Ruth Everett

Proportional assist ventilation in low birth weight infants with acute respiratory disease: A comparison to assist/control and conventional mechanical ventilation ☆ ☆☆ ★

OBJECTIVES To compare the physiologic efficacy and safety aspects of proportional assist (PA), assist/control (A/C), and intermittent mandatory ventilation (IMV) in very low birth weight infants with acute respiratory illness and to test the hypothesis that ventilatory pressure requirements are lower and arterial oxygenation is improved during PA when compared with IMV or A/C at an equivalent inspired oxygen fraction. STUDY DESIGN Randomized, 3-period, crossover design. METHODS Thirty-six infants were stratified by birth weight (600 to 750, 751 to 900, and 901 to 1200 g) and exposed to consecutive 45-minute epochs of the 3 modalities in a sequence chosen at random. Tidal volumes of 4 to 6 mL/kg were targeted during A/C and IMV. The IMV rate was matched to the rate during an A/C test period. PA was adjusted to unload the resistance of the endotracheal tube and the disease-related increase in lung elastic recoil. RESULTS Compared with A/C and IMV, PA maintained similar arterial oxygenation with lower airway and transpulmonary pressures (15% to 44% reduction depending on the index variable). The oxygenation index decreased by 28% during PA. No adverse events were observed. The number and severity of apneic episodes and periods of arterial oxygen desaturations were similar with the 3 modes. Similar results were obtained within each birth weight subgroup. CONCLUSIONS PA safely maintains gas exchange with smaller transpulmonary pressure changes compared with A/C and IMV. It may therefore offer a way of reducing the incidence of chronic lung disease in low birth weight infants.

Mechanisms for episodes of hypoxemia in preterm infants undergoing mechanical ventilation

OBJECTIVE To ascertain possible mechanisms implicated in the development of transient episodes of hypoxemia (oxygen saturation < 85%) frequently observed in preterm infants undergoing mechanical ventilation, even after the acute phase of respiratory failure has passed. STUDY DESIGN Tidal flow, airway and esophageal pressure, and oxygen saturation were continuously recorded in 10 infants (mean +/- SD, birth weight 733 +/- 149 gm, gestational age 25.5 +/- 2.2 weeks, age 26.3 +/- 11.9 days) who had repeated episodes of hypoxemia without any evident cause. Measurements of minute ventilation (VE) inspiratory compliance (Ci), and inspiratory resistance (Ri) were compared before and during episodes of hypoxemia. RESULTS All episodes of hypoxemia were preceded by an active exhalation that produced a mean decrease in end-expiratory lung volume of 6.4 +/- 2.8 ml/kg. The reduction in lung volume was immediately followed by a sudden decrease in tidal flow and volume, despite continuation of mechanical ventilation at the same rate and peak pressure. The resulting hypoventilation was associated with a drop in Ci to approximately one half and an increase in Ri to more than double the baseline values. Approximately 30 seconds after the beginning of hypoventilation, the arterial oxygen saturation reached a hypoxemic level (oxygen saturation < 85%)> CONCLUSION Most hypoxemic episodes were triggered by an expiratory effort that produced a large decrease in lung volume. This reduction in lung volume probably leads to closure of small airways and the development of intrapulmonary shunts, which would explain the rapid development of hypoxemia.

Influence of different methods of synchronized mechanical ventilation on ventilation, gas exchange, patient effort, and blood pressure fluctuations in premature neonates

