Identification of optimal lung volume during high-frequency oscillatory ventilation using respiratory inductive plethysmography

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.

Hemodynamic Effects of a High-Frequency Oscillatory Ventilation Open-Lung Strategy in Critically Ill Children With Acquired or Congenital Cardiac Disease

OBJECTIVES

To study the hemodynamic consequences of an open-lung high-frequency oscillatory ventilation (HFOV) strategy in patients with an underlying cardiac anomaly with or without intracardiac shunt or primary pulmonary hypertension with severe lung injury.

DESIGN

Secondary analysis of prospectively collected data.

SETTING

Medical-surgical PICU.

PATIENTS

Children less than 18 years old with cardiac anomalies (± intracardiac shunt) or primary pulmonary hypertension.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Data from 52 subjects were analyzed, of whom 39 of 52 with cardiac anomaly (23/39 with intracardiac shunt) and 13 of 52 with primary pulmonary hypertension. Fourteen patients were admitted postoperatively, and 26 patients were admitted with acute respiratory failure. Five subjects (9.6%) were canulated for ECMO (of whom four for worsening respiratory status). Ten patients (19.2%) died during PICU stay. Median conventional mechanical ventilation settings prior to HFOV were peak inspiratory pressure 30 cm H 2 O (27-33 cm H 2 O), positive end-expiratory pressure 8 cm H 2 O (6-10 cm H 2 O), and F io2 0.72 (0.56-0.94). After transitioning to HFOV, there was no negative effect on mean arterial blood pressure, central venous pressure, or arterial lactate. Heart rate decreased significantly over time ( p < 0.0001), without group differences. The percentage of subjects receiving a fluid bolus decreased over time ( p = 0.003), especially in those with primary pulmonary hypertension ( p = 0.0155) and without intracardiac shunt ( p = 0.0328). There were no significant differences in the cumulative number of daily boluses over time. Vasoactive Infusion Score did not increase over time. Pa co2 decreased ( p < 0.0002) and arterial pH significantly improved ( p < 0.0001) over time in the whole cohort. Neuromuscular blocking agents were used in all subjects switched to HFOV. Daily cumulative sedative doses were unchanged, and no clinically apparent barotrauma was found.

CONCLUSIONS

No negative hemodynamic consequences occurred with an individualized, physiology-based open-lung HFOV approach in patients with cardiac anomalies or primary pulmonary hypertension suffering from severe lung injury.

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.

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.

High-resolution respiratory inductive plethysmography in rats: validation in anesthetized conditions

Respiratory monitoring is often required in experimental physiological and pharmacological studies in rodents. Currently, the mostly used techniques are direct measurement of airflow on intubated animals and whole body plethysmography.

OBJECTIVE

Although the reliability of these methods has been broadly demonstrated, they also have several drawbacks such as invasiveness, high cost of use or confinement of the animals. Respiratory inductive plethysmography (RIP) is a non-invasive technique already used in medium-sized mammals that has not yet been evaluated in small rodents. The implementation of inductive plethysmography in rats represents an instrumental challenge because of the small inductances that are expected.

APPROACH

A rodent-specific RIP apparatus has been developed and compared to direct airflow measurement provided by a pneumotachograph (PNT) considered as the invasive gold standard for respiratory monitoring. The experiments were carried out on anesthetized rats artificially ventilated at different levels of tidal volumes (V _T) covering the whole physiological range.

MAIN RESULTS

Based on the Euclidian distance between signals, this study shows that after calibration, signals from RIP fit at 93% with PNT values. The Bland and Altman plot evidences differences between RIP and PNT lower than 20% and the values obtained are highly correlated (R  =  0.98, p  <  0.001).

SIGNIFICANCE

This study demonstrates that it is possible to design RIP systems suitable for measurement of tidal volumes and airflow in anesthetized rats. Further studies will now be focused on the validation in extended physiological conditions.

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.

Functional lung imaging during HFV in preterm rabbits

Although high frequency ventilation (HFV) is an effective mode of ventilation, there is limited information available in regard to lung dynamics during HFV. To improve the knowledge of lung function during HFV we have developed a novel lung imaging and analysis technique. The technique can determine complex lung motion information in vivo with a temporal resolution capable of observing HFV dynamics. Using high-speed synchrotron based phase contrast X-ray imaging and cross-correlation analysis, this method is capable of recording data in more than 60 independent regions across a preterm rabbit lung in excess of 300 frames per second (fps). This technique is utilised to determine regional intra-breath lung mechanics of preterm rabbit pups during HFV. Whilst ventilated at fixed pressures, each animal was ventilated at frequencies of 1, 3, 5 and 10 Hz. A 50% decrease in delivered tidal volume was measured at 10 Hz compared to 1 Hz, yet at the higher frequency a 500% increase in minute activity was measured. Additionally, HFV induced greater homogeneity of lung expansion activity suggesting this ventilation strategy potentially minimizes tissue damage and improves gas mixing. The development of this technique permits greater insight and further research into lung mechanics and may have implications for the improvement of ventilation strategies used to support severe pulmonary trauma and disease.

A pilot study of respiratory inductance plethysmography as a safe, noninvasive detector of jet ventilation under general anesthesia

BACKGROUND

High-frequency jet ventilation is an optimal mode of ventilation for many surgical procedures of the trachea and larynx but has limited monitoring modalities to assess adequacy of oxygenation and/or ventilation. Respiratory inductance plethysmography is a noninvasive monitor of chest and abdominal wall movement with well-established applications in the sleep laboratory. We performed an observational pilot study of respiratory inductance plethysmography as a detector of jet ventilation.

