Improved esophageal balloon technique for use in infants

Estimation of change in pleural pressure in assisted and unassisted spontaneous breathing pediatric patients using fluctuation of central venous pressure: A preliminary study

Background

It is important to evaluate the size of respiratory effort to prevent patient self-inflicted lung injury and ventilator-induced diaphragmatic dysfunction. Esophageal pressure (Pes) measurement is the gold standard for estimating respiratory effort, but it is complicated by technical issues. We previously reported that a change in pleural pressure (ΔPpl) could be estimated without measuring Pes using change in CVP (ΔCVP) that has been adjusted with a simple correction among mechanically ventilated, paralyzed pediatric patients. This study aimed to determine whether our method can be used to estimate ΔPpl in assisted and unassisted spontaneous breathing patients during mechanical ventilation.

Methods

The study included hemodynamically stable children (aged <18 years) who were mechanically ventilated, had spontaneous breathing, and had a central venous catheter and esophageal balloon catheter in place. We measured the change in Pes (ΔPes), ΔCVP, and ΔPpl that was calculated using a corrected ΔCVP (cΔCVP-derived ΔPpl) under three pressure support levels (10, 5, and 0 cmH₂O). The cΔCVP-derived ΔPpl value was calculated as follows: cΔCVP-derived ΔPpl = k × ΔCVP, where k was the ratio of the change in airway pressure (ΔPaw) to the ΔCVP during airway occlusion test.

Results

Of the 14 patients enrolled in the study, 6 were excluded because correct positioning of the esophageal balloon could not be confirmed, leaving eight patients for analysis (mean age, 4.8 months). Three variables that reflected ΔPpl (ΔPes, ΔCVP, and cΔCVP-derived ΔPpl) were measured and yielded the following results: -6.7 ± 4.8, − -2.6 ± 1.4, and − -7.3 ± 4.5 cmH2O, respectively. The repeated measures correlation between cΔCVP-derived ΔPpl and ΔPes showed that cΔCVP-derived ΔPpl had good correlation with ΔPes (r = 0.84, p< 0.0001).

Conclusions

ΔPpl can be estimated reasonably accurately by ΔCVP using our method in assisted and unassisted spontaneous breathing children during mechanical ventilation.

How David Hatch became a professor of pediatric anesthesia

This is an account of an interview with David John Hatch who was one of the first Professors of Pediatric Anesthesia in the world. He began his anesthesia career as a medical student administering chloroform and ended it 40 years later as a Consultant at Great Ormond Street Hospital where he developed and led a world renowned research team measuring and assessing lung function in infants and children. These productive years earned him his chair at the Institute of Child Health in London (part of University College London) funded by Portex (currently, a branch of Smiths Medical). His academic achievements include over 110 journal publications, two textbooks and having many honors and awards. Yet he does not think of himself as an academic. In his words "I wanted to be a hard working clinician with an interest in research, and not just academic".

Use of esophageal balloon pressure-volume curve analysis to determine esophageal wall elastance and calibrate raw esophageal pressure: a bench experiment and clinical study

BACKGROUND

Accurate measurement of esophageal pressure (Pes) depends on proper filling of the balloon. Esophageal wall elastance (Ees) may also influence the measurement. We examined the estimation of balloon-surrounding elastance in a bench model and investigated a simplified calibrating procedure of Pes in a balloon with relatively small volume.

METHODS

The Cooper balloon catheter (geometric volume of 2.8 ml) was used in the present study. The balloon was progressively inflated in different gas-tight glass chambers with different inner volumes. Chamber elastance was measured by the fitting of chamber pressure and balloon volume. Balloon pressure-volume (P-V) curves were obtained, and the slope of the intermediate linear section was defined as the estimated chamber elastance. Balloon volume tests were also performed in 40 patients under controlled ventilation. The slope of the intermediate linear section on the end-expiratory esophageal P-V curve was calculated as the Ees. The balloon volume with the largest Pes tidal swing was defined as the best volume. Pressure generated by the esophageal wall during balloon inflation (Pew) was estimated as the product of Ees and best volume. Because the clinical intermediate linear section enclosed filling volume of 0.6 to 1.4 ml in each of the patient, we simplified the estimation of Ees by only using parameters at these two filling volumes.

RESULTS

In the bench experiment, bias (lower and upper limits of agreement) was 0.5 (0.2 to 0.8) cmH₂O/ml between the estimated and measured chamber elastance. The intermediate linear section on the clinical and bench P-V curves resembled each other. Median (interquartile range) Ees was 3.3 (2.5-4.1) cmH₂O/ml. Clinical best volume was 1.0 (0.8-1.2) ml and ranged from 0.6 to 1.4 ml. Estimated Pew at the best volume was 2.8 (2.5-3.5) cmH₂O with a maximum value of 5.2 cmH₂O. Compared with the conventional method, bias (lower and upper limits of agreement) of Ees estimated by the simple method was - 0.1 (- 0.7 to 0.6) cmH₂O/ml.

CONCLUSIONS

The slope of the intermediate linear section on the balloon P-V curve correlated with the balloon-surrounding elastance. The estimation of Ees and calibration of Pes were feasible for a small-volume-balloon.

TRIAL REGISTRATION

Identifier NCT02976844 . Retrospectively registered on 29 November 2016.

Optimal esophageal balloon volume for accurate estimation of pleural pressure at end-expiration and end-inspiration: an in vitro bench experiment

BACKGROUND

Esophageal pressure, used as a surrogate for pleural pressure, is commonly measured by air-filled balloon, and the accuracy of measurement depends on the proper balloon volume. It has been found that larger filling volume is required at higher surrounding pressure. In the present study, we determined the balloon pressure-volume relationship in a bench model simulating the pleural cavity during controlled ventilation. The aim was to confirm whether an optimal balloon volume range existed that could provide accurate measurement at both end-expiration and end-inspiration.

METHODS

We investigated three esophageal balloons with different dimensions and materials: Cooper, SmartCath-G, and Microtek catheters. The balloon was introduced into a glass chamber simulating the pleural cavity and volume-controlled ventilation was initiated. The ventilator was set to obtain respective chamber pressures of 5 and 20 cmH₂O during end-expiratory and end-inspiratory occlusion. Balloon was progressively inflated, and balloon pressure and chamber pressure were measured. Balloon transmural pressure was defined as the difference between balloon and chamber pressure. The balloon pressure-volume curve was fitted by sigmoid regression, and the minimal and maximal balloon volume accurately reflecting the surrounding pressure was estimated using the lower and upper inflection point of the fitted sigmoid curve. Balloon volumes at end-expiratory and end-inspiratory occlusion were explored, and the balloon volume range that provided accurate measurement at both phases was defined as the optimal filling volume.

RESULTS

Sigmoid regression of the balloon pressure-volume curve was justified by the dimensionless variable fitting and residual distribution analysis. All balloon transmural pressures were within ±1.0 cmH₂O at the minimal and maximal balloon volumes. The minimal and maximal balloon volumes during end-inspiratory occlusion were significantly larger than those during end-expiratory occlusion, except for the minimal volume in Cooper catheter. Mean (±standard deviation) of optimal filling volume both suitable for end-expiratory and end-inspiratory measurement ranged 0.7 ± 0.0 to 1.7 ± 0.2 ml in Cooper, 1.9 ± 0.2 to 3.6 ± 0.3 ml in SmartCath-G, and 2.2 ± 0.2 to 4.6 ± 0.1 ml in Microtek catheter.

CONCLUSIONS

In each of the tested balloon, an optimal filling volume range was found that provided accurate measurement during both end-expiratory and end-inspiratory occlusion.

