Indicators of Optimal Lung Volume During High-Frequency Oscillatory Ventilation in Infants*:

Neonatal high-frequency oscillatory ventilation: where are we now?

High-frequency oscillatory ventilation (HFOV) is an established mode of respiratory support in the neonatal intensive care unit. Large clinical trial data is based on first intention use in preterm infants with acute respiratory distress syndrome. Clinical practice has evolved from this narrow population. HFOV is most often reserved for term and preterm infants with severe, and often complex, respiratory failure not responding to conventional modalities of respiratory support. Thus, optimal, and safe, application of HFOV requires the clinician to adapt mean airway pressure, frequency, inspiratory:expiratory ratio and tidal volume to individual patient needs based on pathophysiology, lung volume state and infant size. This narrative review summarises the status of HFOV in neonatal intensive care units today, the lessons that can be learnt from the past, how to apply HFOV in different neonatal populations and conditions and highlights potential new advances. Specifically, we provide guidance on how to apply an open lung approach to mean airway pressure, selecting the correct frequency and use of volume-targeted HFOV.

Lung behavior during a staircase high-frequency oscillatory ventilation recruitment maneuver

BACKGROUND

Lung volume optimization maneuvers (LVOM) are necessary to make physiologic use of high-frequency oscillatory ventilation (HFOV), but lung behavior during such maneuvers has not been studied to determine lung volume changes after initiation of HFOV, to quantify recruitment versus derecruitment during the LVOM and to calculate the time to stabilization after a pressure change.

METHODS

We performed a secondary analysis of prospectively collected data in subjects < 18 years on HFOV. Uncalibrated respiratory inductance plethysmography (RIP) tracings were used to quantify lung recruitment and derecruitment during the LVOM inflation and deflation. The time constant was calculated according to the Niemann model.

RESULTS

RIP data of 51 subjects (median age 3.5 [1.7-13.3] months) with moderate-to-severe pediatric acute respiratory distress syndrome (PARDS) in 85.4% were analyzed. Lung recruitment and derecruitment occurred during the LVOM inflation phase upon start of HFOV and between and within pressure changes. At 90% of maximum inflation pressure, lung derecruitment already started during the deflation phase. Time to stable lung volume (time constant) could only be calculated in 26.2% of all pressure changes during the inflation and in 21.4% during the deflation phase, independent of continuous distending pressure (CDP). Inability to calculate the time constant was due to lack of stabilization of the RIP signal or no change in any direction.

CONCLUSIONS

Significant heterogeneity in lung behavior during a staircase incremental-decremental LVOM occurred, underscoring the need for higher initial inflation pressures when transitioning from conventional mechanical ventilation (CMV) and a longer time between pressure changes to allow for equilibration.

Neonatal high frequency ventilation: Current trends and future directions

High frequency ventilation (HFV) in neonates has been in use for over forty years. Some early HFV ventilators are no longer available, but high frequency oscillatory ventilation (HFOV) and jet ventilators (HFJV) continue to be commonly employed. Advanced HFOV models available outside of the United States are much quieter and easier to use, and are available as options on many conventional ventilators, providing important improvements such as tidal volume measurement and targeting. HFJV excels in treating air leak and non-homogenous lung disease and is often used for other diseases as well. High frequency non-invasive ventilation (hfNIV) is a novel application of HFV that remains under investigation. Similar to bubble CPAP, hfNIV has been applied with a variety of high-frequency ventilators. Efficacy and safety of hfNIV with any device have not yet been established. This article describes the current approaches to these HFV therapies and stresses the importance of understanding how each device works and what disease processes may respond best to the technology employed.

Oscillometry for personalizing continuous distending pressure maneuvers: an observational study in extremely preterm infants

RATIONALE

Lung recruitment and continuous distending pressure (CDP) titration are critical for assuring the efficacy of high-frequency ventilation (HFOV) in preterm infants. The limitation of oxygenation (peripheral oxygen saturation, SpO2) in optimizing CDP calls for evaluating other non-invasive bedside measurements. Respiratory reactance (Xrs) at 10 Hz measured by oscillometry reflects lung volume recruitment and tissue strain. In particular, lung volume recruitment and decreased tissue strain result in increased Xrs values.

OBJECTIVES

In extremely preterm infants treated with HFOV as first intention, we aimed to measure the relationship between CDP and Xrs during SpO2-driven CDP optimization.

METHODS

In this prospective observational study, extremely preterm infants born before 28 weeks of gestation undergoing SpO2-guided lung recruitment maneuvers were included in the study. SpO2 and Xrs were recorded at each CDP step. The optimal CDP identified by oxygenation (CDPOpt_SpO2) was compared to the CDP providing maximal Xrs on the deflation limb of the recruitment maneuver (CDPXrs).

RESULTS

We studied 40 infants (gestational age at birth = 22+ 6-27+ 5 wk; postnatal age = 1-23 days). Measurements were well tolerated and provided reliable results in 96% of cases. On average, Xrs decreased during the inflation limb and increased during the deflation limb. Xrs changes were heterogeneous among the infants for the amount of decrease with increasing CDP, the decrease at the lowest CDP of the deflation limb, and the hysteresis of the Xrs vs. CDP curve. In all but five infants, the hysteresis of the Xrs vs. CDP curve suggested effective lung recruitment. CDPOpt_SpO2 and CDPXrs were highly correlated (ρ = 0.71, p < 0.001) and not statistically different (median difference [range] = -1 [-3; 9] cmH2O). However, CDPXrs were equal to CDPOpt_SpO2 in only 6 infants, greater than CDPOpt_SpO2 in 10, and lower in 24 infants.

CONCLUSIONS

The Xrs changes described provide complementary information to oxygenation. Further investigation is warranted to refine recruitment maneuvers and CPD settings in preterm infants.

Transcutaneous CO2 Monitoring in Extremely Low Birth Weight Premature Infants

Extremely low birth weight (ELBW) premature infants are particularly susceptible to hypocarbia and hypercarbia, which are associated with brain and lung morbidities. Transcutaneous CO2 (TcCO2) monitoring allows for continuous non-invasive CO2 monitoring during invasive and non-invasive ventilation and is becoming more popular in the NICU. We aimed to evaluate the correlation and agreement between CO2 levels measured by a TcCO2 monitor and blood gas CO2 (bgCO2) among ELBW infants. This was a prospective observational multicenter study. All infants < 1000 g admitted to the participating NICUs during the study period were monitored by a TcCO2 monitor, if available. For each bgCO2 measured, a simultaneous TcCO2 measurement was documented. In total, 1828 pairs of TcCO2-bgCO2 values of 94 infants were collected, with a median (IQR) gestational age of 26.4 (26.0, 28.3) weeks and birth weight of 800 (702, 900) g. A moderate correlation (Pearson: r = 0.64) and good agreement (bias (95% limits of agreement)):(2.9 [-11.8, 17.6] mmHg) were found between the TcCO2 and bgCO2 values in the 25-70 mmHg TcCO2 range. The correlation between the TcCO2 and bgCO2 trends was moderate. CO2 measurements by TcCO2 are in good agreement (bias < 5 mmHg) with bgCO2 among premature infants < 1000 g during the first week of life, regardless of day of life, ventilation mode (invasive/non-invasive), and sampling method (arterial/capillary/venous). However, wide limits of agreement and moderate correlation dictate the use of TcCO2 as a complementary tool to blood gas sampling, to assess CO2 levels and trends in individual patients.

