Measurement of dynamic respiratory mechanics in neonatal and pediatric intensive care: the multiple linear regression technique

The impact of PEEP on mechanical power and driving pressure in children with pediatric acute respiratory distress syndrome

BACKGROUND

Positive end-expiratory pressure (PEEP) is widely used to improve oxygenation and avoid alveolar collapse in mechanically ventilated patients with pediatric acute respiratory distress syndrome (PARDS). However, its improper use can be harmful, impacting variables associated with ventilation-induced lung injury, such as mechanical power (MP) and driving pressure (∆P). Our main objective was to assess the impact of increasing PEEP on MP and ∆P in children with PARDS.

INTERVENTIONS

Mechanically ventilated children on pressure-controlled volume-guaranteed mode were prospectively assessed for inclusion. PEEP was sequentially changed to 5, 12, 10, 8, and again to 5 cm H2O. After 10 min at each PEEP level, ventilatory data were collected and then variables of interest were determined. Respiratory system mechanics were measured using the least squares fitting method.

RESULTS

Thirty-one patients were included, with median age and weight of 6 months and 6.3 kg. Most subjects were admitted for acute viral bronchiolitis (45%) or community-acquired pneumonia (32%) and were diagnosed with mild (45%) or moderate (42%) PARDS. There was a significant increase in MP and ∆P at PEEP levels of 10 and 12 cm H2O. When PEEP was increased from 5 to 12 cm H2O, there was a relative increase in MP of 60.7% (IQR 49.3-82.9) and in ΔP of 33.3% (IQR 17.8-65.8). A positive correlation was observed between MP and ΔP (ρ = 0.59).

CONCLUSIONS

Children with mild or moderate PARDS may experience a significant increase in MP and ∆P with increased PEEP. Therefore, respiratory system mechanics and lung recruitability must be carefully evaluated during PEEP titration.

Effect of end-inspiratory pause duration on respiratory system compliance calculation in mechanically ventilated dogs with healthy lungs

OBJECTIVE

To compare respiratory system compliance (CRS), expressed per kilogram of bodyweight (CRSBW), calculated without end-inspiratory pause (EIP) and after three EIP times (0.2, 0.5 and 1 seconds) with that after 3 second EIP (considered the reference EIP for static CRS) and to determine the EIP times that provided CRSBW values in acceptable agreement with static CRSBW during controlled mechanical ventilation (CMV) in anaesthetized dogs.

STUDY DESIGN

Prospective, randomized, nonblinded, crossover clinical study.

ANIMALS

A group of 24 client-owned dogs with healthy lungs undergoing surgery in lateral recumbency.

METHODS

During CMV in dogs undergoing general anaesthesia, five EIPs [0 (no EIP), 0.2, 0.5, 1 and 3 seconds] were consecutively applied in random order. Tidal volume (Vt) was set at 10 mL kg-1 and positive end-expiratory pressure (PEEP) was not applied. Respiratory rate and inspiratory time were established according to each EIP time, setting EIP between 0 and 50% of the inspiratory time. The CRSBW was calculated as [expired Vt/(plateau pressure - PEEP)]/bodyweight and recorded every 15 seconds for 2 minutes after a 5 minute equilibration period with each EIP. One-way anova for repeated measures and the Bland-Altman analysis were used to compare CRSBW and evaluate agreement between EIP times, respectively.

RESULTS

The CRSBW was significantly greater as the EIP time increased up to 1 second (p < 0.05). In the Bland-Altman analysis, none of the tested EIPs (0, 0.2, 0.5 and 1 seconds) provided 95% confidence intervals for limits of agreement within the maximum allowed difference considered for acceptable agreement with 3 second EIP.

CONCLUSIONS

and clinical relevance An EIP ≤ to 1 second does not provide a CRSBW value in acceptable agreement with static CRSBW in healthy dogs. Besides, the application of an EIP ≤ to 0.5 seconds underestimates the static CRSBW to an increasing extent as the EIP time decreases.

Effect of increased resistance on dynamic compliance assessed by two clinical monitors during volume-controlled ventilation: A test-lung study

OBJECTIVE

To evaluate the effect of increased respiratory system resistance (RRS) on dynamic compliance (Cdyn) assessed by the NM3 monitor (Cdyn(NM3)) and the E-CAiOV module (Cdyn(ECAiOV)).

STUDY DESIGN

Prospective laboratory study.

METHODS

A training test lung (TTL) simulated the mechanical ventilation of a mammal with 50 and 300 mL tidal volumes in three conditions of RRS [normal (RBL), moderately increased (R1) and severely increased (R2)] and a wide range of clinically relevant Cdyn. Simulations at increased RRS were paired with simulations at RBL with the same static compliance for comparisons. Pearson's correlation coefficient and concordance correlation coefficient between the measurements at RBL with the ones with increased RRS were calculated. Bland-Altman plots were also used to evaluate the agreement of Cdyn(ECAiOV) and Cdyn(NM3) at RBL (control values) with their paired values at R1 and R2. Relative bias and limits of agreement (LOAs) were calculated and LOAs larger than 30% were considered unacceptable. Trending ability of Cdyn(NM3) and Cdyn(ECAiOV) were evaluated by polar plots. Values of p < 0.05 were considered significant.