Correcting the dynamic response of a commercial esophageal balloon-catheter

It is generally recommended that an esophageal balloon-catheter possess an adequate frequency response up to 15 Hz, such that parameters of respiratory mechanics may be quantified with precision. In our experience, however, we have observed that some commercially available systems do not display an ideal frequency response (<8-10 Hz). We therefore investigated whether the poor frequency response of a commercially available esophageal catheter may be adequately compensated using two numerical techniques: 1) an exponential model correction, and 2) Wiener deconvolution. These two numerical techniques were performed on a commercial balloon-catheter interfaced with 0, 1, and 2 lengths of extension tubing (90 cm each), referred to as configurations L0, L90, and L180, respectively. The frequency response of the balloon-catheter in these configurations was assessed by empirical transfer function analysis, and its "working" range was defined as the frequency beyond which more than 5% amplitude and/or phase distortion was observed. The working frequency range of the uncorrected balloon-catheter extended up to only 10 Hz for L0, and progressively worsened with additional tubing length (L90 = 3 Hz, L180 = 2 Hz). Although both numerical methods of correction adequately enhanced the working frequency range of the balloon-catheter to beyond 25 Hz for all length configurations (L0, L90, and L180), Wiener deconvolution consistently provided more accurate corrections. Our data indicate that Wiener deconvolution provides a superior correction of the balloon-catheter's dynamic response, and is relatively more robust to extensions in catheter tube length compared with the exponential correction method.

Response characteristics of esophageal balloon catheters handmade using latex and nonlatex materials

The measurement of esophageal pressure allows for the calculation of several important and clinically useful parameters of respiratory mechanics. Esophageal pressure is often measured with balloon-tipped catheters. These catheters may be handmade from natural latex condoms and polyethylene tubing. Given the potential of natural latex to cause allergic reaction, it is important to determine whether esophageal catheter balloons can be fabricated, by hand, using nonlatex condoms as construction materials. To determine the static and dynamic response characteristics of esophageal balloon catheters handmade from latex and nonlatex materials, six esophageal catheter balloons were constructed from each of the following condom materials: natural latex, synthetic polyisoprene, and polyurethane (18 total). Static compliance and working volume range of each balloon catheter was obtained from their pressure-volume characteristics in water. The dynamic response of balloon catheters were measured via a pressure “step” test, from which a third-order underdamped transfer function was modeled. The dynamic ranges of balloon catheters were characterized by the frequencies corresponding to ±5% amplitude- and phase-distortion (f_A5% and f_φ5%). Balloon catheters handmade from polyurethane condoms displayed the smallest working volume range and lowest static balloon compliance. Despite this lower compliance, f_A5% and f_φ5% were remarkably similar between all balloon materials. Our findings suggest that polyisoprene condoms are an ideal nonlatex construction material to use when fabricating esophageal catheter balloons by hand.

Crossover study of proportional assist versus assist control ventilation

OBJECTIVE

To test the hypothesis that in very prematurely born infants remaining ventilated beyond the first week, proportional assist ventilation (PAV) compared with assist control ventilation (ACV) would be associated with reduced work of breathing, increased respiratory muscle strength and less ventilator-infant asynchrony which would be associated with improved oxygenation.

DESIGN

Randomised crossover study.

SETTING

Tertiary neonatal unit.

PATIENTS

12 infants with a median gestational age of 25 (range 24-26) weeks were studied at a median of 43 (range 8-86) days.

INTERVENTIONS

Infants were studied for 1 h each on PAV and ACV in random order.

MAIN OUTCOME MEASURES

At the end of each hour, the work of breathing (assessed by measuring the diaphragmatic pressure time product), thoracoabdominal asynchrony and respiratory muscle strength (maximal inspiratory pressure, maximal expiratory pressure (Pemax) and maximal transdiaphragmatic pressure (Pdimax)) were assessed. Blood gas analysis was performed and the oxygenation index (OI) calculated.

RESULTS

After 1 h on PAV compared with 1 h on ACV, the median OI (5.55 (range 5-11) vs 10.10 (range 7-16), p=0.002) and PTP levels were lower (217 (range 59-556) cm H2O.s/min vs 309 (range 55-544) cm H2O.s/min, p=0.005), while Pdimax (44.26 (range 21-66) cm H2O vs 37.9 (range 19-45) cm H2O, p=0.002) and Pemax (25.6 (range 6.5-42) cm H2O vs 15.9 (range 3-35) cm H2O levels p=0.010) were higher.

CONCLUSIONS

These results suggest that PAV compared with ACV may have physiological advantages for prematurely born infants who remain ventilated after the first week after birth.

Liquid- and air-filled catheters without balloon as an alternative to the air-filled balloon catheter for measurement of esophageal pressure

Background

Measuring esophageal pressure (P_es) using an air-filled balloon catheter (BC) is the common approach to estimate pleural pressure and related parameters. However, P_es is not routinely measured in mechanically ventilated patients, partly due to technical and practical limitations and difficulties. This study aimed at comparing the conventional BC with two alternative methods for P_es measurement, liquid-filled and air-filled catheters without balloon (LFC and AFC), during mechanical ventilation with and without spontaneous breathing activity. Seven female juvenile pigs (32–42 kg) were anesthetized, orotracheally intubated, and a bundle of an AFC, LFC, and BC was inserted in the esophagus. Controlled and assisted mechanical ventilation were applied with positive end-expiratory pressures of 5 and 15 cmH₂O, and driving pressures of 10 and 20 cmH₂O, in supine and lateral decubitus.

Main Results

Cardiogenic noise in BC tracings was much larger (up to 25% of total power of P_es signal) than in AFC and LFC (<3%). Lung and chest wall elastance, pressure-time product, inspiratory work of breathing, inspiratory change and end-expiratory value of transpulmonary pressure were estimated. The three catheters allowed detecting similar changes in these parameters between different ventilation settings. However, a non-negligible and significant bias between estimates from BC and those from AFC and LFC was observed in several instances.

Conclusions

In anesthetized and mechanically ventilated pigs, the three catheters are equivalent when the aim is to detect changes in P_es and related parameters between different conditions, but possibly not when the absolute value of the estimated parameters is of paramount importance. Due to a better signal-to-noise ratio, and considering its practical advantages in terms of easier calibration and simpler acquisition setup, LFC may prove interesting for clinical use.

Effort of breathing in children receiving high-flow nasal cannula

OBJECTIVE

High-flow humidified nasal cannula is often used to provide noninvasive respiratory support in children. The effect of high-flow humidified nasal cannula on effort of breathing in children has not been objectively studied, and the mechanism by which respiratory support is provided remains unclear. This study uses an objective measure of effort of breathing (Pressure. Rate Product) to evaluate high-flow humidified nasal cannula in critically ill children.

DESIGN

Prospective cohort study.

SETTING

Quaternary care free-standing academic children's hospital.

PATIENTS

ICU patients younger than 18 years receiving high-flow humidified nasal cannula or whom the medical team planned to extubate to high-flow humidified nasal cannula within 72 hours of enrollment.

INTERVENTIONS

An esophageal pressure monitoring catheter was placed to measure pleural pressures via a Bicore CP-100 pulmonary mechanics monitor. Change in pleural pressure (ΔPes) and respiratory rate were measured on high-flow humidified nasal cannula at 2, 5, and 8 L/min. ΔPes and respiratory rate were multiplied to generate the Pressure.Rate Product, a well-established objective measure of effort of breathing. Baseline Pes, defined as pleural pressure at end exhalation during tidal breathing, reflected the positive pressure generated on each level of respiratory support.

MEASUREMENTS AND MAIN RESULTS

Twenty-five patients had measurements on high-flow humidified nasal cannula. Median age was 6.5 months (interquartile range, 1.3-15.5 mo). Median Pressure,Rate Product was lower on high-flow humidified nasal cannula 8 L/min (median, 329 cm H2O·min; interquartile range, 195-402) compared with high-flow humidified nasal cannula 5 L/min (median, 341; interquartile range, 232-475; p = 0.007) or high-flow humidified nasal cannula 2 L/min (median, 421; interquartile range, 233-621; p < 0.0001) and was lower on high-flow humidified nasal cannula 5 L/min compared with high-flow humidified nasal cannula 2 L/min (p = 0.01). Baseline Pes was higher on high-flow humidified nasal cannula 8 L/min than on high-flow humidified nasal cannula 2 L/min (p = 0.03).

CONCLUSIONS

Increasing flow rates of high-flow humidified nasal cannula decreased effort of breathing in children, with the most significant impact seen from high-flow humidified nasal cannula 2 to 8 L/min. There are likely multiple mechanisms for this clinical effect, including generation of positive pressure and washout of airway dead space.