Estimating Preterm Lung Volume: A Comparison of Lung Ultrasound, Chest Radiography, and Oxygenation

OBJECTIVE

To determine the relationship between lung ultrasound (LUS) examination, chest radiograph (CXR), and radiographic and clinical evaluations in the assessment of lung volume in preterm infants.

STUDY DESIGN

In this prospective cohort study LUS was performed before CXR on 70 preterm infants and graded using (1) a LUS score, (2) an atelectasis score, and (3) measurement of atelectasis depth. Radiographic diaphragm position and radio-opacification were used to determine global and regional radiographic atelectasis. The relationship between LUS, CXR, and oxygenation was assessed using receiver operator characteristic and correlation analysis.

RESULTS

LUS scores, atelectasis scores, and atelectasis depth did not correspond with radiographic global atelectasis (area under receiver operator characteristics curves, 0.54 [95% CI, 0.36-0.71], 0.49 [95% CI, 0.34-0.64], and 0.47 [95% CI, 0.31-0.64], respectively). Radiographic atelectasis of the right upper, right lower, left upper, and left lower quadrants was predicted by LUS scores (0.75 [95% CI, 0.59-0.92], 0.75 [95% CI, 0.62-0.89], 0.69 [95% CI, 0.56-0.82], and 0.63 [95% CI, 0.508-0.751]) and atelectasis depth (0.66 [95% CI, 0.54-0.78], 0.65 [95% CI, 0.53-0.77], 0.63 [95% CI, 0.50-0.76], and 0.56 [95% CI, 0.44-0.70]). LUS findings were moderately correlated with oxygen saturation index (ρ = 0.52 [95% CI, 0.30-0.70]) and saturation to fraction of inspired oxygen ratio (ρ = -0.63 [95% CI, -0.76 to -0.46]). The correlation between radiographic diaphragm position, the oxygenation saturation index, and peripheral oxygen saturation to fraction of inspired oxygen ratio was very weak (ρ = 0.36 [95% CI, 0.11-0.59] and ρ = -0.32 [95% CI, -0.53 to -0.07], respectively).

CONCLUSIONS

LUS assessment of lung volume does not correspond with radiographic diaphragm position preterm infants. However, LUS predicted radiographic regional atelectasis and correlated with oxygenation. The relationship between radiographic diaphragm position and oxygenation was very weak. Although LUS may not replace all radiographic measures of lung volume, LUS more accurately reflects respiratory status in preterm infants.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry: ACTRN12621001119886.

Lung ultrasound of the dependent lung detects real-time changes in lung volume in the preterm lamb

BACKGROUND

Effective lung protective ventilation requires reliable, real-time estimation of lung volume at the bedside. Neonatal clinicians lack a readily available imaging tool for this purpose.

OBJECTIVE

To determine the ability of lung ultrasound (LUS) of the dependent region to detect real-time changes in lung volume, identify opening and closing pressures of the lung, and detect pulmonary hysteresis.

METHODS

LUS was performed on preterm lambs (n=20) during in vivo mapping of the pressure-volume relationship of the respiratory system using the super-syringe method. Electrical impedance tomography was used to derive regional lung volumes. Images were blindly graded using an expanded scoring system. The scores were compared with total and regional lung volumes, and differences in LUS scores between pressure increments were calculated.

RESULTS

Changes in LUS scores correlated moderately with changes in total lung volume (r=0.56, 95% CI 0.47-0.64, p<0.0001) and fairly with right whole (r=0.41, CI 0.30-0.51, p<0.0001), ventral (r=0.39, CI 0.28-0.49, p<0.0001), central (r=0.41, CI 0.31-0.52, p<0.0001) and dorsal (r=0.38, CI 0.27-0.49, p<0.0001) regional lung volumes. The pressure-volume relationship of the lung exhibited hysteresis in all lambs. LUS was able to detect hysteresis in 17 (85%) lambs. The greatest changes in LUS scores occurred at the opening and closing pressures.

CONCLUSION

LUS was able to detect large changes in total and regional lung volume in real time and correctly identified opening and closing pressures but lacked the precision to detect small changes in lung volume. Further work is needed to improve precision prior to translation to clinical practice.

Lung recruitment in neonatal high-frequency oscillatory ventilation with volume-guarantee

BACKGROUND AND OBJECTIVES

The optimal lung volume strategy during high-frequency oscillatory ventilation (HFOV) is reached by performing recruitment maneuvers, usually guided by the response in oxygenation. In animal models, secondary spontaneous change in oscillation pressure amplitude (ΔPhf) associated with a progressive increase in mean airway pressure during HFOV combined with volume guarantee (HFOV-VG) identifies optimal lung recruitment. The aim of this study was to describe recruitment maneuvers in HFOV-VG and analyze whether changes in ΔPhf might be an early predictor for lung recruitment in newborn infants with severe respiratory failure.

DESIGN AND METHODS

The prospective observational study was done in a tertiary-level neonatology department. Changes in ΔPhf were analyzed during standardized lung recruitment after initiating early rescue HFOV-VG in preterm infants with severe respiratory failure.

RESULTS

Twenty-seven patients were included, with a median gestational age of 24 weeks (interquartile range [IQR]: 23-25). Recruitment maneuvers were performed, median baseline mean airway pressure (mPaw) was 11 cm H₂ O (IQR: 10-13), median critical lung opening mPaw during recruitment was 14 cm H₂ O (IRQ: 12-16), and median optimal mPaw was 12 cm H₂ O (IQR: 10-14, p < 0.01). Recruitment maneuvers were associated with an improvement in oxygenation (FiO₂ : 65.0 vs. 45.0, p < 0.01, SpO2/FiO₂ ratio: 117 vs. 217, p < 0.01). ΔPhf decreased significantly after lung recruitment (mean amplitude: 23.0 vs. 16.0, p < 0.01).

CONCLUSION

In preterm infants with severe respiratory failure, the lung recruitment process can be effectively guided by ΔPhf on HFOV-VG.

Quantitative lung ultrasound detects dynamic changes in lung recruitment in the preterm lamb

BACKGROUND

Lung ultrasound (LUS) may not detect small, dynamic changes in lung volume. Mean greyscale measurement using computer-assisted image analysis (Q-LUS_MGV) may improve the precision of these measurements.

METHODS

Preterm lambs (n = 40) underwent LUS of the dependent or non-dependent lung during static pressure-volume curve mapping. Total and regional lung volumes were determined using the super-syringe technique and electrical impedance tomography. Q-LUS_MGV and gold standard measurements of lung volume were compared in 520 images.

RESULTS

Dependent Q-LUS_MGV moderately correlated with total lung volume (rho = 0.60, 95% CI 0.51-0.67) and fairly with right whole (rho = 0.39, 0.27-0.49), central (rho = 0.38, 0.27-0.48), ventral (rho = 0.41, 0.31-0.51) and dorsal regional lung volumes (rho = 0.32, 0.21-0.43). Non-dependent Q-LUS_MGV moderately correlated with total lung volume (rho = 0.57, 0.48-0.65) and fairly with right whole (rho = 0.43, 0.32-0.52), central (rho = 0.46, 0.35-0.55), ventral (rho = 0.36, 0.25-0.47) and dorsal lung volumes (rho = 0.36, 0.25-0.47). All correlation coefficients were statistically significant. Distinct inflation and deflation limbs, and sonographic pulmonary hysteresis occurred in 95% of lambs. The greatest changes in Q-LUS_MGV occurred at the opening and closing pressures.