RESULTS

The effect of increased RRS was more pronounced for Cdyn(ECAiOV) than for Cdyn(NM3). Unacceptable agreement was only observed in Cdyn(NM3) at R2 in the 300 mL simulation (bias = -18.3% and lower LOA = -45%). For Cdyn(ECAiOV), agreement was unacceptable for all tested RRS in both simulations, being the worst at R2 in the 300 mL simulation (bias = -54.7% and lower LOA = -100.2%). Both levels of increased RRS caused poor trending ability for Cdyn(ECAiOV), whereas the same effect was only observed for Cdyn(NM3) at R2.

CONCLUSIONS AND CLINICAL RELEVANCE

In the presence of increased RRS, Cdyn estimated by the NM3 monitor presented better capability to distinguish between changes in RRS from changes in respiratory system compliance.

Respiratory mechanics and gas exchange in an ovine model of congenital heart disease with increased pulmonary blood flow and pressure

In a model of congenital heart disease (CHD), we evaluated if chronically increased pulmonary blood flow and pressure were associated with altered respiratory mechanics and gas exchange. Respiratory mechanics and gas exchange were evaluated in 6 shunt, 7 SHAM, and 7 control age-matched lambs. Lambs were anesthetized and mechanically ventilated for 15 min with tidal volume of 10 mL/kg, positive end-expiratory pressure of 5 cmH2O, and inspired oxygen fraction of 0.21. Respiratory system, lung and chest wall compliances (Crs, CL and Ccw, respectively) and resistances (Rrs, RL and Rcw, respectively), and the profile of the elastic pressure-volume curve (%E2) were evaluated. Arterial blood gases and volumetric capnography variables were collected. Comparisons between groups were performed by one-way ANOVA followed by Tukey-Kramer test for normally distributed data and with Kruskal-Wallis test followed by Steel-Dwass test for non-normally distributed data. Average Crs and CL in shunt lambs were 30% and 58% lower than in control, and 56% and 68% lower than in SHAM lambs, respectively. Ccw was 52% and 47% higher and Rcw was 53% and 40% lower in shunt lambs compared to controls and SHAMs, respectively. No difference in %E2 was identified between groups. No difference in respiratory mechanics was observed between control and SHAM lambs. In shunt lambs, Rcw, Crs and CL were decreased and Ccw was increased when compared to control and SHAM lambs. Pulmonary gas exchange did not seem to be impaired in shunt lambs when compared to controls and SHAMs.

Precision of continuous neonatal ventilator respiratory mechanics is improved with selected optimal respiratory cycles

Given their high apparent variability, bedside continuous respiratory mechanics (RM) parameters [excepting tidal volume (V (T))] remain infrequently used for adjustment of neonatal ventilatory settings. RM parameters provided by ventilator (VRC) from ten recordings of newborns [10 min in synchronised intermittent mandatory ventilation and 10 min in assist/control (A/C)] were compared to those computed from visually selected assisted leak-free optimal respiratory cycles (SRC). Mean values, variability and ability to distinguish patients were compared between VRC and SRC. Dynamic resistances were more correlated (r(2) = 0.95) than compliances (r (2) = 0.42). V (T)s were correlated only in A/C (r(2) = 0.78). C20/C was significantly higher in VRC (1.81 ± 0.67) than in SRC (1.23 ± 0.36) and frequently out of neonatal reference range. In A/C ventilation, V(T) was higher in VRC (5.6 ± 1.8 ml/kg) than in SRC (4.8 ± 1.0 ml/kg) (p < 0.05). Displayed V (T)s do not reflect those found in optimal assisted breaths and therefore have incomplete value in assessing adequacy of ventilator settings. The variability of RM parameters provided by the ventilator is large, and coefficients of variation were significantly lower with optimal respiratory cycles (for resistance, compliance, V (T) and C20/C; 27%, 26%, 18%, 24% in SRC and 36%, 35%, 40% and 33% in VRC). Selecting optimal cycles yields RM with two to three times higher discriminating power between patients.

CONCLUSION

Current ventilator's RM parameters have limited clinical use. Using optimal breaths to calculate RM parameters improves precision and discriminating power. For integration to ventilatory care, automation of this selection must be implemented first.