Comparison Study of Airway Reactivity Outcomes due to a Pharmacologic Challenge Test: Impulse Oscillometry versus Least Mean Squared Analysis Techniques

The technique of measuring transpulmonary pressure and respiratory airflow with manometry and pneumotachography using the least mean squared analysis (LMS) has been used broadly in both preclinical and clinical settings for the evaluation of neonatal respiratory function during tidal volume breathing for lung tissue and airway frictional mechanical properties measurements. Whereas the technique of measuring respiratory function using the impulse oscillation technique (IOS) involves the assessment of the relationship between pressure and flow using an impulse signal with a range of frequencies, requires less cooperation and provides more information on total respiratory system resistance (chest wall, lung tissue, and airways). The present study represents a preclinical animal study to determine whether these respiratory function techniques (LMS and IOS) are comparable in detecting changes in respiratory resistance derived from a direct pharmacological challenge.

Prediction of extubation outcome in preterm infants by composite extubation indices

OBJECTIVE

To determine whether composite extubation indices can predict extubation outcome in preterm infants.

DESIGN

Prospective observational study.

SETTING

Level III neonatal intensive care unit.

PATIENTS

Fifty-six preterm infants cared for in the neonatal intensive care unit of a tertiary teaching hospital during 2007 and 2008.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The study consisted of two parts. In the first part, different extubation indices were evaluated in a group of 28 neonates (derivation group). These indices included the diaphragmatic pressure-time index, the respiratory muscle pressure-time index, the maximal transdiaphragmatic pressure, the maximal inspiratory pressure, the airway pressure generated 100 milliseconds after an occlusion/maximal transdiaphragmatic pressure ratio, the airway pressure generated 100 milliseconds after an occlusion/maximal inspiratory pressure ratio, the tidal volume, and the respiratory rate to tidal volume ratio. After exploratory analysis, the best performing indices and the optimal threshold values to predict extubation outcome were selected. In the second part of the study, these indices were validated at the predetermined threshold values in an additional group of 28 preterm neonates (validation group). Four infants (14.3%) in the derivation group and four in the validation group (14.3%) failed extubation. Receiver operator characteristic curve analysis revealed that a diaphragmatic pressure-time index of ≤0.12, a respiratory muscle pressure-time index ≤0.10, a airway pressure generated 100 milliseconds after an occlusion/maximal transdiaphragmatic pressure of ≤0.14, and a airway pressure generated 100 milliseconds after an occlusion/maximal inspiratory pressure of ≤0.09 were the most accurate predictors of extubation outcome in the derivation group. In the validation group, a diaphragmatic pressure-time index of ≤0.12 and a respiratory muscle pressure-time index of ≤0.10 both had zero false-positive results, predicting with accuracy successful extubation.

CONCLUSION

Composite extubation indices such as the diaphragmatic pressure-time index and the noninvasive respiratory muscle pressure-time index can accurately predict extubation outcome in preterm neonates.

Pulmonary function assessment in an infant with Barnes syndrome: proactive evaluation for surgical intervention

Our aim for this study was to report pulmonary mechanics in a neonate with a severe case of Barnes syndrome, a rare form of thoracolaryngopelvic dysplasia, and to use these data to guide ventilatory support and serve as a presurgical screening tool. A comprehensive pulmonary function evaluation was performed on a 36-day-old patient with Barnes syndrome who was being mechanically ventilated because of severe pulmonary distress secondary to thoracic dystrophy. The measurements consisted of respiratory volumes including functional residual capacity, ventilatory mechanics including compliance and resistance, and thoracoabdominal synchrony. Chest wall compliance was 64% below normal, and the thoracoabdominal motion was indicative of predominantly abdominal displacement during inspiratory breaths. The lungs were functioning at a low functional residual capacity, resulting in low lung compliance and increased pulmonary resistance. As a result of the evaluation, the patient was recommended for lateral thoracic expansion surgery and the ventilatory management was adjusted to focus on end-distending pressure support.

Maintained inspiratory activity during proportional assist ventilation in surfactant-depleted cats early after surfactant instillation: phrenic nerve and pulmonary stretch receptor activity

BACKGROUND

Inspiratory activity is a prerequisite for successful application of patient triggered ventilation such as proportional assist ventilation (PAV). It has recently been reported that surfactant instillation increases the activity of slowly adapting pulmonary stretch receptors (PSRs) followed by a shorter inspiratory time (Sindelar et al, J Appl Physiol, 2005 [Epub ahead of print]). Changes in lung mechanics, as observed in preterm infants with respiratory distress syndrome and after surfactant treatment, might therefore influence the inspiratory activity when applying PAV early after surfactant treatment.

OBJECTIVE

To investigate the regulation of breathing and ventilatory response in surfactant-depleted young cats during PAV and during continuous positive airway pressure (CPAP) early after surfactant instillation in relation to phrenic nerve activity (PNA) and the activity of PSRs.

METHODS

Seven anesthetized, endotracheally intubated young cats were exposed to periods of CPAP and PAV with the same end-expiratory pressure (0.2-0.5 kPa) before and after lung lavage and after surfactant instillation. PAV was set to compensate for 75% of the lung elastic recoil.

RESULTS

Tidal volume and respiratory rate were higher with lower PaCO2 and higher PaO2 during PAV than during CPAP both before and after surfactant instillation (p < 0.05; both conditions). As an indicator of breathing effort, esophageal deflection pressure and PNA were lower during PAV than during CPAP in both conditions (p < 0.02). Peak PSR activity was higher and occurred earlier during PAV than during CPAP (p < 0.01), and correlated linearly with PNA duration in all conditions studied (p < 0.001). The inspiratory time decreased as tidal volume increased when CPAP was changed to PAV, with the highest correlation observed after surfactant instillation (r = -0.769). No apneic periods could be observed.

CONCLUSION

PSR activity and the control of breathing are maintained during PAV in surfactant-depleted cats early after surfactant instillation, with a higher ventilatory response and a lower breathing effort than during CPAP.

Surfactant replacement partially restores the activity of pulmonary stretch receptors in surfactant-depleted cats

Single units of slowly adapting pulmonary stretch receptors (PSRs) were investigated in anesthetized cats during spontaneous breathing on continuous positive airway pressure (2–5 cmH2O), before and after lung lavage and then after instillation of surfactant to determine the PSR response to surfactant replacement. PSRs were classified as high threshold (HT) and low threshold (LT), and their instantaneous impulse frequency ( fimp) was related to transpulmonary pressure (Ptp) and tidal volume (Vt). Both the total number of impulses and maximal fimp of HT and LT PSRs decreased after lung lavage (55 and 45%, respectively) in the presence of increased Ptp and decreased Vt. While Ptp decreased markedly and Vt remained unchanged after surfactant instillation, all except one PSR responded with increased total number of impulses and maximal fimp (42 and 26%, respectively). Some HT PSRs ceased to discharge after lung lavage but recovered after surfactant instillation. The end-expiratory activity of LT PSRs increased or was regained after surfactant instillation. After instillation of surfactant, respiratory rate increased further with a shorter inspiratory time, resulting in a lower inspiratory-to-expiratory time ratio. Arterial pH decreased (7.31 ± 0.04 vs. 7.22 ± 0.06) and Pco2 increased (5.5 ± 0.7 vs. 7.2 ± 1.3 kPa) after lung lavage, but they were the same after as before instillation of surfactant (pH = 7.21 ± 0.08 and Pco2 = 7.6 ± 1.4 kPa) during spontaneous breathing. In conclusion, surfactant instillation increased lung compliance, which, in turn, increased the activity of both HT and LT PSRs. A further increase in respiratory rate due to a shorter inspiratory time after surfactant instillation suggests that the partially restored PSR activity after surfactant instillation affected the breathing pattern.