CONCLUSION

Q-LUS_MGV detected changes in total and regional lung volume and offers objective quantification of LUS images, and may improve bedside discrimination of real-time changes in lung volume.

IMPACT

Lung ultrasound (LUS) offers continuous, radiation-free imaging that may play a role in assessing lung recruitment but may not detect small changes in lung volume. Mean greyscale image analysis using computer-assisted quantitative LUS (Q-LUS_MGV) moderately correlated with changes in total and regional lung volume. Q-LUS_MGV identified opening and closing pressure and pulmonary hysteresis in 95% of lambs. Computer-assisted image analysis may enhance LUS estimation of lung recruitment at the bedside. Future research should focus on improving precision prior to clinical translation.

Epidemiology of Neonatal Acute Respiratory Distress Syndrome: Prospective, Multicenter, International Cohort Study

OBJECTIVES

Age-specific definitions for acute respiratory distress syndrome (ARDS) are available, including a specific definition for neonates (the "Montreux definition"). The epidemiology of neonatal ARDS is unknown. The objective of this study was to describe the epidemiology, clinical course, treatment, and outcomes of neonatal ARDS.

DESIGN

Prospective, international, observational, cohort study.

SETTING

Fifteen academic neonatal ICUs.

PATIENTS

Consecutive sample of neonates of any gestational age admitted to participating sites who met the neonatal ARDS Montreux definition criteria.

MEASUREMENTS AND MAIN RESULTS

Neonatal ARDS was classified as direct or indirect, infectious or noninfectious, and perinatal (≤ 72 hr after birth) or late in onset. Primary outcomes were: 1) survival at 30 days from diagnosis, 2) inhospital survival, and 3) extracorporeal membrane oxygenation (ECMO)-free survival at 30 days from diagnosis. Secondary outcomes included respiratory complications and common neonatal extrapulmonary morbidities. A total of 239 neonates met criteria for the diagnosis of neonatal ARDS. The median prevalence was 1.5% of neonatal ICU admissions with male/female ratio of 1.5. Respiratory treatments were similar across gestational ages. Direct neonatal ARDS (51.5% of neonates) was more common in term neonates and the perinatal period. Indirect neonatal ARDS was often triggered by an infection and was more common in preterm neonates. Thirty-day, inhospital, and 30-day ECMO-free survival were 83.3%, 76.2%, and 79.5%, respectively. Direct neonatal ARDS was associated with better survival outcomes than indirect neonatal ARDS. Direct and noninfectious neonatal ARDS were associated with the poorest respiratory outcomes at 36 and 40 weeks' postmenstrual age. Gestational age was not associated with any primary outcome on multivariate analyses.

CONCLUSIONS

Prevalence and survival of neonatal ARDS are similar to those of pediatric ARDS. The neonatal ARDS subtypes used in the current definition may be associated with distinct clinical outcomes and a different distribution for term and preterm neonates.

Time to Lung Volume Stability After Pressure Change During High-Frequency Oscillatory Ventilation

Supplemental Digital Content is available in the text.

OBJECTIVES:

Clinicians have little guidance on the time needed before assessing the effect of a mean airway pressure change during high-frequency oscillatory ventilation. We aimed to determine: 1) time to stable lung volume after a mean airway pressure change during high-frequency oscillatory ventilation and 2) the relationship between time to volume stability and the volume state of the lung.

DESIGN:

Prospective observational study.

SETTING:

Regional quaternary teaching hospital neonatal ICU.

PATIENTS:

Thirteen term or near-term infants receiving high-frequency oscillatory ventilation and muscle relaxants.

INTERVENTIONS:

One to two cm H₂O mean airway pressure changes every 10 minutes as part of an open lung strategy based on oxygen response.

MEASUREMENTS AND MAIN RESULTS:

Continuous lung volume measurements (respiratory inductive plethysmography) were made during the mean airway pressure changes. Volume signals were analyzed with a biexponential model to calculate the time to stable lung volume if the model R² was greater than 0.6. If volume stability did not occur within 10 minutes, the model was extrapolated to maximum 3,600 s. One-hundred ninety-six mean airway pressure changes were made, with no volume change in 33 occurrences (17%). One-hundred twenty-five volume signals met modeling criteria for inclusion; median (interquartile range) R², 0.96 (0.91–0.98). The time to stable lung volume was 1,131 seconds (718–1,959 s) (mean airway pressure increases) and 647 seconds (439–1,309 s) (mean airway pressure decreases), with only 17 (14%) occurring within 10 minutes and time to stability being longer when the lung was atelectatic.

CONCLUSIONS:

During high-frequency oscillatory ventilation, the time to stable lung volume after a mean airway pressure change is variable, often requires more than 10 minutes, and is dependent on the preceding volume state.

New indicators for optimal lung recruitment during high frequency oscillator ventilation

Previous research has demonstrated the potential benefit derived from the combination of high frequency oscillatory ventilation and volume guarantee mode (HFOV-VG), a procedure that allows us to explore and control very low tidal volumes. We hypothesized that secondary spontaneous change in oscillation pressure amplitude (∆Phf), while increasing the mean airway pressure (MAP) using HFOV-VG can target the lung recruitment.

METHODS

A two-step animal distress model study was designed; in the first-step (ex vivo model), the animal's lungs were isolated to visually check lung recruitment and, in the second one (in vivo model), they were checked through arterial oxygen partial pressure improvement. Baseline measurements were performed, ventilation was set for 10 min and followed by bronchoalveolar lavage with isotonic saline to induce depletion of surfactant and thereby achieve a low compliance lung model. The high-frequency tidal volume and frequency remained constant and the MAP was increased by 2 cmH₂ O (ex vivo) and 3 cmH₂ O steps (in vivo) every 2 min. Changes in ΔPhf to achieve the fixed volume were recorded at the end of each interval to describe the maximum drop point as the recruitment point.

RESULTS

Fourteen Wistar Han rats were included, seven on each sub-study described. After gradual MAP increments, a progressive decrease in ΔPhf related to recruited lung regions was visually demonstrated. In the in vivo model we detected a significant comparative decrease of ΔPhf, when measured against the previous value, after reaching a MAP of 11 cmH₂ O up to 17 cmH₂ O, correlating with a significant improvement in oxygenation.

CONCLUSION

The changes in ∆Phf, linked to a progressive increase in MAP during HFOV-VG, might identify optimal lung recruitment and could potentially be used as an additional lung recruitment marker.

Physiologic responses to a staircase lung volume optimization maneuver in pediatric high-frequency oscillatory ventilation

BACKGROUND

Titration of the continuous distending pressure during a staircase incremental-decremental pressure lung volume optimization maneuver in children on high-frequency oscillatory ventilation is traditionally driven by oxygenation and hemodynamic responses, although validity of these metrics has not been confirmed.