Adaptive SLICE method: an enhanced method to determine nonlinear dynamic respiratory system mechanics

The objective of this paper is to introduce and evaluate the adaptive SLICE method (ASM) for continuous determination of intratidal nonlinear dynamic compliance and resistance. The tidal volume is subdivided into a series of volume intervals called slices. For each slice, one compliance and one resistance are calculated by applying a least-squares-fit method. The volume window (width) covered by each slice is determined based on the confidence interval of the parameter estimation. The method was compared to the original SLICE method and evaluated using simulation and animal data. The ASM was also challenged with separate analysis of dynamic compliance during inspiration. If the signal-to-noise ratio (SNR) in the respiratory data decreased from +∞ to 10 dB, the relative errors of compliance increased from 0.1% to 22% for the ASM and from 0.2% to 227% for the SLICE method. Fewer differences were found in resistance. When the SNR was larger than 40 dB, the ASM delivered over 40 parameter estimates (42.2 ± 1.3). When analyzing the compliance during inspiration separately, the estimates calculated with the ASM were more stable. The adaptive determination of slice bounds results in consistent and reliable parameter values. Online analysis of nonlinear respiratory mechanics will profit from such an adaptive selection of interval size.

Limitation of measurements of expiratory tidal volume and expiratory compliance under conditions of endotracheal tube leaks

OBJECTIVE

Endotracheal tube leaks (ETTLs) occur in neonates ventilated with uncuffed tubes. Assuming that the influence of ETTLs might be neglected during expiration, only expiratory tidal volume is measured for calculation of expiratory compliance in cases of large ETTLs. However, expiratory ETTL might be substantial. Therefore, we evaluated the effect of ETTL size on expiratory tidal volume and compliance.

DESIGN

Prospective laboratory study and retrospective clinical study.

SETTING

University research laboratory and neonatal intensive care unit.

PATIENTS

Sixty ventilated neonates (weight 640-2160 g, gestational age 25-33 wks) were investigated.

INTERVENTIONS

The impact of increasing ETTLs on inspiratory and expiratory measured tidal volume (Vm), corrected tidal volume (Vc), and leak volume (Vl) was investigated in a ventilated neonatal lung model. The range of ETTLs (1% to 95%) was subdivided into five groups of 12 infants each. Furthermore, the relationships between standard ETTL size and inspiratory and expiratory ETTLs were evaluated using nonlinear regression. Standard ETTL size was defined as the difference between measured inspiratory and expiratory tidal volume (Vm) related to inspiratory Vm.

MEASUREMENTS AND MAIN RESULTS

The size of a standard ETTL was 40% when expiratory ETTL reached 10% and was 12% when the inspiratory ETTL reached 10%. In infants, the differences between Vm and Vc were statistically significant during inspiration in the group beginning at a standard ETTL of 41% and during expiration in the group beginning at a standard ETTL of 69% (p < .05). Results of nonlinear regression showed that the standard ETTL was 33% (95% confidence interval, 28% to 36%) when expiratory ETTL reached 10% and was 13% (95% confidence interval, 12% to 15%) when inspiratory ETTL reached 10%.

CONCLUSIONS

Expiratory Vl has a relevant impact if a certain ETTL size is reached.

Respiratory system inertance corresponds to extravascular lung water in surfactant-deficient piglets

In various cardio-pulmonary diseases lung mass is considerably increased due to intrapulmonary fluid accumulation, i.e. extravascular lung water (EVLW). Generally, inertance is a physical system parameter that is mass-dependent. We hypothesized that changes in lung mass influence the inertive behavior of the respiratory system. EVLW and intrathoracic blood volume (ITBV) were compared with respiratory system inertance (I(rs)) in four piglets before and after broncho-alveolar lavage (BAL) that induced surfactant deficiency with interstitial edema. EVLW and ITBV were determined using the double-indicator dilution technique, I(rs) by multiple linear regression analysis. Measurements were taken before, and 1 and 2 h after BAL. EVLW increased threefold (from 6.2+/-0.8 mL/kg at baseline to 17.7+/-0.9 mL/kg (p < 0.001) after BAL). I(rs) increased by 35% (from 0.17+/-0.02 to 0.23+/-0.04 cmH(2)O s(2)/L (p = 0.036) after BAL) and was tightly correlated to EVLW (r(2) = 0.95, p < 0.023). ITBV did not change significantly after BAL. We conclude that I(rs) reflects actual changes in lung mass and thus hints at fluid accumulation within the lung.

Lung function measurements in a preterm animal model of respiratory failure: comparison of two different neonatal ventilators