Neonatal chest wall suspension splint: a novel and noninvasive method for support of lung volume

Surfactant and musculoskeletal immaturity results in lower compliance of the lung relative to the chest wall, with clinical manifestations of low lung volume, marked chest wall retractions (CWR), and thoracoabdominal asynchrony. Inspiratory efforts are dissipated on distorting the chest wall inward rather than recruiting lung volumes. The current study tests the hypothesis that a novel neonatal chest wall suspension splint (SP), designed to provide stability to the compliant chest wall, would reduce inspiratory chest wall retractions and improve lung volumes. Nine preterm infants (29 +/- 1 SE weeks of gestation; 1.59 +/- 0.27 SE kg study weight) were studied at 16 +/- 5 SE days of life at baseline (BL) and following application of the front plate (FP) and the full SP (Hug n Snug Neonatal Chest Splint, Respironics, Inc.). Phase angle of thoracoabdominal motion, CWR, functional residual capacity (FRC), and pulmonary function were evaluated during spontaneous breathing. Compared to BL, there was a significant decrease in anterior CWR (2.21 +/- 0.91 SE vs. 0.25 +/- 0.09 SE mm; P < 0.05), an increase in FRC (16.6 +/- 2.8 SE vs. 27.8 +/- 5.5 SE ml/kg; P < 0.05) and tidal volume (4.8 +/- 1.5 SE vs. 7.3 +/- 1.4 SE ml/kg; P < 0.05), minimal effect on pulmonary compliance (1.98 +/- 0.50 SE vs. 1.72 +/- 0.30 SE ml/cmH2O/kg), and a trend for a decrease in phase angle (128.4 +/- 10.9 SE vs. 111.8 +/- 19.3 SE) with the application of the splint. FRC correlated inversely with severity of CWR across all conditions (P < 0.05, r = -0.68). Phase angle was directly correlated to anterior CWR (r = 0.72; P < 0.05) and correlated inversely with FRC (P < 0.005; r = -0.56). We speculate that by improving CW stability, the use of this splint may reduce the energetic requirements of breathing and, potentially, the need for more invasive ventilatory support in the neonate.

Technical protocol for the use of esophageal manometry in the diagnosis of sleep-related breathing disorders

A time-tested protocol for intrathoracic pressure monitoring during sleep is described. This method of esophageal manometry uses a fluid-filled catheter to measure variations in transmitted intrathoracic pressure with respiration. Esophageal manometry is an invaluable tool for the sleep specialist in the diagnosis of sleep-related breathing disorders, especially for detecting cases of upper airway resistance syndrome and for distinguishing subtle central apneas from obstructive events. The methods for scoring esophageal pressure, the indications and contraindications for esophageal manometry, the use of esophageal manometry as the 'gold standard' for the measurement of respiratory effort, and directions for future research are also discussed.

Validity of esophageal pressure measurements with positive end-expiratory pressure in preterm infants

Previous research suggested that esophageal pressure changes (DeltaP(es)) may not reflect pleural pressure changes (DeltaP(pl)) in the presence of positive end-expiratory pressure (PEEP), making assessments of dynamic lung mechanics invalid in these circumstances. To test this hypothesis, we measured DeltaP(es) using a water-filled catheter in 18 preterm infants with lung disease (9 intubated), and adjusted the catheter position to achieve a valid occlusion test. End-expiratory occlusions were then carried out at PEEP (cm H(2)O) of 0, 4, and 8, and plots of DeltaP(es) against DeltaP(ao) during airway occlusion were examined to derive the ratio DeltaP(es)/DeltaP(ao) and the r value (as a measure of linearity). There was no significant change in DeltaP(es)/DeltaP(ao), which remained close to 1.0 as PEEP was increased from 0 to 8 cm H(2)O, and r also remained close to unity, indicating no appreciable hysteresis or alinearity of the plots. Our results show that DeltaP(es), when measured with an appropriately placed water-filled catheter, continues to reflect DeltaP(pl) accurately when lung volume is raised by applying PEEP up to 8 cm H(2)O.

Pulmonary function in technology-dependent children 2 years and older with bronchopulmonary dysplasia

Somatic and pulmonary growth coincide with resolution of hypoxemia by 2 years of age in most children with bronchopulmonary dysplasia (BPD). However, a distinct subgroup of children with BPD continue to require mechanical ventilation and/or supplemental oxygen beyond 2 years of age. This study tested the hypothesis that indices of pulmonary function would be significantly worse in children with BPD 2 years and older who remained technology-dependent secondary to hypoxemia, compared to those of age-matched children with BPD who were normoxemic. We measured pulmonary mechanics in 21 oxygen- or ventilator-dependent children with BPD 2 years and older (BPDO2 group; mean age+/-SD, 30.2+/-6.5 months) and in 19 children with BPD who had been weaned off mechanical ventilation and supplemental oxygen for at least 6 months (control group; mean age, 30.1+/-5.5 months). Respiratory rate and tidal volume were measured after sedation with chloral hydrate, and dynamic compliance and expiratory conductance were calculated using the esophageal catheter technique. Maximal flow at FRC (V'(maxFRC)) and ratio of forced-to-tidal flows at midtidal volume were obtained by the rapid thoracic compression technique. FRC was determined by nitrogen washout. There were no statistically significant differences in most measured indices of pulmonary mechanics between the BPDO2 and control groups. However, V'(maxFRC)/FRC was higher in controls compared to subjects in the BPDO2 group (0.81+/-0.40 sec(-1) vs. 0.34+/-0.21 sec(-1), P<0.003). We conclude that most indices of pulmonary function in children with BPD 2 years and older do not reflect the need for mechanical ventilation or supplemental oxygen. We speculate that measurements of lung elastic recoil and tests of distribution of ventilation and pulmonary perfusion may be more sensitive in differentiating normoxemic and hypoxemic children with BPD 2 years and older.

Characterisation of the Hering-Breuer deflation reflex in the human neonate

Human infants have been observed making inspiratory efforts in response to chest compression. These may be a manifestation of the Hering-Breuer deflation reflex. We sought to stimulate the reflex in 33 term infants by rapidly reducing lung volume using an inflatable jacket. The effect of altering the timing, magnitude or rate of application of the lung deflation on the strength of the inspiratory response was investigated. Inspiratory effort was quantified by measuring (1) the rate of fall in oesophageal pressure on inspiration; and (2) the mean inspiratory flow (MIF) in response to lung deflation. Variables which significantly affected (1) and resulted in increased inspiratory effort were, in order of importance: larger rises in oesophageal pressure on chest compression (38%) (percentage of variance explained), greater reductions in lung volume below functional residual capacity (FRC) (26%), faster rates of lung deflation (19%) and slower respiratory rates (11%). Increased inspiratory efforts, as assessed by response (2), were generated by greater reductions in FRC (23%), larger rises in oesophageal pressure (11%) and faster rates of lung deflation (10%). Increasing deflation pressures eventually resulted in a plateau in both measures of inspiratory response. These results were consistent with the Hering-Breuer deflation reflex being activated which could have a role in protecting the FRC of the newborn infant.

Magnetic phrenic nerve stimulation to assess diaphragm function in children following liver transplantation

BACKGROUND: An accurate and practical test of diaphragm function in children in the intensive care unit is desirable. Diaphragm dysfunction can occur after liver transplantation and may be a contributory factor in the respiratory complications after the procedure. OBJECTIVE: Our aim was to assess if magnetic stimulation (MS) of the phrenic nerves could be used to assess diaphragm strength in children receiving intensive care. SETTING: Pediatric intensive care unit. PATIENTS: Eight supine, ventilated, sedated children (mean age, 7.3; range, 0.6-15 yrs) were studied within 12 hrs of liver transplantation. INTERVENTIONS: MS was performed using either 90-mm double circular coils or 43-mm figure of eight coils placed over the phrenic nerves on the anterior aspect of the neck. Measurements: The produced diaphragm force was assessed by measuring the transdiaphragmatic pressure (Pdi) with balloons in the mid-esophagus and stomach. During MS, the endotracheal tube was briefly occluded by using a pneumatic valve. MAIN RESULTS: Supramaximal diaphragm force responses were obtained in all subjects; mean Pdi, 7.8 (sd, 3.1) cmH(2)O for left, 5.2 (sd, 3.4) cmH(2)O for right, and 14.8 (sd, 9.2) cmH(2)O for bilateral stimulation. Bland and Altman analysis indicated close agreement between esophageal and airway pressure during MS (mean difference, -0.76 [sd, 0.99] cmH(2)O for left stimulation, 0.81 [sd, 1.25] cmH(2)O for right stimulation, and -0.63 [sd, 1.55] cmH(2)O for bilateral stimulation). In three children, there was a >50% difference between the Pdi generated after left and right unilateral MS and the results of MS indicated complete right hemidiaphragm paralysis in one child. CONCLUSION: MS of the phrenic nerves provides a practical technique for assessing diaphragm function in children receiving intensive care. Measurement of airway pressure during MS may provide a noninvasive technique for assessing diaphragm strength when the use of balloon catheters is contraindicated.