METHODS

Respiratory inductance plethysmography values were used construct pressure-volume loops during the lung volume optimization maneuver. The maneuver outcome was evaluated by three independent investigators and labeled positive if there was an increase in respiratory inductance plethysmography values at the end of the incremental phase. Metrics for oxygenation (SpO₂, FiO₂), proximal pressure amplitude, tidal volume and transcutaneous measured pCO₂ (p_tcCO₂) obtained during the incremental phase were compared between outcome maneuvers labeled positive and negative to calculate sensitivity, specificity, and the area under the receiver operating characteristic curve. Ventilation efficacy was assessed during and after the maneuver by measuring arterial pH and PaCO₂. Hemodynamic responses during and after the maneuver were quantified by analyzing heart rate, mean arterial blood pressure and arterial lactate.

RESULTS

41/54 patients (75.9%) had a positive maneuver albeit that changes in respiratory inductance plethysmography values were very heterogeneous. During the incremental phase of the maneuver, metrics for oxygenation and tidal volume showed good sensitivity (> 80%) but poor sensitivity. The sensitivity of the SpO₂/FiO₂ ratio increased to 92.7% one hour after the maneuver. The proximal pressure amplitude showed poor sensitivity during the maneuver, whereas tidal volume showed good sensitivity but poor specificity. PaCO₂ decreased and pH increased in patients with a positive and negative maneuver outcome. No new barotrauma or hemodynamic instability (increase in age-adjusted heart rate, decrease in age-adjusted mean arterial blood pressure or lactate > 2.0 mmol/L) occurred as a result of the maneuver.

CONCLUSIONS

Absence of improvements in oxygenation during a lung volume optimization maneuver did not indicate that there were no increases in lung volume quantified using respiratory inductance plethysmography. Increases in SpO₂/FiO₂ one hour after the maneuver may suggest ongoing lung volume recruitment. Ventilation was not impaired and there was no new barotrauma or hemodynamic instability. The heterogeneous responses in lung volume changes underscore the need for monitoring tools during high-frequency oscillatory ventilation.

Transcutaneous blood gas monitoring among neonatal intensive care units in Japan

BACKGROUND

This study aimed to investigate the utility of transcutaneous (tc) measurements of partial pressure of oxygen (tcPO₂ ) and carbon dioxide (tcPCO₂ ) monitoring in neonatal intensive care units (NICUs) in Japan.

METHODS

At the end of 2016,we sent a survey questionnaire on tc monitoring to all 106 NICUs registered with the Japanese Neonatologist Association. The questions included usage, subjects, methods, management, and the practical usefulness of tc monitoring.

RESULTS

The questionnaire was returned by 69 NICUs (65.1% of response rate). Seventeen institutions (24.6%) measured both tcPCO₂ and tcPO₂ , and 42 (60.9%) measured tcPCO₂ alone. Transcutaneous PCO₂ or tcPO₂ monitoring was applied for "pre-viable" infants born at 22-23 weeks' gestational age (18.6% vs 23.5%), and infants of <500 g birthweight (30.5% vs 17.6%). The tcPCO₂ and tcPO₂ monitoring was started at birth in 49.2% and 70.6% of the newborn infants, respectively. The temperature of the sensor was set at <38°C for tcPCO₂ in 54.3% and >42°C for tcPO₂ in 58.9% of NICUs. The accuracy for tcPO₂ was rated as good in 35.3% or moderate in 64.7%, of institutions but or for tcPCO₂ as 1.7% or 93.2%of institutions , respectively.

CONCLUSION

Transcutaneous monitoring was widely, but limitedly, used for preterm infants. The lower temperature of the tcPCO₂ sensor compared to that reported in other developed countries might compromise the accuracy but increase the feasibility of tc monitoring in Japan.

Volume Guarantee High-Frequency Oscillatory Ventilation in Preterm Infants With RDS: Tidal Volume and DCO2 Levels for Optimal Ventilation Using Open-Lung Strategies

High frequency oscillatory ventilation with volume-guarantee (HFOV-VG) is a promising lung protective ventilator mode for the treatment of respiratory failure in newborns. However, indicators of optimal ventilation during HFOV-VG mode are not identified yet. In this study, we aimed to evaluate optimal high-frequency tidal volume (VThf) and the dissociation coefficient of CO₂ (DCO₂) levels to achieve normocapnia during HFOV-VG after lung recruitment in very low birthweight infants with respiratory distress syndrome (RDS). Preterm babies under the 32nd postmenstrual week with severe RDS that received HFOV-VG using open-lung strategy between January 2014 and January 2019 were retrospectively evaluated. All included patients were treated with the Dräger Babylog VN500 ventilator in the HFOV-VG mode. In total, 53 infants with a mean gestational age of 26.8 ± 2.3 weeks were evaluated. HFOV mean optimal airway pressure (MAPhf) level after lung recruitment was found to be 10.2 ± 1.7 mbar. Overall, the mean applied VThf per kg was 1.64 ± 0.25 mL/kg in the study sample. To provide normocapnia, the mean VThf was 1.61 ± 0.25 mL/kg and the mean DCO₂corr was 29.84 ± 7.88 [mL/kg]²/s. No significant correlation was found between pCO₂ levels with VThf (per kg) or DCO₂corr levels. VThf levels to maintain normocarbia were significantly lower with 12 Hz frequency compared to 10 Hz frequency (1.50 ± 0.24 vs. 1.65 ± 0.25 mL/ kg, p < 0.001, respectively). A weak but significant positive correlation was found between mean airway pressure (MAPhf) and VThf levels. To our knowledge, this is the largest study to evaluate the optimal HFOV-VG settings in premature infants with RDS, using the open-lung strategy. According to the results, a specific set of numbers could not be recommended to achieve normocarbia. Following the trend of each patient and small adjustments according to the closely monitored pCO₂ levels seems logical.

Aeration strategy at birth influences the physiological response to surfactant in preterm lambs

BACKGROUND

The influence of pressure strategies to promote lung aeration at birth on the subsequent physiological response to exogenous surfactant therapy has not been investigated.

OBJECTIVES

To compare the effect of sustained inflation (SI) and a dynamic positive end-expiratory pressure (PEEP) manoeuvre at birth on the subsequent physiological response to exogenous surfactant therapy in preterm lambs.

METHODS

Steroid-exposed preterm lambs (124-127 days' gestation; n=71) were randomly assigned from birth to either (1) positive-pressure ventilation (PPV) with no recruitment manoeuvre; (2) SI until stable aeration; or (3) 3 min dynamic stepwise PEEP strategy (maximum 14-20 cmH₂O; dynamic PEEP (DynPEEP)), followed by PPV for 60 min using a standardised protocol. Surfactant (200 mg/kg poractant alfa) was administered at 10 min. Dynamic compliance, gas exchange and regional ventilation and aeration characteristics (electrical impedance tomography) were measured throughout and compared between groups, and with a historical group (n=38) managed using the same strategies without surfactant.

RESULTS

Compliance increased after surfactant only in the DynPEEP group (p<0.0001, repeated measures analysis of variance), being 0.17 (0.10, 0.23) mL/kg/cmH₂O higher at 60 min than the SI group. An SI resulted in the least uniform aeration, and unlike the no-recruitment and DynPEEP groups, the distribution of aeration and tidal ventilation did not improve with surfactant. All groups had similar improvements in oxygenation post-surfactant compared with the corresponding groups not treated with surfactant.