A variety of ventilators are used in the NICU. Ventilator and lung function measures are often applied in weaning protocols or as outcome variables. The effect of different ventilators on these measures has not been well studied. Our objective was to compare ventilator and lung function measurements in a chronic preterm animal model managed with two different neonatal ventilators. Timed baboon pregnancies exposed to antenatal steroids were delivered by C/S at 125 days (term = 185 days). Infants were immediately intubated, given surfactant, and ventilated with low tidal volumes (4-6 ml/kg) for 6-14 days using well-defined protocols. One group was ventilated via InfantStar (IS) and the other by VIP-Bird (VIP). Physiologic and pulmonary function data were serially recorded with the VitalTrends plethysmography system. Between ventilator comparisons were made. InfantStar (IS) was used on 22 infants in 2002-03, VIP was used on 29 infants in 2004-05. No differences were found for gestation, birth weight, gender, paO(2), paCO(2), FiO(2), arterial/alveolar ratio, dynamic compliance, inspiratory resistance, or tidal volumes. From 24 to 336 h, peak and mean airway pressure, ventilator rate, and ventilatory efficiency index (VEI: PIP x R x CO(2)/1,000) were significantly greater in the VIP group at multiple time points. VIP use was associated with a significant increase in expiratory airway resistance (Rexp - cmH(2)O/L/s) at all but one-time points studied. Compared to the IS, use of the VIP-Bird ventilator in surfactant treated immature baboons with RDS was associated with increased expiratory airway resistance and indices of impaired ventilation, but not oxygenation. Ventilator management in the NICU, especially weaning, may be affected by the specific ventilator in use.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Inertance measurements by jet pulses in ventilated small lungs after perfluorochemical liquid (PFC) applications

Perfluorochemical liquid (PFC) liquids or aerosols are used for assisted ventilation, drug delivery, lung cancer hyperthermia and pulmonary imaging. The aim of this study was to investigate the effect of PFC liquid on the inertance (I) of the respiratory system in newborn piglets using partial liquid ventilation (PLV) with different volumes of liquid. End-inspiratory (I(in)) and end-expiratory (I(ex)) inertance were measured in 15 ventilated newborn piglets (age < 12 h, mean weight 724 +/- 93 g) by brief flow pulses before and 80 min after PLV using a PFC volume (PF5080, 3 M) of 10 ml kg(-1) (N = 5) or 30 ml kg(-1) (N = 10). I was calculated from the imaginary part of the measured respiratory input impedance by regression analysis. Straight tubes with 2-4 mm inner diameter were used to validate the equipment in vitro by comparison with the analytically calculated values. In vitro measurements showed that the measuring error of I was <5% and that the reproducibility was better than 1.5%. The correlation coefficient of the regression model to determine I was >0.988 in all piglets. During gas ventilation, I(in) and I(ex) (mean +/- SD) were 31.7 +/- 0.8 Pa l(-1) s(2) and 33.3 +/- 2.1 Pa l(-1) s(2) in the 10 ml group and 32.4 +/- 0.8 Pa l(-1) s(2) and 34.0 +/- 2.5 Pa l(-1) s(2) in the 30 ml group. However, I of the 3 mm endotracheal tube (ETT) used was already 26.4 Pa l(-1) s(2) (about 80% of measured I). During PLV, there was a minimal increase of I(in) to 33.1 +/- 2.5 Pa l(-1) s(2) in the 10 ml group and to 34.5 +/- 2.7 Pa l(-1) s(2) in the 30 ml group. In contrast, the increase of I(ex) was dramatically larger (p < 0.001) to 67.7 +/- 13.3 Pa l(-1) s(2) and to 74.8 +/- 9.3 Pa l(-1) s(2) in the 10 ml and 30 ml groups, respectively. Measurements of I by jet pulses in intubated small animals are reproducible. PFC increases the respiratory inertance, but the magnitude depends considerably on its spatial distribution which changes during the breathing cycle. Large differences between I(in) and I(ex) are an indicator for liquid in airways or the ETT.

Influence of respiratory system impedance on volume and pressure delivered at the Y piece in ventilated infants

OBJECTIVES

Tidal volume (VT) delivered to infants' airways are overestimated and pressure underestimated when measured in the ventilator and not at the Y piece. This study aimed at evaluating the influence of respiratory system impedance on expiratory VT (VTE) and pressure measurement difference.

DESIGN

Prospective observational study.

SETTING

Pediatric intensive care unit at a university hospital.

PATIENTS

Data were collected between February 2000 and October 2001 for 30 infants (range, 1-23 months) ventilated in the pressure-controlled or volume-controlled mode.

INTERVENTIONS

Measurements of VTE, pressure obtained at the same time at the Y piece and on the ventilator Servo 300, were collected in ventilated infants. Respiratory system impedance was calculated from data obtained at the Y piece. Circuit compliance was measured in vitro. VTEs were corrected for compressible volume.

MEASUREMENTS AND RESULTS

VTEs were overestimated by the Servo 300 in the pressure-controlled and volume-controlled modes (from 5% to 62% of the value displayed on Servo 300). Maximal inspiratory pressures were underestimated by the Servo 300 in the pressure-controlled mode (difference from -2 to +19 cm H(2)O). Measurement difference increased with increasing respiratory system impedance. Ventilator VTE corrected for circuit compliance did not offer a sufficiently accurate estimation of VTE at the Y piece.

CONCLUSIONS

VT and pressure measurements must be performed at the Y piece, especially in infants with increased respiratory system impedance (i.e., decreased respiratory system compliance or increased resistance). Correcting VTE for circuit compliance cannot replace measurement of VT at the Y piece.