Sniff nasal inspiratory pressure in children

Maximal static inspiratory pressure (P(IMAX)) generated at the mouth is one of the tests of inspiratory muscle strength in children. In adults, inspiratory muscle strength has also been assessed using sniff nasal inspiratory pressure (SNIP). This test is easier to perform than P(IMAX) and might therefore be applicable to children. To test this hypothesis, we measured P(IMAX) and SNIP in 116 children aged 4 to 11 years (54 girls, 62 boys). P(IMAX) was measured using a tube mouthpiece and SNIP using a tightly fitting plug in one nostril, while a sniff was performed through the other nostril. Both tests were performed from functional residual capacity (FRC). Pressure was measured with a differential pressure transducer and displayed in real time on a computer screen. Weight, standing height, sitting height, gender, and age were recorded. There was a significant difference (P < 0.01) in group mean (SD) data between SNIP (81.3 (27.4) cmH(2)O) and P(IMAX) (67.9 (28.1) cmH(2)O). Bland/Altman analysis demonstrated a mean difference of -13.5 cmH(2)O (SD 21.4) between the techniques. Regression analysis indicated highly significant relations (P < 0.01) between SNIP and P(IMAX), and between weight, standing and sitting height, and age for SNIP, and between weight, standing height, and age for P(IMAX). SNIP and P(IMAX) were greater in boys than girls (83.2 vs. 79.2 cmH(2)O SNIP; 72.9 vs. 62.0 cmH(2)O P(IMAX)), but this difference was only significant for P(IMAX) (P < 0.05). SNIP was significantly greater than P(IMAX) (P < 0.01) in both boys and girls. These data suggest that SNIP provides a simple, noninvasive additional test to P(IMAX) for assessing inspiratory muscle strength in children.

Pulmonary function in hospitalized infants and toddlers with cystic fibrosis

In older children with cystic fibrosis (CF), well-documented improvements in lung function occur during hospitalization for treatment of pulmonary exacerbations.

OBJECTIVES

(1) To test the hypothesis that improvement in lung function occurs in infants and toddlers hospitalized because of CF pulmonary exacerbations. (2) To compare changes in lung function measured during forced expiratory flow and tidal breathing.

STUDY DESIGN

Seventeen infants and toddlers with CF were evaluated at the beginning and end of hospitalization by the rapid thoracic compression technique to yield maximal flow at forced residual capacity. Tidal mechanics were measured by the esophageal balloon technique to yield lung conductance and compliance.

RESULTS

Lung function improved during the course of hospitalization. The greatest change was observed in measurements of maximal flow at functional residual capacity (.VmaxFRC), increasing from 38.5% +/- 6% predicted (mean +/- SEM) to 59.8% +/- 6% at the end (p < 0.005). Lung conductance (GL) increased from 60% +/- 6% to 78% +/- 8% (p < 0.02); lung compliance (CL) increased from 66% +/- 5% to 75% +/- 5% (p < 0.03). The degree of improvement of .VmaxFRC, GL, and CL was related to baseline measurements; those with poorer pulmonary function at baseline had the greatest degree of improvement during hospitalization.

CONCLUSION

Assessments of airflow obstruction from measurements of .VmaxFRC and GL do not necessarily demonstrate similar findings in a given infant with CF, perhaps because these two techniques measure different physiologic properties. Changes in .VmaxFRC may best reflect the predominant pathophysiology of lung disease in infants and toddlers with CF.

Frequency responses of infant air-balloon versus liquid-filled catheters for intra-esophageal pressure measurement

Amplitude and phase frequency response characteristics of infant air-balloon catheters (IABC) of differing French gauge (FG) sizes and brands were quantified to determine their suitability for measuring dynamic intra-esophageal pressure (Pes) accurately. Frequency response performances of matching IABC and water-filled catheters (WFC) were also compared using the swept sine wave technique. The maximum respiratory rate within which IABCs could potentially measure Pes within a 5% error limit was calculated (FRR). Frequency responses of IABCs greater than FG size 5 exhibited underdamped resonant properties, while smaller FG size IABCs exhibited near-critical damping or overdamping. IABCs maintained uniform amplitude frequency responses up to 25 Hz, demonstrating the ability to measure Pes potentially up to 148 breaths/min within a 5% error limit. The frequency response performance of FG size 6 IABCs was similar to that of FG size 10 IABCs. Compared with matching WFCs, the frequency response performance of IABCs was significantly superior, the frequency response variability within IABC samples was lower, and IABC correlation between FG size and FRR was advantageously lower than for WFCs. FRR values for differing IABC brands and FG sizes are presented. We conclude that IABCs manufactured to infant-appropriate balloon specifications exhibit significantly superior frequency response characteristics compared with matching WFCs. Measurement accuracy is not improved using IABCs greater than FG size 6. Inexpensive intra-esophageal IABCs are technical suitable for the accurate measurement of dynamic Pes during high-frequency respiratory mechanics encountered during infant artificial ventilation.

Correctly selecting a liquid-filled nasogastric infant feeding catheter to measure intraesophageal pressure

Polyvinyl chloride (PVC) nasogastric feeding catheters are used clinically to measure intraesophageal pressure as an estimate of pleural pressure for calculating lung compliance in infants. The accuracy of pressure measurement of 4 French gauge (FG) catheter sizes and three brands of liquid-filled catheter manometer systems (CMS) was evaluated by determining their resonance-frequency amplitude and phase properties. All CMS were underdamped and resonated. No CMS exhibited a uniform mean frequency response above 11 Hz. The maximum respiratory rate (Frr) within which CMS could potentially measure dynamic intraesophageal pressure within a 5% error limit was determined (Frr): the highest mean Frr recorded reliably in large-diameter catheters was 82 breaths/min. Significant CMS differences in accuracy existed between catheter FG sizes and between catheters of similar diameters but differing brands. Correlation (r2) between catheter inner diameter and CMS Frr was 0.66 across brands. In conclusion, intraesophageal PVC liquid-filled feeding catheters are suitable for estimating pleural pressures in subjects mechanically ventilated without sharp inspiratory waveforms or high respiratory rates. Quantitative frequency response characterization of different nasogastric catheter brands and different diameters is mandatory prior to their utilization.

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.

Comparison of dynamic and passive measurements of respiratory mechanics in ventilated newborn infants

Pulmonary mechanics may differ in intubated and ventilated infants depending on whether they are measured by a dynamic or passive method. The objective of this study was to compare respiratory mechanics measured by a dynamic technique with those obtained by a single-breath occlusion technique in ventilated newborn infants. Thirty-one preterm and 15 term infants (mean +/- SD: gestational age, 29.3 +/- 2.3 and 39.5 +/- 1.4 weeks; birth weight, 1.2 +/- 0.5 and 3.4 +/- 0.4 kg; postnatal age, 12 +/- 13 and 5 +/- 4 days, respectively) were studied. Flows were measured through a pneumotachometer placed between the endotracheal tube and the ventilator circuit: tidal volume by integration of flow, and airway pressure directly with a pressure transducer. Airway occlusion was performed with a Neonatal Occlusion Valve (Bicore pulmonary monitor) at the end of inspiration, and the following relaxed exhalation was analyzed to give passive respiratory system compliance (Crs) and resistance (Rrs). These values were compared with dynamic respiratory system compliance (Cdyn) and dynamic expiratory resistance (Re) obtained with the PEDS system (P) within 1 hour, without an esophageal balloon and on the same ventilator settings. Dynamic respiratory system compliance and resistance measured with the PEDS and the Bicore systems did not differ significantly and were well correlated. Mean Cdyn (P) values in preterm and term infants were 77% and 77% of Crs; the equation of the regression line was Cdyn = 0.75 Crs + 0.02 and Cdyn = 0.78 Crs - 0.02; and standard error of the estimate (SEE) was 0.2 and 0.3 mL/cmH2O with a correlation coefficient (r) of 0.89 and 0.89 (P < 0.0001), respectively. The mean Re(P) values in preterm and term infants were 68% and 64% of Rrs, and the equation of the regression line was Re = 0.3 Rrs + 63 and Re = 0.5 Rrs + 20, with SEE of 25 and 20 cmH2O/L/sec, and r of 0.65 and 0.69 (P < 0.0001, P < 0.005), respectively. The two methods are non-invasive and were well tolerated. We conclude that passive and dynamic respiratory compliance and resistance measured in intubated infants are highly correlated, although the values measured by the passive technique are higher than those obtained by the dynamic technique.