CONCLUSIONS

A DynPEEP strategy at birth may improve the response to early surfactant therapy, whereas rapid lung inflation with SI creates non-uniform aeration that appears to inhibit surfactant efficacy.

Continuous Noninvasive Carbon Dioxide Monitoring in Neonates: From Theory to Standard of Care

Ventilatory support may affect the short- and long-term neurologic and respiratory morbidities of preterm infants. Ongoing monitoring of oxygenation and ventilation and control of adequate levels of oxygen, pressures, and volumes can decrease the incidence of such adverse outcomes. Use of pulse oximetry became a standard of care for titrating oxygen delivery, but continuous noninvasive monitoring of carbon dioxide (CO₂) is not routinely used in NICUs. Continuous monitoring of CO₂ level may be crucial because hypocarbia and hypercarbia in extremely preterm infants are associated with lung and brain morbidities, specifically bronchopulmonary dysplasia, intraventricular hemorrhage, and cystic periventricular leukomalacia. It is shown that continuous monitoring of CO₂ levels helps in maintaining stable CO₂ values within an accepted target range. Continuous monitoring of CO₂ levels can be used in the delivery room, during transport, and in infants receiving invasive or noninvasive respiratory support in the NICU. It is logical to hypothesize that this will result in better outcome for extremely preterm infants. In this article, we review the different noninvasive CO₂ monitoring alternatives and devices, their advantages and disadvantages, and the available clinical data supporting or negating their use as a standard of care in NICUs.

Weight-correction of carbon dioxide diffusion coefficient (DCO2 ) reduces its inter-individual variability and improves its correlation with blood carbon dioxide levels in neonates receiving high-frequency oscillatory ventilation

BACKGROUND

Carbon-dioxide elimination during high-frequency oscillatory ventilation (HFOV) is thought to be proportional to the carbon dioxide diffusion coefficient (DCO₂ ) which is calculated as frequency x (tidal volume)² . DCO₂ can be used to as an indicator of CO₂ elimination but values obtained in different patients cannot be directly compared.

OBJECTIVES

To analyze the relationship between DCO₂ , the weight-corrected DCO₂ (DCO₂ corr) and blood gas PCO₂ values obtained from infants receiving HFOV.

METHODS

DCO₂ data were obtained from 14 infants at 1/s sampling rate and the mean DCO₂ was determined over 10 min periods preceding the time of the blood gas. DCO₂ corr was calculated by dividing the DCO₂ by the square of the body weight in kg.

RESULTS

Weight-correction significantly reduced the inter-individual variability of DCO₂ . When data from all the babies were combined, standard DCO₂ showed no correlation with PCO₂ but DCO₂ corr showed a weak but statistically significant inverse correlation. The correlation was better when the endotracheal leak was <10%. There was significant inverse but weaker correlation between the HFOV tidal volume (VThf) and the PCO₂ . In any baby, DCO₂ corr >50 mL² /sec/kg² or VThf > 2.5 mL/kg was rarely needed to avoid hypercapnia.

CONCLUSIONS

Weight-correction of DCO₂ values improved its comparability between patients. Weight-corrected DCO₂ correlated better with PCO₂ than uncorrected DCO₂ but the correlation was weak.

Time to lung aeration during a sustained inflation at birth is influenced by gestation in lambs

BackgroundCurrent sustained lung inflation (SI) approaches use uniform pressures and durations. We hypothesized that gestational-age-related mechanical and developmental differences would affect the time required to achieve optimal lung aeration, and resultant lung volumes, during SI delivery at birth in lambs.Methods49 lambs, in five cohorts between 118 and 139 days of gestation (term 142 d), received a standardized 40 cmH₂O SI, which was delivered until 10 s after lung volume stability (optimal aeration) was visualized on real-time electrical impedance tomography (EIT), or to a maximum duration of 180 s. Time to stable lung aeration (T_stable) within the whole lung, gravity-dependent, and non-gravity-dependent regions, was determined from EIT recordings.ResultsT_stable was inversely related to gestation (P<0.0001, Kruskal-Wallis test), with the median (range) being 229 (85,306) s and 72 (50,162) s in the 118-d and 139-d cohorts, respectively. Lung volume at T_stable increased with gestation from a mean (SD) of 20 (17) ml/kg at 118 d to 56 (13) ml/kg at 139 d (P=0.002, one-way ANOVA). There were no gravity-dependent regional differences in T_stable or aeration.ConclusionsThe trajectory of aeration during an SI at birth is influenced by gestational age in lambs. An understanding of this may assist in developing SI protocols that optimize lung aeration for all infants.

Tidal Volume Delivery during the Anesthetic Management of Neonates Is Variable

OBJECTIVES

To describe expiratory tidal volume (V_T) during routine anesthetic management of neonates at a single tertiary neonatal surgical center, as well as the proportion of V_T values within the range of 4.0-8.0 mL/kg.

STUDY DESIGN

A total of 26 neonates needing surgery under general anesthesia were studied, of whom 18 were intubated postoperatively. V_T was measured continuously during normal clinical care using a dedicated neonatal respiratory function monitor (RFM), with clinicians blinded to values. V_T, pressure, and cardiorespiratory variables were recorded regularly while intubated intraoperatively, during postoperative transport, and for 15 minutes after returning to the neonatal intensive care unit (NICU). In addition, paired V_T values from the anesthetic machine were documented intraoperatively.

RESULTS

A total of 2597 V_T measures were recorded from 26 neonates. Intraoperative and postoperative transport expiratory V_T values were highly variable compared with the NICU V_T (P < .0001, Kruskal-Wallis test), with 51% of inflations outside the 4.0-8.0 mL/kg range (35% and 38% of V_T >8.0 mL/kg, respectively), compared with 29% in the NICU (P < .001, χ² test). The use of a flow-inflating bag resulted in a median (range) V_T of 8.5 mL/kg (range, 5.3-11.4 mL/kg) vs 5.6 ml/kg (range, 4.3-7.9 mL/kg) using a Neopuff T-piece system (P < .0001, Mann-Whitney U test). The mean anesthetic machine expiratory V_T was 3.2 mL/kg (95% CI, -4.5 to 10.8 mL/kg) above RFM.

CONCLUSIONS

V_T is highly variable during the anesthetic care of neonates, and potentially injurious V_T is frequently delivered; thus, we suggest close V_T monitoring using a dedicated neonatal RFM.

Relationship between Mean Airways Pressure, Lung Mechanics, and Right Ventricular Output during High-Frequency Oscillatory Ventilation in Infants

OBJECTIVE

To characterize changes in lung mechanics and right ventricular output (RVO) during incremental/decremental continuous distending pressure (CDP) maneuvers in newborn infants receiving high-frequency oscillatory ventilation, with the aim of evaluating when open lung maneuvers are needed and whether they are beneficial.

STUDY DESIGN

Thirteen infants on high-frequency oscillatory ventilation were studied with a median (IQR) gestational age of 26¹ (25³-29¹) weeks and median (IQR) body weight of 810 (600-1020) g. CDP was increased stepwise from 8 cmH₂O to a maximum pressure and subsequently decreased until oxygenation deteriorated or a CDP of 8 cmH₂O was reached. The lowest CDP that maintained good oxygenation was considered the clinically optimal CDP. At each CDP, the following variables were evaluated: oxygenation, respiratory system reactance (Xrs), and RVO by Doppler echocardiography.