Measurement of changes in respiratory mechanics during partial liquid ventilation using jet pulses

OBJECTIVE

To compare the changes in respiratory mechanics within the breathing cycle in healthy lungs between gas ventilation and partial liquid ventilation using a special forced-oscillation technique.

DESIGN

Prospective animal trial.

SETTINGS

Animal laboratory in a university setting.

SUBJECTS

A total of 12 newborn piglets (age, <12 hrs; mean weight, 725 g).

INTERVENTIONS

After intubation and instrumentation, lung mechanics of the anesthetized piglets were measured by forced-oscillation technique at the end of inspiration and the end of expiration. The measurements were performed during gas ventilation and 80 mins after instillation of 30 mL/kg perfluorocarbon PF 5080.

MEASUREMENTS AND MAIN RESULTS

Brief flow pulses (width, 10 msec; peak flow, 16 L/min) were generated by a jet generator to measure the end-inspiratory and the end-expiratory respiratory input impedance in the frequency range of 4-32 Hz. The mechanical variables resistance, inertance, and compliance were determined by model fitting, using the method of least squares. At least in the lower frequency range, respiratory mechanics could be described adequately by an RIC single-compartment model in all piglets. During gas ventilation, the respiratory variables resistance and inertance did not differ significantly between end-inspiratory and end-expiratory measurements (mean [sd]: 4.2 [0.7] vs. 4.1 [0.6] kPa x L(-1) x sec, 30.0 [3.2] vs. 30.7 [3.1] Pa x L(-1) x sec2, respectively), whereas compliance decreased during inspiration from 14.8 (2.0) to 10.2 (2.4) mL x kPa(-1) x kg(-1) due to a slight lung overdistension. During partial liquid ventilation, the end-inspiratory respiratory mechanics was not different from the end-inspiratory respiratory mechanics measured during gas ventilation. However, in contrast to gas ventilation during partial liquid ventilation, compliance rose from 8.2 (1.0) to 13.0 (3.0) mL x kPa(-1) x kg(-1) during inspiration. During expiration, when perfluorocarbon came into the upper airways, both resistance and inertance increased considerably (mean with 95% confidence interval) by 34.3% (23.1%-45.8%) and 104.1% (96.0%-112.1%), respectively.

CONCLUSIONS

The changes in the respiratory mechanics within the breathing cycle are considerably higher during partial liquid ventilation compared with gas ventilation. This dependence of lung mechanics from the pulmonary gas volume hampers the comparability of dynamic measurements during partial liquid ventilation, and the magnitude of these changes cannot be detected by conventional respiratory-mechanical analysis using time-averaged variables.

The flow-pressure plot: a new look on the patient-ventilator interaction in neonatal care

Most modern neonatal ventilators have now a built-in flow sensor and, as a spin-off of their mechanical action, provide some information about lung function characteristics as compliance and resistance after computation of the flow and pressure signals. Additionally, respiratory graphics as volume-pressure and flow-volume plots can be displayed. In clinical practice, however, they are rarely used to refine the ventilator setting. A nonconventional flow-pressure plot is presented in this article, constructed from the volume or flow, and pressure outputs of a Babylog 8000 (Dräger, Lübeck, Germany). This flow-pressure diagram appears to be useful in the real-time computation of respiratory mechanics based on the Rahn's law of respiratory motion. Its major advantage, however, is the easy pattern recognition of subtle changes in infant-ventilator interaction, ie, excessive triggering, fighting against the ventilator, augmented breath, tube subobstruction. It may be useful to add the flow-pressure plot to the classical respiratory graphics allowing to monitor mechanical ventilation more accurately and to fine tune the ventilator setting accordingly.

Role of lung function testing in the management of mechanically ventilated infants

The mechanical characteristics of the ventilated lung can only be interpreted when the volume of the lung, the elastic properties, and the degree of airway obstruction have been accurately quantified by pulmonary function testing. More gentle ventilation strategies (permissive hypercapnia) are used, and the efficacy of mechanical ventilation can be verified in the intensive care unit. Pulmonary function testing brings new insights, awareness, and applications, but its limitations need to be taken into account when interpreting the acquired data.

Salbutamol prevents the increase of respiratory resistance caused by tracheal intubation during sevoflurane anesthesia in asthmatic children

Asthmatic children having their tracheas intubated with sevoflurane often have an increase in respiratory system resistance (Rrs). In this randomized, placebo-controlled, double-blinded study, we investigated the protective effect of an inhaled beta2-adrenergic agonist. Either salbutamol or placebo was administered 30 to 60 min before anesthesia to 30 mildly to moderately asthmatic children scheduled for elective surgery. Induction was performed with sevoflurane in a mixture of 50% nitrous oxide in oxygen and maintained at 3%, with children breathing spontaneously via a face mask and Jackson-Rees modification of the T-piece. Airway opening pressure and flow were measured before and after insertion of an oral endotracheal tube. Rrs and respiratory system compliance were calculated with multilinear regression analysis. The groups were comparable with respect to age, weight, asthma history, and breathing pattern. Intubation induced a different Rrs response in the two groups: children treated with salbutamol showed a 6.0% (-25.2% to +13.2%) decrease (mean, 95% confidence interval), whereas in the Placebo group there was a 17.7% (+4.4% to +30.9%) increase (P = 0.04). Neither asthma history nor the serum inflammation marker eosinophilic cationic protein was predictive for this response. We conclude that when using sevoflurane in mildly to moderately asthmatic children, a preanesthetic treatment with inhaled salbutamol is protective of an increase in Rrs.