In vitro assessment of infant pulmonary function equipment

Commercially available automated pulmonary monitors are used increasingly in neonatal intensive care units. However, detailed information regarding the static and dynamic accuracy of these monitors is rarely available. Collaboration between scientists, clinicians, and manufacturers is essential to establish improved technical standards and protocols for testing of equipment and for the development of more reliable neonatal pulmonary monitors. The aim of this study was to develop a protocol for the in vitro assessment of commercial infant pulmonary function equipment which could be applied within the laboratory to provide rapid feedback to the manufacturer. A recently released neonatal pulmonary monitor, the Bicore CP100 (software version 3.3), was selected for the development of this protocol. The deadspace and resistance of the measuring device were determined. The flow and airway pressure measuring systems were evaluated alone and connected to a tracheal tube for both static accuracy and frequency response. The pressure-volume relationship of the esophageal balloon was determined and its static accuracy and frequency response were assessed. The algorithms for on-line calculations were checked and their correct application confirmed by examination of an ASCII data print out. Finally, the pulmonary monitor was tested during intermittent positive pressure ventilation of a neonatal lung model of known compliance and resistance.

Continuous negative extrathoracic pressure and positive end-expiratory pressure. A comparative study in Escherichia coli endotoxin-treated neonatal piglets

Recent clinical studies have suggested that improvement in pulmonary gas exchange with the use of continuous negative extrathoracic pressure (CNEP) in conjunction with intermittent mandatory ventilation (IMV) may be due to increased pulmonary blood flow. Accordingly, we investigated the effects of CNEP vs positive end-expiratory pressure (PEEP) in ventilated neonatal piglets after Escherichia coli endotoxin was administered to induce pulmonary hypertension. Two experimental groups of piglets with six in each, were subjected to three 30-min alternating periods--6 cm H2O CNEP with 6 cm H2O PEEP, beginning 2 h after endotoxin infusion. End-expiratory lung volume (EELV) increased similarly from baseline (13 +/- 2 mL/kg) with both CNEP (28 +/- 2 mL/kg) and PEEP (29 +/- 2 mL/kg). In addition, the increase in PaO2 from baseline with CNEP (106 +/- 9 to 135 +/- 7 mm Hg) was similar to that with PEEP (114 +/- 11 to 132 +/- 6 mm Hg). Further, no differences were found in dynamic lung compliance, EELV, lung resistance, blood gas indexes, or hemodynamics, including transmural pulmonary artery pressure and pulmonary vascular resistance between CNEP and PEEP. With transpulmonary pressure and transrespiratory pressure equal, CNEP in tandem with IMV is physiologically equivalent to PEEP and IMV.

Heart rate variability during sleep in infants with bronchopulmonary dysplasia. Effects of mild decrease in oxygen saturation

We sought to determine whether abnormal heart rate modulation by the autonomic nervous system occurs in patients with severe bronchopulmonary dysplasia (BPD) in relation to sleep stages and mild changes in arterial oxygen saturation SaO2. On 10 oxygen-dependent 7- to 29-month-old infants with BPD, polygraphic recordings, including heart and respiratory rate and body movement detection, were performed. Heart rate variability was evaluated in high (HF), mid, and low (LF) frequency bands. Parameters were analyzed in two ranges of SaO2: normal range, (SaO2 greater than 95%), and mild decrease in (SaO2, values of 90 to 94%). In contrast to what is normally observed, LF at normal SaO2 was less marked in rapid eye movement, (REM) sleep than in non-rapid eye movement (NREM) sleep stage 2. A mild decrease in SaO2, as compared with a normal SaO2 value, was associated with: (1) a heart and respiratory rate acceleration, (2) a decrease in HF in REM sleep (p < 0.02); (3) an increase in LF in NREM sleep stage 2 (p < 0.02), intensifying the change observed in a normal SaO2 level. These data show that a mild decrease in SaO2 increases modifications of autonomic control observed in infants with severe BPD.

The Hering-Breuer deflationary reflex in the newborn infant

Initial observations on healthy term infants using the forced partial expiratory flow-volume technique with an inflatable jacket suggested that this technique was stimulating the Hering-Breuer deflationary reflex, a reflex which has not been systematically studied in man. To investigate this fully, esophageal pressure, jacket pressure, flow and volume at the mouth were recorded during the forced partial expiratory flow-volume maneuver on 10 infants (mean age, 3.1 days; birth weight, 3533 g; gestation, 39.8 weeks). A total of 186 measurements were performed at different points in the respiratory cycle. In 159 (85%) recordings inspiratory effort was evident with a fall in esophageal pressure within 166 msec; in some cases this occurred before the jacket was fully inflated. This was associated with a reduction of 23.4 cm H2O in mean intrathoracic pressure, which was 2.5 times that occurring during normal tidal breathing. In the remaining 27 measurements a plateau pressure was associated with closure of the upper airway. When the squeeze was applied at low lung volumes (end-expiration) the inspiratory effort occurred significantly earlier (133 msec) and stronger (reducing peak intrathoracic pressure to 15.8 cm H2O) than when applied at end-inspiration (181 msec with a reduction in intrathoracic pressure to 25.2 cm H2O). The observed inspiratory response was highly consistent, representing the deflationary reflex as described by Hering and Breuer in 1868. The stronger and more rapid onset of inspiration at low lung volume supports the claim made by Breuer that it has a protective role on functional residual capacity (FRC) in young infants.

Randomized trial of long-term diuretic therapy for infants with oxygen-dependent bronchopulmonary dysplasia

STUDY OBJECTIVE

To determine whether long-term oral diuretic therapy would improve the pulmonary function of preterm infants with bronchopulmonary dysplasia.

DESIGN

Randomized, double-blind, placebo-controlled study.

SETTING

Level III intensive care nursery.

INTERVENTION

We randomly selected 43 stable patients with oxygen-dependent bronchopulmonary dysplasia to receive either orally administered spironolactone and chlorothiazide or placebo. These drugs were continued until the patients no longer required supplemental oxygen. Both groups received furosemide as needed.

MEASUREMENTS AND RESULTS

Each infant had pulmonary function tests at study entry, 4 weeks after study entry, 1 week and 8 weeks after being weaned to room air and off study drugs, and at 1 year of corrected age. Pulmonary function tests include dynamic pulmonary compliance, airway resistance, thoracic gas volume, and maximal expiratory flow at functional residual capacity; most of the infants had functional residual capacity measured. Between the first and second pulmonary function tests (while the infants were receiving diuretic or placebo), the infants in the diuretic group had a significant improvement in dynamic pulmonary compliance (46%; p < 0.001) and airway resistance (31%; p < 0.05); there were no changes in compliance or resistance in the placebo group. Although patients in both the diuretic and the placebo groups required progressively less supplemental oxygen, by 4 weeks after study entry the patients in the diuretic group needed less supplemental oxygen than did those in the placebo group (p < 0.01). There were no significant differences in results of serial pulmonary function tests in either group after discontinuation of diuretic therapy. Despite the significant differences in pulmonary function between the two groups, there was no significant difference between them in the total number of days that supplemental oxygen was required. Significantly more infantsin the placebo group received more than 10 doses of furosemide on an as-needed basis.