RESULTS

At maximal CDP reached during the trial, 19 [1] cmH₂O (mean [SEM]), oxygenation markedly improved, and Xrs and RVO decreased. During deflation, oxygenation remained stable over a wide range of CDP settings, Xrs returned to the baseline values, and RVO increased but the baseline values were not readily restored in all patients.

CONCLUSION

These results suggest that Xrs and RVO are more sensitive than oxygenation to overdistension and they may be useful in clinical practice to guide open lung maneuvers.

Effectiveness of individualized lung recruitment strategies at birth: an experimental study in preterm lambs

Respiratory transition at birth involves rapidly clearing fetal lung liquid and preventing efflux back into the lung while aeration is established. We have developed a sustained inflation (SI_OPT) individualized to volume response and a dynamic tidal positive end-expiratory pressure (PEEP) (open lung volume, OLV) strategy that both enhance this process. We aimed to compare the effect of each with a group managed with PEEP of 8 cmH₂O and no recruitment maneuver (No-RM), on gas exchange, lung mechanics, spatiotemporal aeration, and lung injury in 127 ± 1 day preterm lambs. Forty-eight fetal-instrumented lambs exposed to antenatal steroids were ventilated for 60 min after application of the allocated strategy. Spatiotemporal aeration and lung mechanics were measured with electrical impedance tomography and forced-oscillation, respectively. At study completion, molecular and histological markers of lung injury were analyzed. Mean (SD) aeration at the end of the SI_OPT and OLV groups was 32 (22) and 38 (15) ml/kg, compared with 17 (10) ml/kg (180 s) in the No-RM (P = 0.024, 1-way ANOVA). This translated into better oxygenation at 60 min (P = 0.047; 2-way ANOVA) resulting from better distal lung tissue aeration in SI_OPT and OLV. There was no difference in lung injury. Neither SI_OPT nor OLV achieved homogeneous aeration. Histological injury and mRNA biomarker upregulation were more likely in the regions with better initial aeration, suggesting volutrauma. Tidal ventilation or an SI achieves similar aeration if optimized, suggesting that preventing fluid efflux after lung liquid clearance is at least as important as fluid clearance during the initial inflation at birth.

Special Considerations in Neonatal Mechanical Ventilation

Care of infants supported with mechanical ventilation is complex, time intensive, and requires constant vigilance by an expertly prepared health care team. Current evidence must guide nursing practice regarding ventilated neonates. This article highlights the importance of common language to establish a shared mental model and enhance clear communication among the interprofessional team. Knowledge regarding the underpinnings of an open lung strategy and the interplay between the pathophysiology and individual infant's response to a specific ventilator strategy is most likely to result in a positive clinical outcome.

The interrelationship of recruitment maneuver at birth, antenatal steroids, and exogenous surfactant on compliance and oxygenation in preterm lambs

BACKGROUND

To describe the interrelationship between antenatal steroids, exogenous surfactant, and two approaches to lung recruitment at birth on oxygenation and respiratory system compliance (Cdyn) in preterm lambs.

METHODS

Lambs (n = 63; gestational age 127 ± 1 d) received either surfactant at 10-min life (Surfactant), antenatal corticosteroids (Steroid), or neither (Control). Within each epoch lambs were randomly assigned to a 30-s 40 cmH2O sustained inflation (SI) or an initial stepwise positive end-expiratory pressure (PEEP) open lung ventilation (OLV) maneuver at birth. All lambs then received the same management for 60-min with alveolar-arterial oxygen difference (AaDO2) and Cdyn measured at regular time points.

RESULTS

Overall, the OLV strategy improved Cdyn and AaDO2 (all epochs except Surfactant) compared to SI (all P < 0.05; two-way ANOVA). Irrespective of strategy, Cdyn was better in the Steroid group in the first 10 min (all P < 0.05). Thereafter, Cdyn was similar to Steroid epoch in the OLV + Surfactant, but not SI + Surfactant group. OLV influenced the effect of steroid and surfactant (P = 0.005) on AaDO2 more than SI (P = 0.235).

CONCLUSIONS

The antenatal state of the lung influences the type and impact of a recruitment maneuver at birth. The effectiveness of surfactant maybe enhanced using PEEP-based time-dependent recruitment strategies rather than approaches solely aimed at initial lung liquid clearance.

Diagnostic accuracy of capnography during high-frequency ventilation in neonatal intensive care units

BACKGROUND AND OBJECTIVE

High-frequency ventilation (HFV) is a powerful tool for CO2 elimination, and thus requires careful monitoring of CO2 . Our aim was to assess the diagnostic accuracy (correlation, agreement, and trending) of continuous distal capnography (dCap) with PaCO2 in infants ventilated with HFV.

DESIGN

This was a prospective, observational, multicenter study. dCap was compared with simultaneous PaCO2 ("gold standard") drawn from indwelling arterial line for patient care in term and preterm infants ventilated with HFV. dCap was obtained via the side-port of a double-lumen endotracheal-tube by a Microstream capnograph with specially designed software for HFV.

RESULTS

Twenty-four infants participated in the study (median [range] gestational age [GA]: 26.8 [23.6-38.6] weeks). Analysis included 332 measurements. dCap was in correlation (r = 0.70, P < 0.001) but with less than adequate agreement (mean difference ± SD of the differences: -11.7 ± 10.3 mmHg) with PaCO2 . Comparable findings were found in the subgroup of infants <1,000 g (n = 240 measurements). Correlations were maintained in severe lung disease. Changes in dCap and in PaCO2 for consecutive measurements within each patient were correlated (r = 0.63, P < 0.001). Area under the receiver operating curves (ROC) for dCap to detect high (>60 mmHg) or low (<30 mmHg) PaCO2 was 0.83 (CI: 0.76-0.90) and 0.88 (CI: 0.79-0.97), respectively; P < 0.001.

CONCLUSIONS

Our prospective study suggests that continuous dCap in infants ventilated with HFV may be helpful for trends and alarm for unsafe levels of PaCO2 . dCap is only a complimentary tool and cannot replace PaCO2 sampling because the agreement between these measurements was less than adequate.

Using very high frequencies with very low lung volumes during high-frequency oscillatory ventilation to protect the immature lung. A pilot study

OBJECTIVE

High-frequency oscillatory ventilation (HFOV) has been described as a rescue therapy in severe respiratory distress syndrome (RDS) with a potential protective effect in immature lungs. In recent times, HFOV combined with the use of volume guarantee (VG) strategy has demonstrated an independent effect of the frequency on tidal volume to increase carbon-dioxide (CO2) elimination. The aim of this study was to demonstrate the feasibility of using the lowest tidal volume on HFOV+VG to prevent lung damage, maintaining a constant CO2 elimination by increasing the frequency.

STUDY DESIGN

Newborn infants with RDS on HFOV were prospectively included. After adequate and stable ventilation using a standard HFOV strategy, the tidal volume was fixed using VG and decreased while the frequency was increased to the highest possible to maintain a constant CO2 elimination. Pre- and post-PCO2, delta pressure and tidal volume obtained in each situation were compared.

RESULT

Twenty-three newborn infants were included. It was possible to increase the frequency while decreasing the tidal volume in all patients, maintaining a similar CO2 elimination, with a tendency to a lower mean PCO2 after reaching the highest frequency. High-frequency tidal volume was significantly lower, 2.20 ml kg(-1) before vs 1.59 ml kg(-1) at the highest frequency.