IMPLICATIONS

Tracheal intubation with sevoflurane as the sole anesthetic is now often performed in children. It can induce an increase in respiratory system resistance in children with asthma. This study shows that in children with mild to moderate asthma, a preanesthetic treatment with inhaled salbutamol can prevent the increase of respiratory system resistance.

Dynamic respiratory system mechanics in infants during pressure and volume controlled ventilation

Dynamic respiratory system mechanics can be determined using multiple linear regression (MLR) analysis. There is no need for a particular ventilator setting or for a special ventilatory manoeuvre. The purpose of this study was to investigate whether or not different ventilator modes and the flow-dependent resistance of the endotracheal tube (ETT) influence the determination of resistance and compliance by MLR. Ten paediatric patients who were on controlled mechanical ventilation for various disorders were investigated. The ventilator modes were changed between pressure control (PC) and volume control (VC). Flow and airway pressure were measured and tracheal pressure was continuously calculated. Each mode was applied for 3 min, and 10 consecutive breaths at the end of each period were analysed. Respiratory mechanics were determined by MLR based on either airway pressure, thus including the resistance of the ETT, or tracheal pressure. Resistance was found to be slightly higher in PC than in VC. There was no effect on determination of compliance between the different modes. Elimination of the flow-dependent resistance of the ETT preserved the differences between the modes. The authors conclude that using multiple linear regression compliance is not affected by the actual ventilator mode, whereas resistance is.

Influence of jacket placement on respiratory compliance during raised lung volume measurements in infants

SUMMARY. Recent introduction of the raised lung volume rapid thoraco-abdominal compression (RVRTC) technique for measuring forced expiratory maneuvers in infants provides the potential opportunity to assess respiratory mechanics simultaneously by using multiple linear regression (MLR) of the relaxed breaths preceding jacket inflation to force expiration. This study was undertaken to investigate whether data obtained from raised lung volume are influenced by placement of the rapid thoraco-abdominal compression (RTC) squeeze jacket. Paired measurements of tidal volume (V(T)) and respiratory rate (RR) during tidal breathing, and of inflation volume (V(inf)), respiratory system compliance (C(rs)), and resistance (R(rs)) during passive lung inflations were made in 60 (30 male) healthy term infants with and without a fastened, but uninflated RTC jacket in place. Jacket placement was associated with a significant reduction (P < 0.0001) in weight-corrected V(inf) [-1.86 (95% confidence interval, -2.46, -1.27) mL.kg(-1)] and C(rs) [-0.77 (-1.04, -0.49) mL.kPa(-1).kg(-1)]. This represented a reduction in weight-corrected C(rs) from 9.00 to 8.24 mL.kPa(-1).kg(-1), with the fall being >10% in 42% of infants studied. There was no significant change in R(rs) or weight-corrected V(T). If passive respiratory mechanics are to be measured during raised lung volume maneuvers, they should be performed prior to the jacket being fastened, unless considerable care is taken with each infant to ensure that the jacket does not restrict chest wall movement during maximum inflation.

Compliance is nonlinear over tidal volume irrespective of positive end-expiratory pressure level in surfactant-depleted piglets

Between the lower and the upper inflection point of a quasistatic pressure-volume (PV) curve, a segment usually appears in which the PV relationship is steep and linear (i.e., compliance is high, with maximal volume change per pressure change, and is constant). Traditionally it is assumed that when positive end-expiratory pressure (PEEP) and tidal volume (V T) are titrated such that the end-inspiratory volume is positioned at this linear segment of the PV curve, compliance is constant over VT during ongoing ventilation. The validity of this assumption was addressed in this study. In 14 surfactant-deficient piglets, PEEP was increased from 3 cm H(2)O to 24 cm H(2)O, and the compliance associated with 10 consecutive volume increments up to full VT was determined with a modified multiple-occlusion method at the different PEEP levels. With PEEP at approximately the lower inflection point, compliance was minimal in most lungs and decreased markedly over VT, indicating overdistension. Compliance both increased and decreased within the same breath at intermediate PEEP levels. It is concluded that a PEEP that results in constant compliance over the full VT range is difficult to find, and cannot be derived from conventional respiratory-mechanical analyses; nor does this PEEP level coincide with maximal gas exchange.