CONCLUSIONS

Long-term diuretic therapy in stable infants with oxygen-dependent bronchopulmonary dysplasia, after extubation, improves their pulmonary function and decreases their fractional inspired oxygen requirement, but does not decrease the number of days that they require supplemental oxygen. The improvement in pulmonary function associated with diuretic therapy is not maintained after treatment is discontinued.

Effects of intermittent mandatory ventilation on respiratory timing in preterm infants

Twenty preterm infants (25-36 weeks' gestation) were studied during intermittent mandatory ventilation (IMV) at rates of 10-34 inflations/min. Airway pressure and abdominal capsule signals were recorded at varying postnatal ages. Spontaneous interbreath interval (IBI), inspiratory (Ti) and expiratory (Te) duration were measured over 100-1000 ventilator cycles. Baseline Te was 0.48 s (+/- 0.129) and increased to 0.65 s (+/- 0.182) when associated with a mechanical inflation. Baseline Ti was 0.34 s (+/- 0.062) and increased to 0.38 s (+/- 0.081) with inflation. IBI increased from 0.82 s (+/- 0.161) to 1.03 s (+/- 0.201) with inflation. 1:1 entrainment (phase-locking) was observed at rates of ventilation below the spontaneous respiratory rate but spontaneous inspiration and mechanical inflation were always out of phase. A linear relationship was noted between the prolongation of IBI and the timing of inflation within the spontaneous respiratory cycle during IMV. This relationship could be used to estimate the range of rates of mechanical inflation capable of inducing 1:1 entrainment.

Continuous negative extrathoracic pressure versus positive end-expiratory pressure in piglets after saline lung lavage

Recent reports have suggested that substituting continuous negative extrathoracic pressure (CNEP) for positive end-expiratory pressure (PEEP) may result in clinical benefits to infants with pulmonary disease. Other studies have suggested potential hemodynamic advantages. We compared the effects of CNEP and PEEP in 13 mechanically ventilated newborn piglets after acute lung injury induced by saline lavage. The piglets were instrumented, saline-lavaged, and exposed to 15 minute periods of incremental CNEP (-3, -6, -9, -12 cmH2O) (n = 7) or PEEP (3, 6, 9, 12 cmH2O) (n = 6). We measured and/or calculated dynamic lung compliance (CLdyn), lung resistance (RL), end-expiratory lung volume (EELV), blood gases, cardiac output (CO), heart rate (HR), transmural vascular pressures, and pulmonary and systemic vascular resistance. Pulmonary function abnormalities after saline lavage included decreased PaO2, CLdyn, EELV, and increased PaCO2 and RL (P < 0.05). Except for decreased CO, lung inflation with both CNEP and PEEP resulted in large increases in PaO2 without major pulmonary or hemodynamic effects. Other than differences in EELV at 3, 6, and 9 cmH2O distending pressure, there were no differences in pulmonary function or hemodynamics between sequences of incremental CNEP and PEEP. We conclude that CNEP and PEEP are physiologically equivalent in this model of acute lung injury.

The effects of reversing distending pressure sequences in the neonatal piglet

We studied different sequences of lung inflation in ventilated newborn piglets with normal lungs in order to determine the effects of sequence, magnitude and duration of distending pressure on pulmonary function, and/or hemodynamics. End-expiratory pressure was varied using a continuous negative extrathoracic pressure (CNEP) device. Three groups of ventilated piglets with normal lungs were exposed to 2 cmH2O increments of CNEP from -2 to -12 cmH2O, and to decrements from -12 to -2 cmH2O, or to only -6 cmH2O. Lung inflation sequence, magnitude of inflation pressure, and duration of inflation had significant effects on end-expiratory lung volume and lung compliance at numerically equivalent pressure levels. End-expiratory lung volume and lung compliance varied (at four and five of six inflation pressures studied) by as much as 68% and 104%, respectively. Hemodynamic effects of the lung inflation sequence were more variable; those found to be different at numerically equivalent pressure levels were associated with changes in lung compliance and ventilation. Differences in pulmonary mechanics can best be explained by the effects of lung inflation on alveolar recruitment versus overinflation.

Validation of esophageal pressure occlusion test after paralysis

Measurements of respiratory mechanics are frequently made in ventilated infants and children. Esophageal pressure measurements (Pes) using a balloon on a catheter have been used to partition the respiratory mechanics into lung and chest wall components. Appropriate positioning of this balloon is crucial to obtain accurate estimates of pleural pressure. Traditionally, in spontaneously breathing subjects the balloon position is assessed with an occlusion test. In ventilated subjects, it is not always possible to perform an occlusion test prior to paralysis, and even if such a test is performed it may be relevant under conditions of positive pressure ventilation. By occluding the airway opening and applying gentle pressure to the abdomen or rib cage, positive swings in pressure can be measured by both Pes and airway opening pressure (Pao). We compared traditional occlusion tests measured in 16 spontaneously breathing puppies to the positive pressure occlusion test performed after paralysis. In 2 pups we were unable to obtain a reasonable traditional occlusion test (> 15% difference between Pes and Pao) but we obtained 10 traditional occlusion tests in each of the remaining 14 pups (2.1-14 kg). In 11 of these animals delta Pes was within 10% of delta Pao. This compared well to positive pressure occlusion test using abdominal pressure performed after analysis, where delta Pes was within 10% of delta Pao in 10 animals. In 9 of these pups occlusion tests were also performed by applying pressure on the rib cage, where delta Pes was within 10% of delta Pao in 6 animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of the frequency-tracking locus in estimating the degree of respiratory entrainment in preterm infants

In order to define the complex interactions between external stimuli and non-linear physiological systems, a technique (the frequency-tracking locus, FTL) was devised that describes the cycle-by-cycle changes in phase angle and amplitude between two signals. Qualitative assessment of the nature of interactions between the signals can be made by examining the FTL. Quantitation of the extent of entrainment of the spontaneous physiological rhythm is possible after deriving a numerical index (the path-length index, PLI) describing the departure of the system from a fully entrained state. The FTL was applied to the study of interactions between spontaneous respiratory effort and mechanical inflation in preterm newborn babies undergoing mechanical ventilation. Stable and unstable states of 1:1 interaction were noted while integer-ratio relationships were seen at low rates of mechanical ventilation. Stable states of entrainment corresponded to a PLI value near unity, and the value of PLI increased rapidly as interactions became unstable. The FTL may be used to describe complex interactions in physiological systems, and may be used as a guide to baby-ventilator matching during mechanical ventilation of the newborn.

The effect of instrumental dead space on measurement of breathing pattern and pulmonary mechanics in the newborn

The effect of the instrumental dead space on breathing pattern and the values of pulmonary mechanics was evaluated because of concern about the relatively large dead space of 26 mL in a commercially available system. Sixty-three healthy newborn infants were studied with a system as commercially supplied, and with the dead space eliminated using a 2 L/min biased flow. This led to a significant reduction in mean (+/- SD) values of respiratory rate from 56.8 (+/- 11.7) to 48.2 (+/- 11.7) breath/min (P < 0.0001), tidal volume from 5.2 (+/- 1.3) to 4.9 (+/- 0.9) mL/kg (P < 0.05), minute volume from 284 (+/- 68) to 220 (+/- 63) mL/min/kg (P < 0.0001), and work of breathing from 13.7 (+/- 6.6) to 11.8 (+/- 7.6) g.cm/kg (P < 0.02). There was a significant increase in dynamic lung compliance from 5.2 (+/- 1.5) to 5.6 (+/- 1.2) mL/cm H2O (P < 0.01) but no difference for total pulmonary resistance 39.6 (+/- 22.8) and 38.8 (+/- 22.2) cm H2O/L/sec. This shows that the instrumental dead space prevents measurement of the basal breathing patterns and alters the values of pulmonary mechanics. It is, therefore, important to use equipment with low dead space or make efforts to remove it by using a biased flow system such as we describe when measuring breathing patterns and pulmonary mechanics in the newborn.