We studied the effects of two methods of synchronized mechanical ventilation [synchronized intermittent mandatory ventilation (SIMV) and assist/control (A/C)] on ventilation, gas exchange, patient effort, and arterial blood pressure (ABP) fluctuations. SIMV and A/C were applied in random order in 12 preterm neonates (gestational age, 29.7 ± 2.3 weeks; birth weight, 1,217 ± 402 g). We measured total (Vetot) and mechanical (Vemech) minute ventilation, spontaneous (Vtspont) and ventilator supported (Vtmech) tidal volume, transcutaneous oxygen saturation (SpO2), transculaneous PO2 (TcPO2), and PCO2, (TcPCO2), mean airway pressure (Paw), phasic esophageal pressure deflections (Pe) as an estimate of inspiratory effort, mean arterial blood pressure (ABP), and beat‐to‐beat ABP fluctuations. The measurements obtained during conventional intermittent mandatory ventilation (IMV) were compared with the recordings during SIMV and A/C. To make the measurement conditions comparable and to prevent hyperventilation, peak inspiratory pressure was reduced during the A/C mode so that Vetot remained in the same range as during the IMV mode. Whereas Vetot was similar in all three conditions by study design, Vemech was larger during SIMV and A/C than during IMV. Vtmech increased during SIMV and by study design was smaller during A/C than during IMV. Pe decreased during SIMV and A/C compared with IMV, and Paw was higher during A/C than during IMV or SIMV. Beat‐to‐beat ABP fluctuations were reduced during SIMV and A/C compared with IMV and showed a close positive correlation with Pe changes. We conclude that SIMV increases Vemech and reduces Pe compared with IMV, resulting in smaller intrathoracic and ABP fluctuations. During A/C, a substantial portion of the spontaneous respiratory effort is shifted to the ventilator, resulting in a further decrease in Pe and ABP fluctuations Pediatr Pulmonol. 1996; 22:305–313.

Randomized crossover comparison of proportional assist ventilation and patient-triggered ventilation in extremely low birth weight infants with evolving chronic lung disease

Background: Refinement of ventilatory techniques remains a challenge given the persistence of chronic lung disease of preterm infants. Objective: To test the hypothesis that proportional assist ventilation (PAV) will allow to lower the ventilator pressure at equivalent fractions of inspiratory oxygen (FiO2) and arterial hemoglobin oxygen saturation in ventilator-dependent extremely low birth weight infants in comparison with standard patient-triggered ventilation (PTV). Methods:Design: Randomized crossover design. Setting:Two level-3 university perinatal centers. Patients: 22 infants (mean (SD): birth weight, 705 g (215); gestational age, 25.6 weeks (2.0); age at study, 22.9 days (15.6)). Interventions: One 4-hour period of PAV was applied on each of 2 consecutive days and compared with epochs of standard PTV. Results: Mean airway pressure was 5.64 (SD, 0.81) cm H2O during PAV and 6.59 (SD, 1.26) cm H2O during PTV (p < 0.0001), the mean peak inspiratory pressure was 10.3 (SD, 2.48) cm H2O and 15.1 (SD, 3.64) cm H2O (p < 0.001), respectively. The FiO2 (0.34 (0.13) vs. 0.34 (0.14)) and pulse oximetry readings were not significantly different. The incidence of arterial oxygen desaturations was not different (3.48 (3.2) vs. 3.34 (3.0) episodes/h) but desaturations lasted longer during PAV (2.60 (2.8) vs. 1.85 (2.2) min of desaturation/h, p = 0.049). PaCO2 measured transcutaneously in a subgroup of 12 infants was similar. One infant met prespecified PAV failure criteria. No adverse events occurred during the 164 cumulative hours of PAV application. Conclusions: PAV safely maintains gas exchange at lower mean airway pressures compared with PTV without adverse effects in this population. Backup conventional ventilation breaths must be provided to prevent apnea-related desaturations.