Analysis of nonlinear volume-dependent respiratory system mechanics in pediatric patients

OBJECTIVE

Analysis of dynamic respiratory system mechanics is generally based on a resistance-compliance model in which nonlinearities of the respiratory mechanics indices are not considered. The recently developed SLICE method analyzing consecutive volume slices of the tidal volume was used for determination of non-linear volume-dependent respiratory system mechanics. Volume-dependent compliance C(Slice) and resistance R(Slice) were compared with C(MLR) and R(MLR) obtained by standard multiple linear regression analysis (MLR).

DESIGN

Prospective observational study.

SETTING

Pediatric intensive care unit in a university hospital.

PATIENTS

Fifteen pediatric patients, aged 24 days to 9.6 yrs, weighing 3-67.5 kg.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

With respect to their pulmonary status, the patients were grouped into three clinical groups: patients with no lung diseases, patients with restrictive lung diseases, and patients with obstructive lung diseases. All patients were mechanically ventilated via a cuffed endotracheal tube in the pressure-controlled mode. Flow and airway pressure were measured at the proximal end of the tube and tracheal pressure was continuously calculated. Respiratory mechanics were determined either with the SLICE method or, as reference, by using standard MLR. In most patients, the pressure-volume relationship was nonlinear, particularly in patients with restrictive and obstructive lung diseases. In the presence of considerable nonlinearity, the volume-dependent respiratory mechanics indices obtained by the SLICE method showed better agreement between recalculated and original pressure-volume loops compared with the MLR results. Furthermore, signs of overdistension of the patient's lung became obvious when using the SLICE method, whereas they were undetected by MLR.

CONCLUSIONS

The SLICE method is well suited for the analysis of nonlinear volume-dependent respiratory system mechanics in pediatric patients. The SLICE method may be used as a first step toward an adaptation of ventilator settings with respect to the actual mechanical status of the patient's respiratory system, and, to prevent pulmonary overdistension.

Pulmonary vascular-bronchial interactions: acute reduction in pulmonary blood flow alters lung mechanics

BACKGROUND

Postoperative pulmonary hypertension in children after congenital heart surgery is a risk factor for death and is associated with severe acute changes in both pulmonary vascular resistance and lung mechanics.

OBJECTIVE

To examine the impact of changes in pulmonary blood flow on lung mechanics in preoperative children with congenital heart disease, in order to assess the cause-effect relation of pulmonary vascular-bronchial interactions.

DESIGN

Prospective, cross sectional study.

SETTING

Cardiac catheterisation laboratory, general anaesthesia with mechanical ventilation.

INTERVENTIONS

Variation of pulmonary blood flow (Qp) by either balloon occlusion of an atrial septal defect before interventional closure, or by complete occlusion of the pulmonary artery during balloon pulmonary valvuloplasty for pulmonary valve stenosis.

MAIN OUTCOME MEASURES

Ventilatory tidal volume (Vt), dynamic respiratory system compliance (Cdyn), respiratory system resistance (Rrs).

RESULTS

28 occlusions were examined in nine patients with atrial septal defect (median age 9.5 years) and 22 in eight patients with pulmonary stenosis (median age 1.2 years). Normalisation of Qp during balloon occlusion of atrial septal defect caused no significant change in airway pressures and Rrs, but there was a small decrease in Vt (mean (SD): 9.61 (0.85) to 9.52 (0.97) ml/kg; p < 0.05) and Cdyn (0.64 (0.11) to 0.59 (0.10) ml/cm H(2)O*kg; p < 0.01). These changes were more pronounced when there was complete cessation of Qp during balloon valvuloplasty in pulmonary stenosis, with a fall in Vt (9.71 (2.95) to 9.32 (2.84) ml/kg; p < 0.05) and Cdyn (0.72 (0.29) to 0.64 (0.26) ml/cm H(2)O*kg; p < 0.001), and there was also an increase in Rrs (25.1 (1. 7) to 28.8 (1.6) cm H(2)O/litre*s; p < 0.01). All these changes exceeded the variability of the baseline measurements more than threefold.

CONCLUSIONS

Acute changes in pulmonary blood flow are associated with simultaneous changes in lung mechanics. While these changes are small they may represent a valid model to explain the pathophysiological impact of spontaneous changes in pulmonary blood flow in clinically more critical situations in children with congenital heart disease.

Changes in respiratory mechanics during abdominal laparoscopic surgery in children

We designed this study in order to measure the changes in respiratory mechanics during laparoscopic surgery in children. Ventilation parameters (Flow (Ví) and Peak Pressure (Pmax)) were measured and total respiratory system mechanics (resistance (Rrs) and compliance (Crs)) were derived using multiple linear regression analysis in 11 children aged 8 months to 11 years. The Pmax increased by 26.6% and the Rrs increased by 20.2% whilst the Crs decreased by 38.9% after pneumoperitoneum. These findings suggest that clinically important changes in respiratory mechanics occur as a result of the pneumoperitoneum produced during abdominal laparoscopic surgery.

