of the respiratory system. Although slightly altered by these discrepancies, the multiple linear regression-elastance curves did fit the observed quasi-static pressure/volume characteristics for use in clinical practice. The multiple linear regression technique may prove to be clinically useful by continuous monitoring of respiratory system mechanics during mechanical ventilation.