Proportional Assist Ventilation Decreases Thoracoabdominal Asynchrony and Chest Wall Distortion in Preterm Infants

Thoracoabdominal asynchrony (TAA) and chest wall distortion (CWD) are commonly seen in preterm infants secondary to a highly compliant rib cage and poor compensation of distorting forces by inspiratory rib cage muscles. Continuous positive airway pressure (CPAP) reduces TAA and CWD by stenting the chest wall. We hypothesized that application of positive airway pressure only during inspiration and in proportion to an infants inspiratory effort should have a similar but more pronounced effect than CPAP alone. A ventilator providing airway pressure changes in proportion to flow and volume generated by an infant (proportional assist ventilation) was used to unload the respiratory pump during inspiration. Ten preterm infants were studied [birth weight, 745 (635–1175) g; gestational age, 26.5 (24–31) wk; postnatal age 3 (1–7) d; medium (range)]. TAA and CWD were determined by respiratory inductive plethysmography. TAA was expressed as the phase angle between the rib cage and abdominal motion and CWD as the total compartmental displacement ratio. In addition, we measured tidal volume with a pneumotachograph and esophageal and airway pressure deflections with pressure transducers. Measurements were obtained during alternating periods of CPAP and two different degrees of support (Gain 1 = 1.09 ± 0.68, Gain 2 = 1.84 ± 0.84 cm H2O/mL) that were provided by a proportional assist ventilator. Phase angle and the total compartmental displacement ratio decreased with increasing gain compared with CPAP alone. Peak airway pressure increased from 0.6 to 3.8 to 7.6 cm H2O above positive end-expiratory pressure (PEEP) with CPAP, Gain 1, and Gain 2, respectively, as tidal volume increased from 2.8 to 4.1 to 4.7 mL/kg. Esophageal pressure changes decreased only little with increasing gain. Chest wall excursion increased and abdominal movement decreased, indicating a redistribution of tidal volume between chest and abdomen. We conclude that proportional assist ventilation reduces TAA and CWD by generating a small increase in airway pressure that occurs in synchrony and in proportion to each inspiratory effort.

Computer-controlled minute ventilation in preterm infants undergoing mechanical ventilation.

INTRODUCTION Computer-controlled minute ventilation (CCMV) continuously adjusts the ventilator rate to changes in spontaneous respiratory drive and pulmonary mechanics to maintain a preset total minute ventilation. HYPOTHESIS We hypothesized that CCMV would maintain ventilation and oxygenation with fewer mechanical breaths than conventional intermittent mandatory ventilation in very low birth weight infants. METHODS Very low birth weight infants in clinically stable condition who were undergoing mechanical ventilation were enrolled. The number of mechanical breaths, total and mechanical expiratory minute ventilation, mean airway pressure, oxygen hemoglobin saturation by pulse oximetry, and transcutaneous partial carbon dioxide and partial oxygen tensions were obtained during intermittent mandatory ventilation and CCMV (45 to 60 minutes) and compared by paired t test. RESULTS Fifteen infants were studied. Birth weight (median, range) was 700 gm (550 to 1205 gm), gestational age 26 weeks (23 to 34 weeks), age 21 days (3 to 50 days). When switched from intermittent mandatory ventilation to CCMV, the number of mechanical breaths was reduced (15 +/- 2.8 to 8.6 +/- 2.9 breaths per minute, p < 0.001), leading to lower airway pressure (3.97 +/- 1.00 to 3.45 +/- 1.00 cm H2O, p < 0.001) and lower expiratory minute ventilation generated by the mechanical ventilator (116 +/- 31 to 65 +/- 28 ml/min per kilogram, p < 0.001), while total expiratory minute ventilation remained unchanged. Mean transcutaneous partial carbon dioxide and oxygen tensions, oxygen hemoglobin saturation, and the time spent within different oxygen hemoglobin saturation ranges did not differ between both ventilatory modes. CONCLUSION CCMV maintained adequate ventilation and oxygenation with lower mechanical ventilatory support than IMV. CCMV may reduce barotrauma and chronic lung disease during long-term use.