CONCLUSION

It is possible to use lower delivered tidal volumes during HFOV combined with VG and higher frequencies with adequate ventilation to allow minimizing lung injury.

Transcutaneous carbon dioxide monitoring for the prevention of neonatal morbidity and mortality

BACKGROUND

Carbon dioxide (CO2) measurement is a fundamental evaluation in a neonatal intensive care unit (NICU), as both low and high values of CO2 might have detrimental effects on neonatal morbidity and mortality. Though measurement of CO2 in the arterial blood gas is the most accurate way to assess the amount of CO2, it requires blood sampling and it does not provide a continuous monitoring of CO2.

OBJECTIVES

To assess whether the use of continuous transcutaneous CO2 (tcCO2) monitoring in newborn infants reduces mortality and improves short and long term respiratory and neurodevelopmental outcomes.

SEARCH METHODS

We used the standard search strategy of the Cochrane Neonatal Review group to search the Cochrane Central Register of Controlled Trials (CENTRAL 2015, Issue 11), MEDLINE via PubMed (1966 to November 1, 2015), EMBASE (1980 to November 1, 2015), and CINAHL (1982 to November 1, 2015). We also searched clinical trials databases, conference proceedings, and the reference lists of retrieved articles for randomized controlled trials and quasi-randomized trials.

SELECTION CRITERIA

Randomized, quasi-randomized and cluster randomized controlled trials comparing different strategies regarding tcCO2 monitoring in newborns. Three comparisons were considered, that is, continuous tcCO2 monitoring versus 1) any intermittent modalities to measure CO2; 2) other continuous CO2 monitoring; and 3) with or without intermittent CO2 monitoring.

DATA COLLECTION AND ANALYSIS

We used the standard methods of the Cochrane Neonatal Review Group. Two review authors independently assessed studies identified by the search strategy for inclusion.

MAIN RESULTS

Our search strategy yielded 106 references. Two review authors independently assessed all references for inclusion. We did not find any completed studies for inclusion, nor ongoing trials.

AUTHORS' CONCLUSIONS

There was no evidence to recommend or refute the use of transcutaneous CO2 monitoring in neonates. Well-designed, adequately powered randomized controlled studies are necessary to address efficacy and safety of transcutaneous CO2 monitoring in neonates.

An authentic animal model of the very preterm infant on nasal continuous positive airway pressure

BACKGROUND

The surge in uptake of nasal continuous positive airway pressure (CPAP) for respiratory support in preterm infants has occurred in the absence of an authentic animal model. Such a model would allow investigation of research questions of physiological and therapeutic importance. We therefore aimed to develop a preterm lamb model of the non-intubated very preterm infant on CPAP.

METHODS

After staged exteriorisation and instrumentation, preterm lambs were delivered from anaesthetised ewes at 131 to 133 days gestation. Via a single nasal prong (4-mm internal diameter, 6- to 7-cm depth), positive pressure was delivered from the outset, with nasal intermittent positive pressure ventilation (NIPPV) used until transition to nasal CPAP was attempted, and periodically thereafter for hypoventilation. Caffeine and doxapram were used as respiratory stimulants. Gastric distension was prevented with an oesophageal balloon. Cardiorespiratory parameters and results of arterial blood gas analyses were monitored throughout the study period, which continued for 150 min after first transition to CPAP.

RESULTS

Ten preterm lambs were studied, at gestation 132 ± 1 days (mean ± SD) and birth weight 3.6 ± 0.45 kg. After stabilisation on NIPPV, transition to nasal CPAP was first attempted at 28 ± 11 min. There was transient respiratory acidosis, with gradual resolution as spontaneous respiratory activity increased. In the final hour, 79% ± 33% of time was spent on CPAP alone, with typical respiratory rates around 60 breaths per minute. PaCO2 at end-experiment was 58 ± 36 mmHg.

CONCLUSIONS

Non-intubated preterm lambs can be effectively transitioned to nasal CPAP soon after birth. This animal model will be valuable for further research.

Are All Oscillators Created Equal? In vitro Performance Characteristics of Eight High-Frequency Oscillatory Ventilators

BACKGROUND

The mode of waveform generation and circuit characteristics differ between high-frequency oscillators. It is unknown if this influences performance.

OBJECTIVES

To describe the relationships between set and delivered pressure amplitude (x0394;P), and the interaction with frequency and endotracheal tube (ETT) diameter, in eight high-frequency oscillators.

METHODS

Oscillators were evaluated using a 70-ml test lung at 1.0 and 2.0 ml/cm H2O compliance, with mean airway pressures (PAW) of 10 and 20 cm H2O, frequencies of 5, 10 and 15 Hz, and an ETT diameter of 2.5 and 3.5 mm. At each permutation of PAW, frequency and ETT, the set x0394;P was sequentially increased from 15 to 50 cm H2O, or from 20 to 100% maximum amplitude (10% increments) depending on the oscillator design. The x0394;P at the ventilator (x0394;PVENT), airway opening (x0394;PAO) and within the test lung (x0394;PTRACH), and tidal volume (V(T)) at the airway opening were determined at each set x0394;P.

RESULTS

In two oscillators the relationships between set and delivered x0394;P were non-linear, with a plateau in x0394;P thresholds noted at all frequencies (Dräger Babylog 8000) or ≥10 Hz (Dräger VN500). In all other devices there was a linear relationship between x0394;PVENT, x0394;PAO and x0394;PTRACH (all r2 >0.93), with differing attenuation of the pressure wave. Delivered V(T) at the different settings tested varied between devices, with some unable to deliver V(T) >3 ml at 15 Hz, and others generating V(T)>20 ml at 5 Hz and a 1:1 inspiratory-to-expiratory time ratio.

CONCLUSIONS

Clinicians should be aware that modern high-frequency oscillators exhibit important differences in the delivered x0394;P and V(T).

Monitoring Lung Volumes During Mechanical Ventilation

Respiratory inductive plethysmography (RIP) is a non-invasive method of measuring change in lung volume which is well-established as a monitor of tidal ventilation and thus respiratory patterns in sleep medicine. As RIP is leak independent, can measure end-expiratory lung volume as well as tidal volume and is applicable to both the ventilated and spontaneously breathing patient, there has been a recent interest in its use as a bedside tool in the intensive care unit.

Options for assessing and measuring chest wall motion

Assessing chest wall motion is a basic and vital component in managing the child with respiratory problems, whether these are due to pathology in the lungs, airways, chest wall or muscles. Since the 1960s, clinical assessment has been supplemented with an ever-growing range of technological options for measuring chest wall motion, each with unique advantages and disadvantages. Measurements of chest wall motion can be used to: (1) Assess respiratory airflow and volume change, as a non-invasive alternative to measurement at the airway opening, (2) Monitor breathing over long periods of time, to identify apnoea and other types of sleep-disordered breathing, (3)Identify and quantify patterns of abnormal chest wall movement, whether between ribcage and abdominal components (thoracoabdominal asynchrony) or between different regions of the ribcage (eg in scoliosis and pectus excavatum). Measuring chest wall motion allows us to do things which simply cannot be done by more mainstream respiratory function techniques measuring flow at the airway opening: it allows respiratory airflow to be measured when it would otherwise be impossible, and it tells us how the different parts of the chest wall (eg ribcage vs abdomen, right vs left) are moving in order to generate that airflow. The basis of the different techniques available to assess and measure chest wall motion will be reviewed and compared, and their relevance to paediatric respiratory practice assessed.