A simplified method for determining the frequency response of pneumotachographs used in infants

In all rapidly changing systems, an appropriate response time of all sensing devices is essential for the accurate conversion of a physiological parameters to a proportional electrical signal. The frequency response of equipment is the ability to accurately reflect both the magnitude and temporal relationship of dynamic events over a defined frequency range. The phase lag expresses the temporal delay between the physical event being measured and the output signal. The attenuation expresses diminution of the amplitude ratio of the output signal in relation to the input signal over a range of frequencies. We have developed a method that specifically addresses the measurement of frequency response and attenuation of pneumotachographs and low pressure transducers. The system consists of a 81 liter rectangular box separated in the middle by a 30.5 cm acoustic loudspeaker, the cone sealed by latex, and driven by a signal generator coupled to a low frequency amplifier. This system can produce an undistorted sinusoidal signal between 0 and 20 Hz, and the peak flow through the pneumotachograph is only minimally affected by changes in frequency. Rapid analysis is possible using an oscilloscope to produce Lissajou loops. There is no measurable attenuation between the electrical signal and the pressure generated over frequencies from 1 to 20 Hz. The system is accurate at low frequencies and can generate appropriate signals over the frequency range of interest for respiratory applications, and it can be inexpensively constructed.

Asynchronous chest wall movements during non-rapid eye movement and rapid eye movement sleep in children with bronchopulmonary dysplasia

The aim of this study was to assess whether age-related changes in thoracic shape modify patterns of thoracoabdominal asynchrony (TAA) or applicability of phase angle analysis during sleep in young children with increased respiratory loads. We assessed TAA during polysomnographic monitoring in 14 young children (mean age 32 months, range 19 to 46; mean weight 12.5 kg, range 9.3 to 17) with severe bronchopulmonary dysplasia (BPD). Of the patients 10 were severely enough affected to require tracheostomy. We measured asynchrony of rib cage (RC) and abdominal (AB) movements at midinspiration and the corresponding phase angle from oscillographic recordings during both non-REM and REM sleep. We measured the amplitude of "paradoxical" displacement of either RC or AB during inspiration and expressed this as a percentage of the total displacement of the compartment. Of 9 children who manifested early inspiratory AB paradox during non-REM sleep, 7 showed a figure eight on the Konno-Mead diagram. The magnitude of abdominal paradox during non-REM sleep was significantly positively correlated with age (n = 14, r = 0.68; p < 0.01). Phase angle was significantly negatively correlated with dynamic lung compliance (n = 14, r = -0.66; p < 0.01). During REM sleep, expiratory abdominal muscle activity was abolished and all patients with abdominal paradox "converted" to an open loop with RC paradox during inspiration. Graphic assessment of the Lissajous figure on the Konno-Mead diagram indicated when midinspiratory phase angle analysis did not reflect the severity of TAA and can be used to infer patterns of respiratory muscle recruitment. We conclude that young children manifest patterns of TAA that differ from the early inspiratory RC paradox commonly observed in infants. Comparison of RC-AB loops between non-REM and REM sleep in the same child can assess increased thoracic inspiratory efforts and expiratory muscle activity as potential mechanisms for abdominal paradox, as distinct from diaphragm ineffectiveness.

Inspiratory pressures with CO2 stimulation and weaning from mechanical ventilation in children

Maximal transdiaphragmatic (Pdimax) and airway occlusion pressures (PaOmax) have been used to predict weaning from mechanical assisted ventilation in adults, but criteria for weaning are still based on trial and error in infants and young children. Because infants and young children cannot cooperate, crying Pdi and PaOmax against an occlusion have been used, but these may not yield maximal values. We hypothesized that breathing CO2 would achieve better Pdimax and PaOmax values by maximizing respiratory drive and help in establishing weaning criteria. To test this, we measured tidal breathing and occluded Pdi and PaOmax in 27 patients (mean age, 15.0 +/- 31.5 SD months) who required prolonged assisted mechanical ventilation and had failed previous weaning attempts. Measurements were performed while patients were breathing spontaneously 100% O2 and 5% and 7% CO2 in O2. The patients achieved higher Pdimax breathing 5% CO2 (73.2 +/- 24.4 cm H2O) than in O2 (61.6 +/- 24.4 cm H2O; p < 0.0001) or in 7% CO2 (69.1 +/- 23.4 cm H2O; p < 0.0001). They also achieved higher PaOmax in 5% CO2 (81.7 +/- 23.5 cm H2O) than with the other gases (69.9 +/- 25.5 in O2, and 77.5 +/- 24.1 in 7% CO2; p < 0.001); 19 patients (70%) were weaned from assisted ventilation within 3.2 +/- 1.9 wk. In 5% CO2, all patients who were weaned achieved Pdimax > 60 cm H2O and could sustain > 60% Pdimax for more than five successive occluded breaths (100% sensitivity; 100% specificity; p < 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary sequelae at six months following extracorporeal membrane oxygenation

Mechanical assisted ventilation for neonatal respiratory failure is associated with residual lung disease. Because ECMO rests the lungs, it has been suggested that ECMO will prevent chronic lung disease in survivors. To determine whether or not ECMO survivors have evidence of pulmonary sequelae, we studied 19 infants who were treated with ECMO for neonatal respiratory failure. Ten infants still required supplemental oxygen or pulmonary medications or both to treat clinical lung disease during the first six months of life. Thoracic gas volume was normal. Pulmonary mechanics in ECMO survivors were compared with those of 13 preterm infants with BPD at similar age. We conclude that a significant proportion of ECMO survivors have residual abnormalities in pulmonary mechanics at 6 months of age. We speculate that neonatal lung injury due to meconium aspiration and other causes is a more important determinant of abnormal pulmonary sequelae than the method of treatment.

Variability of dynamic compliance measurements in spontaneously breathing and ventilated newborn infants

We studied reproducibility and variability of dynamic pulmonary compliance (Cdyn) by making measurements with the esophageal balloon at multiple locations within the esophagus, in both spontaneously breathing and mechanically ventilated newborn infants. Reliable measurements could be obtained over a range similar to that reported for measurements with a liquid-filled catheter. In spontaneously breathing infants Cdyn was found to be highly variable. This variability was unrelated to catheter position but was associated with concomitant changes in pulmonary resistance. Probably because of the high variability, the correlation of Cdyn with a measurement of respiratory system compliance (Crs) was rather poor (r = 0.63). Cdyn measured in mechanically ventilated infants was significantly less variable and compared favorably to Crs (r = 0.86), but its accuracy could not be adequately assessed since the comparison of esophageal and airway occlusion pressure was not feasible in all infants. In addition, significant differences in Cdyn were found between spontaneous and ventilated breaths during mechanical ventilation. Further studies in both ventilated and spontaneously breathing infants are needed to assess the variability of Cdyn over extended time periods.

Immediate improvement in lung volume after exogenous surfactant: alveolar recruitment versus increased distention

To determine whether changes in lung volume may be responsible for the clinical improvement in preterm infants given exogenous surfactant, we measured functional residual capacity (FRC), lung mechanics, and partial pressure of oxygen in seven ventilated neonates (birth weight 1080 +/- 361 gm (mean +/- SD); gestational age 28.3 +/- 2.6 weeks) less than 9 hours of age who had findings typical of hyaline membrane disease. All patients received 100 mg/kg calf lung surfactant extract. FRC was measured by a closed-circuit helium-dilution technique, and lung mechanics were determined by least mean squares analysis. FRC increased in all patients (range 56% to 330%; p less than 0.03). Dynamic lung compliance and total airway conductance did not change. Mean +/- SEM specific lung compliance (dynamic lung compliance/FRC) decreased 55.93% +/- 4.27% (p less than 0.02) and mean specific conductance (total airway conductance/FRC) decreased 45.91% +/- 9.74% (p less than 0.009). Mean alveolar/arterial partial pressure of oxygen ratio decreased 51.0% +/- 8.67% (p less than 0.01). These data indicate that the immediate improvement in oxygenation after surfactant administration is related to increased lung volumes. The decrease in specific lung compliance and specific airway conductance is suggestive of increased distention rather than recruitment of functional alveoli.