Effects of respiratory mechanical unloading on thoracoabdominal motion in meconium-injured piglets and rabbits

Impaired pulmonary mechanics can cause chest wall distortion (CWD) so that work of breathing is dissipated in deforming the rib cage. We hypothesized that respiratory mechanical unloading as a technique of assisted mechanical ventilation would reduce CWD in animals with injured lungs. We studied five piglets and five adult rabbits to test across different ages and chest configurations. As a result of intratracheal meconium instillation, lung compliance decreased from 21 (median; range 17-35) to 9.5 (6.7-14) mL/kPa/kg in rabbits and from 26 (18-31) to 7.9 (4.9-11) in piglets. Airway resistance increased from 5.0 (4.6-6.1) to 6.9 (5.8-7.9) kPa/L/s in rabbits only. Respiratory inductive plethysmography was used to measure the phase shift between the rib cage and abdominal compartment movements and the total compartmental displacement ratio. We aimed at unloading at least three-fourths of lung elastance in all animals and 2.0 kPa/L/s of resistance in rabbits. Elastic unloading decreased the phase shift in all but one animal. It reduced the total compartmental displacement ratio from 1.27 (1.14-3.73) to 1.16(1.02-1.82) in piglets and from 1.77 (1.45-5.24) to 1.37 (1.11-4.78) in rabbits. The inspiratory rib cage expansion increased, whereas abdominal expansion did not. The tidal esophageal pressure deflection decreased. Tidal volume increased, whereas respiratory rate remained unaffected so that the partial pressure of arterial CO2 decreased. Resistive unloading as an adjunct to elastic unloading further reduced CWD and induced a more rapid, shallower breathing. We conclude that respiratory unloading as a mechanical support to spontaneous breathing reduces CWD. We speculate that the decrease in CWD increases ventilatory efficiency for a given diaphragmatic effort.

Patient-triggered ventilation: a comparison of tidal volume and chestwall and abdominal motion as trigger signals.

Patient‐triggered synchronized ventilation requires reliable and early detection of the infants inspiratory effort. Several trigger methods have been developed that frequently lack the sensitivity to detect inspiration in small preterm infants (trigger failure), or show a high rate of breaths triggered by artifacts in the respiratory signal (autotrigger). The purpose of this study was to determine the effectiveness of the following trigger signals: abdominal movement sensed by a newly developed induction technique, chestwall motion detected by changes in transthoracic impedance, and tidal volume measured by anemometry at the endotracheal tube connector. Ten preterm infants (birth weight, 580–1,424 g; median weight, 943 g; study weight, 535–1,415 g; median weight, 838 g; gestation age, 26–32 weeks, median gestational age, 28 weeks, study age, 1–50 days, median study age, 11 days) were included in the study. A Sechrist SAVI ventilator was triggered by one of three signals: chestwall or abdominal movement, or tidal volume generated by the infants. Response time between beginning of inspiratory flow, the occurrence of the trigger signal (signal delay), and the onset of the triggered breath (trigger delay) were determined for each of the three signals. The signal response time was −13.5 msec (95% Cl, −33 to −2 msec) for the abdominal movement signal, indicating that it started before inspiratory flow; 0.0 msec for the volume signal; and 44.0 msec (95% Cl, 29–73 msec) for the chestwall signal (P < 0.002); this long delay was secondary to chestwall distortion and a subsequent delay in outward ribcage movement in many infants. The trigger delay for the abdominal signal was 90.0 msec (95% Cl, 55–104 msec), 135.5 msec (95% Cl: 82–186 msec) for the volume signal, and 176.5 msec (95% Cl: 165–232 msec) for the chestwall signal, indicating that there was a difference in the rise time of signal voltage between the three methods (P < 0.01). The rate of autotriggered breaths was 3.2% (95% Cl,0.3–9.3%) when using the abdominal signal, 0.55% (95% Cl, 0.0–2.1%) for the tidal volume signal, and 11.25% (95% Cl, 0.5–27.8%) for the chestwall signal (P < 0.05). The incidence of trigger failure was low with all three signals and was not significantly different between the techniques. In summary, the chestwall signal had a long trigger delay and was highly susceptible to false triggering. It is, therefore, not a reliable trigger signal for synchronized mechanical ventilation in preterm infants. In contrast, tidal volume and abdominal movement signals had an acceptable trigger delay and a low rate of autotriggering, making them useful clinical trigger signals. Pediatr Pulmonol. 1996;22:28–34.