Pressure and flow waveform characteristics of eight high-frequency oscillators

OBJECTIVES

The differences in performance of early generation high-frequency oscillators have been attributed to their distinct pressure and flow waveforms. Recently, five new oscillators have been commercially released. The objective of this study was to characterize the pressure and flow waveforms of eight commercially available oscillators.

DESIGN

In vitro benchtop study.

SETTING

Tertiary pediatric teaching hospital.

INTERVENTIONS

Eight oscillators were evaluated using a test lung; mean airway pressure 10 and 20 cm H2O; frequencies 5, 10, and 15 Hz; pressure amplitude 30 cm H2O (or equivalent); compliance 1.0 mL/cm H2O; and endotracheal tube 3.5 mm. Ventilators tested were Sensormedics 3100A and B (Carefusion), SLE5000 (SLE), Fabian (Acutronic), Leonie+ (Heinen+Löwenstein), Sophie (Stephan), and VN500 and Babylog 8000 (Dräger).

MEASUREMENTS AND MAIN RESULTS

Pressure (airway opening, at oscillator and within the test lung) and airway opening flow waveforms were recorded. Airway opening waveforms were characterized by type (square or sine) and by determining power spectral density analysis. The Sensormedics A and B and the SLE5000 delivered square waves; all other oscillators generated sine waves. Sensormedics, the SLE5000, and the Sophie had a characteristic inspiratory slope (incisura). The pressure waveform within the test lung was a sine wave for all oscillators. Oscillators with square waves or an inspiratory incisura exhibited the highest number of nonfundamental frequency components on power spectral density analysis, suggesting more complex harmonic waveforms with potentially greater transmissive power to the lungs. At frequencies of 5 and 10 Hz, all ventilators, except Babylog 8000, generated airway pressure amplitudes greater than 28.6 cm H2O and tidal volumes greater than 6 mL at the airway opening.

CONCLUSIONS

Current high-frequency oscillators deliver different waveforms. As these may result in variable clinical performance, operators should be aware that these differences exist.

Optimal mean airway pressure during high-frequency oscillatory ventilation determined by measurement of respiratory system reactance

BACKGROUND

The aims of the present study were (i) to characterize the relationship between mean airway pressure (PAW) and reactance measured at 5 Hz (reactance of the respiratory system (X RS), forced oscillation technique) and (ii) to compare optimal PAW (P opt) defined by X RS, oxygenation, lung volume (VL), and tidal volume (VT) in preterm lambs receiving high-frequency oscillatory ventilation (HFOV).

METHODS

Nine 132-d gestation lambs were commenced on HFOV at PAW of 14 cmH2O (P start). PAW was increased stepwise to a maximum pressure (P max) and subsequently sequentially decreased to the closing pressure (Pcl, oxygenation deteriorated) or a minimum of 6 cmH2O, using an oxygenation-based recruitment maneuver. X RS, regional V L (electrical impedance tomography), and V T were measured immediately after (t 0 min) and 2 min after (t 2 min) each PAW decrement. P opt defined by oxygenation, X RS, V L, and V T were determined.

RESULTS

The PAW-X RS and PAW-VT relationships were dome shaped with a maximum at Pcl+6 cmH2O, the same point as P opt defined by VL. Below Pcl+6 cmH2O, X RS became unstable between t 0 min and t 2 min and was associated with derecruitment in the dependent lung. P opt, as defined by oxygenation, was lower than the P opt defined by X RS, V L, or V T.

CONCLUSION

X RS has the potential as a bedside tool for optimizing PAW during HFOV.

Effect of sustained inflation vs. stepwise PEEP strategy at birth on gas exchange and lung mechanics in preterm lambs

BACKGROUND

Sustained inflation (SI) at birth facilitates establishment of functional residual capacity (FRC) in the preterm lung, but the ideal lung recruitment strategy is unclear. We have compared the effect of SI and a stepwise positive end-expiratory pressure (PEEP; SEP) strategy in a preterm model.

METHODS

127 d gestation lambs received either 20-s SI (n = 9) or 2 cmH2O stepwise PEEP increases to 20 cmH2O every 10 inflations, and then decreases to 6 cmH2O (n = 10). Ventilation continued for 70 min, with surfactant administered at 10 min. Alveolar-arterial oxygen gradient (AaDO2), compliance (C(dyn)), end-expiratory thoracic volume (EEVRIP; respiratory inductive plethysmography), and EEV and C(dyn) in the gravity-dependent and nondependent hemithoraces (electrical impedance tomography) were measured throughout. Early mRNA markers of lung injury were analyzed using quantitative real-time PCR.

RESULTS

From 15 min of life, AaDO2 was lower in SEP group (P < 0.005; two-way ANOVA). SEP resulted in higher and more homogeneous C(dyn) (P < 0.0001). Mean (SD) EEVRIP at 5 min was 18 (9) ml/kg and 6 (5) ml/kg following SEP and SI, respectively (P = 0.021; Bonferroni posttest); this difference was due to a greater nondependent hemithorax EEV. There was no difference in markers of lung injury.

CONCLUSION

An SEP at birth improved gas exchange, lung mechanics, and EEV, without increasing lung injury, compared to the SI strategy used.

Surfactant before the first inflation at birth improves spatial distribution of ventilation and reduces lung injury in preterm lambs

The interrelationship between the role of surfactant and a sustained inflation (SI) to aid ex utero transition of the preterm lung is unknown. We compared the effect of surfactant administered before and after an initial SI on gas exchange, lung mechanics, spatial distribution of ventilation, and lung injury in preterm lambs. Gestational-age lambs (127 days; 9 per group) received 100 mg/kg of a surfactant (Curosurf) either prior (Surf+SI) or 10 min after birth (SI+Surf). At birth, a 20-s, 35 cmH2O SI was applied, followed by 70 min of positive pressure ventilation. Oxygenation, carbon dioxide removal, respiratory system compliance, end-expiratory thoracic volume (via respiratory inductive plethysmography), and distribution of end-expiratory volume and ventilation (via electrical impedance tomography) were measured throughout. Early markers of lung injury were analyzed using quantitative RT-PCR. During the first 15 min, oxygenation, carbon dioxide removal, and compliance were better in the Surf+SI group (all P < 0.05). End-expiratory volume on completion of the sustained inflation was higher in the Surf+SI group than the SI+Surf group; 11 ± 1 ml/kg vs. 7 ± 1 ml/kg (mean ± SE) (P = 0.043; t-test), but was not different at later time points. Although neither achieved homogenous aeration, spatial ventilation was more uniform in the Surf+SI group throughout; 50.1 ± 10.9% of total ventilation in the left hemithorax at 70 min vs. 42.6 ± 11.1% in the SI+Surf group. Surf+SI resulted in lower mRNA levels of CYR61 and EGR1 compared with SI+Surf (P < 0.001, one-way ANOVA). Surfactant status of the fetal preterm lung at birth influences the mechanical and injury response to a sustained inflation and ventilation by changing surface tension of the air/fluid interface.