Respiratory oscillation mechanics in infants with bronchiolitis during mechanical ventilation

The aim of the study was to describe the pattern of respiratory oscillation mechanics and responses to positive end-expiratory pressure (PEEP) in bronchiolitis. Six infants were studied during the course of mechanical ventilation. A 20 Hz sinusoidal pressure variation was applied at the endotracheal tube where flow was measured with a pneumotachograph. Resistance and reactance obtained from the complex pressure-flow ratio were separated during inspiration (R(rs,i); X(rs,i)) and expiration (R(rs,e); X(rs,e)), and the differences between R(rs,i) and R(rs,e) (deltaR(rs)) and X(rs,i) and X(rs,e) (deltaX(rs)) were calculated. The data were corrected for the mechanical characteristics of the endotracheal tube. The measurements were repeated while PEEP was varied between 0 and 8 hPa. Two infants were found to have normal R(rs) and near-zero X(rs) and both parameters exhibited little change within the respiratory cycle or with varying PEEP. Four infants had high R(rs) at zero PEEP. In two, R(rs,i) was markedly elevated (108.5 and 85.2 hPa.s/L, respectively), and X(rs,i) was markedly negative (-25.0 and -22.5 hPa.s/L, respectively) at zero PEEP, while deltaR(rs) and deltaX(rs) were small. R(rs,i) and the absolute value of X(rs,i) decreased with increasing PEEP. This pattern of oscillation mechanics was consistent with low lung volumes and atelectasis, being reversed by increasing PEEP. In the remaining two subjects, R(rs,i) was moderately elevated (57.8 and 53.6 hPa.s/L, respectively) and X(rs,i) moderately negative (-12.5 and -7.7 hPa.s/L, respectively) at zero PEEP. DeltaR(rs) (-59.8 and -56.5 hPa.s/L, respectively) and delta(rs) (28.1 and 48.7 hPa.s/L, respectively) were large, but were dramatically reduced by increasing PEEP. These patterns were consistent with expiratory airflow limitation. Measurements of respiratory impedance are, therefore, informative in regard to the pathophysiological mechanisms occurring in bronchiolitis during mechanical ventilation, and they may be helpful in setting the level and assessing the effect of PEEP.

Respiratory mechanics during mechanical ventilation: a model study on the effects of leak around a tracheal tube

Air leak around a tracheal tube (TT) during mechanical ventilation is likely to occur during the inspiratory phase because airway pressure is high for a prolonged period. The presence of a leak may introduce errors in measurements of respiratory mechanics made at the airway opening. If so, respiratory mechanics can be measured more accurately when data are collected during the expiratory phase of ventilation. We examined this phenomenon in a lung model. When a leak was introduced into the model, simulating a leak around the TT, the leak occurred predominantly during the inspiratory phase of respiration. As the magnitude of the leak increased, the overestimation of resistance progressively increased, when calculated from pressure and flow measured at the airway opening. A large leak (38%) resulted in an overestimation of respiratory system resistance by 51% and an underestimation of elastance (Ers) by 23% when calculated from the entire ventilatory cycle. However, there was no under- or overestimation in Rrs when calculated from the expiratory phase only, and ERS was overestimated by only 6.1%. Varying peak inspiratory pressure, end-expiratory pressure, and expiratory time did influence the effect of leak, however, respiratory mechanics could still be calculated accurately from the expiratory phase under these conditions. We conclude that measurements of lung mechanics from the expiratory phase is a promising approach to dealing with the problem of measuring respiratory mechanics in mechanically ventilated infants with leaks around the tracheal tube.

A new microtransducer catheter for measuring esophageal pressure in infants

Measurement of esophageal pressure, as a reflection of pleural pressure, is essential for assessment of dynamic lung mechanics in neonates and infants. Conventionally, an esophageal balloon or a fluid-filled catheter is used, but considerable skill is required to obtain accurate results. Both devices have problems, and failure to achieve valid occlusion tests have been reported, particularly in small infants with lung disease. Recently, a flexible #3 French gauge (FG) microtransducer catheter (MTC, Dräger Netherlands) has become available for medical monitoring. We have assessed the accuracy and feasibility of using this device for measuring lung mechanics in 51 spontaneously breathing infants and small children aged 1 day to 24 months (weight 1.35 to 12.0 kg), 9 of whom were healthy neonates, the remainder suffering from a variety of cardio-respiratory diseases, and in 18 sick ventilated infants (weight 0.6 to 4.0 kg). Positioning of the catheter was well tolerated by all infants. The ratio of esophageal to airway opening pressure changes (delta Pes:delta Pao) ranged from 0.94 to 1.09 [mean (SD) 1.013 (0.03)] for the spontaneously breathing infants and from 0.98 to 1.06 [mean (SD) 1.003 (0.02)] In the ventilated infants with no significant difference in this ratio between the two groups (p = 0.16). This new generation of catheter tip pressure transducers may provide a simpler and more reliable tool for assessing transpulmonary pressure changes in infants than has previously been available.