Intrasubject variability of repeated pulmonary function measurements in preterm ventilated infants

This study set out to describe the variability and assess the reproducibility of repeated pulmonary function measurements in ventilated preterm infants. We measured tidal volume (VT), lung compliance (CL), and resistance (RL) in 16 infants (mean ± SD: birthweight 1222 ± 343 g) during spontaneous breathing and during mechanical ventilation, suppressing breathing efforts by mild hyperventilation. CL and RL were calculated from the equation of motion using linear regression analysis (LR), and by the Mead and Wittenberger method (MW). Flow and transpulmonary pressure were recorded for at least two consecutive periods, after which the esophageal tube was removed and replaced 1 hour later for a second set of recordings. The mean percent change (% Δ) between the initial and the repeated measurements with their respective 95% confidence intervals were calculated. Reproducibility was assessed by the intraclass correlation coefficient (ICC) (total agreement = 1, good reproducibility ≥0.75). The mean % Δ between initial and repeat measurements during spontaneous breathing ranged from 11% to 14% for CL and VT, and from 22% to 32% for RL. The variation for RL was even higher when the analysis was done separately for the inspiratory and expiratory phase. CL and VT had good reproducibility (ICC >0.9), while RL was significantly less reproducible (ICC <0.75). Measurements obtained from mechanical breaths had less variability than from spontaneous breaths, ranging from 8% to 15% for CL and VT, and from 13% to 21% for RL. Reproducibility assessed by the ICC was good for most measurements during mechanical breaths. The variability and reproducibility of measurements were similar for both methods of analysis during mechanical ventilation, but during spontaneous breathing showed larger variability with the MW method than with LR analysis. We concluded that VT and CL were reproducible during spontaneous and mechanical breathing. However, RL measurements were reproducible only during mechanical ventilation. The high variability of RL in spontaneously breathing preterm infants may reduce the clinical usefulness of this measurement for individual patients. Pediatr Pulmonol. 1996; 21:35–41.

Acute effects of inhaled nitric oxide on pulmonary and cardiac function in preterm infants with evolving bronchopulmonary dysplasia.

BACKGROUND: Inhaled nitric oxide (iNO) reduces pulmonary vascular resistance by preferential vasodilation in ventilated lung units. In experimental animals, iNO also reduces airway resistance by smooth muscle relaxation. Hence, there may be a therapeutic role for iNO in evolving bronchopulmonary dysplasia (BPD).OBJECTIVE: To evaluate the acute effects of low-dose iNO on lung mechanics, ventilation distribution, oxygenation, and cardiac function in preterm infants with evolving BPD.METHODS: Measurements of lung compliance (CL), airway resistance (RL), ventilation-distribution (N2 clearance in multiple-breath washout), oxygenation (SpO2), left ventricular ejection fraction (LVEF) and right ventricular shortening fraction were obtained before and during 2 hours of iNO (10 ppm) in a group of ventilated preterm infants with evolving BPD.RESULTS: A total of 13 preterm infants with (mean±SD) BW: 663.8±116 g, GA: 24.9±1.2 weeks, age: 32±14 days, mean airway pressure: 6.7±0.9 cmH2O and fraction of inspired oxygen: 0.35±0.06 were studied. iNO did not affect CL, RL or N2 clearance. There was a small increase in LVEF. Mean SpO2 remained unchanged, but the duration of spontaneous hypoxemic episodes increased during iNO.CONCLUSION: Low-dose iNO had no acute effects on lung function, cardiac function and oxygenation in evolving BPD.