METHODS

Twenty-five patients underwent microdirect suspension laryngoscopy with high-frequency jet ventilation under general anesthesia with total IV anesthesia. Inductotrace® bands (Ambulatory Monitoring Inc., Ardsley, NY) were applied to the chest and abdomen in all patients and data collected from oxygen administration through emergence at 50-Hz sampling frequency in the DC mode using a 12-bit A-D converter and custom programmed LabVIEW interface. The raw data were filtered and a detector was developed based on a type I, IIR peak comb filter to differentiate apnea, cardiogenic oscillations, and jet ventilation- associated respiratory excursion. The primary end point was the ability of the detector to identify the presence of jet ventilation. Receiver operating characteristic curves were generated for the aggregate data of all patients.

RESULTS

Respiratory inductance plethysmography reliably detected jet ventilation. The data analysis program effectively extracted a relatively small amplitude jet ventilation signal from a baseline signal contaminated by cardiogenic noise. Sensitivity was in the range of 85%, with a filter bandwidth of 0.055 Hz. Increased sensitivity with increasing filter bandwidth was offset by a detection delay of 12.5 seconds.

CONCLUSIONS

Respiratory inductance plethysmography was successfully used to detect high-frequency jet ventilation in patients undergoing laryngotracheal surgery. This pilot study demonstrates the feasibility of respiratory inductance plethysmography as a monitor for use during jet ventilation.

The effect of suction method, catheter size, and suction pressure on lung volume changes during endotracheal suction in piglets

We aimed to identify the effect of suction pressure and catheter size on change in lung volume during open and closed endotracheal suction. Anesthetized piglets (n = 12) were intubated with a 4.0-mm endotracheal tube. Lung injury was induced with saline lavage. Three suction methods (open, closed in-line, and closed with a side-port adaptor) were performed in random order using 6, 7, and 8 French gauge (FG) catheters, at vacuum pressures of 80, 140, and 200 mm Hg. Lung volume change was measured with respiratory inductive plethysmography. Overall, open suction resulted in greater lung volume loss during and at 60-s postsuction than either closed method (p < 0.001). When open and closed methods were analyzed separately, volume change was independent of catheter size and suction pressure with open suction. With closed suction, volume loss increased with larger catheter sizes and higher suction pressures (p < 0.001). With an 8-FG catheter and suction pressure of 140 or 200 mm Hg, volume loss was equivalent with open and closed suction. Lung volume changes are influenced by catheter size and suction pressure, as well as suction method. With commonly used suction pressures and catheter sizes, closed suction has no advantage in preventing loss of volume in this animal model.

Respiratory inductive plethysmography accuracy at varying PEEP levels and degrees of acute lung injury

BACKGROUND AND OBJECTIVE

This study was performed to assess the accuracy of respiratory inductive plethysmographic (RIP) estimated lung volume changes at varying positive end-expiratory pressures (PEEP) during different degrees of acute respiratory failure.

METHODS

Measurements of inspiratory tidal volume were validated in eight piglets during constant volume ventilation at incremental and decremental PEEP levels and with increasing severity of pulmonary injury. RIP accuracy was assessed with calibration from the healthy state, from the disease state as the measurement error was assessed, and at various PEEP levels.

RESULTS

Best results (bias 3%, precision 7%) were obtained in healthy animals. RIP accuracy decreased with progressing degrees of acute respiratory failure and was PEEP dependent, unless RIP was calibrated again. When calibration was performed in the disease state as the measurement error was assessed, bias was reduced but precision did not improve (bias -2%, precision 9%).

CONCLUSIONS

RIP accuracy is within the accuracy range found in monitoring devices currently in clinical use. Most reliable results with RIP are obtained when measurements are preceded by calibration in pulmonary conditions that are comparable to the measurement period. When RIP calibration is not possible, fixed weighting of the RIP signals with species and subject size adequate factors is an alternative. Measurement errors should be taken into account with interpretation of small volume changes.

Differential effect of recruitment maneuvres on pulmonary blood flow and oxygenation during HFOV in preterm lambs

The effects of lung volume recruitment manouvres on pulmonary blood flow (PBF) during high-frequency oscillatory ventilation (HFOV) in preterm neonates are unknown. Since increased airway pressure adversely affects PBF, we compared the effects of two HFOV recruitment strategies on PBF and oxygenation index (OI). Preterm lambs (128 ± 1 day gestation; term ∼150 days) were anesthetized and ventilated using HFOV (10 Hz, 33% tI) with a mean airway pressure (Pao) of 15 cmH2O. Lung volume was recruited by either increasing Pao to 25 cmH2O for 1 min, repeated five times at 5-min intervals (Sigh group; n = 5) or stepwise (5 cmH2O) changes in Pao at 5-min intervals incrementing up to 30 cmH2O then decrementing back to 15 cmH2O (Ramp group; n = 6). Controls ( n = 5) received constant HFOV at 15 cmH2O. PBF progressively decreased (by 45 ± 4%) and OI increased (by 15 ± 6%, indicating reduced oxygenation) in controls during HFOV, which was similar to the changes observed in the Sigh group of lambs. In the Ramp group, PBF fell (by 54 ± 10%) as airway pressure increased ( r2 = 0.99), although the PBF did not increase again as the Pao was subsequently reduced. The OI decreased (by 47 ± 9%), reflecting improved oxygenation at high Pao levels during HFOV in the Ramp group. However, high Pao restored retrograde PBF during diastole in four of six lambs, indicating the restoration of right-to-left shunting through the ductus arteriosus. Thus the choice of volume recruitment maneuvre influences the magnitude of change in OI and PBF that occurs during HFOV. Despite significantly improving OI, the ramp recruitment approach causes sustained changes in PBF.

Assessment of neonatal ventilation during high-frequency oscillatory ventilation

OBJECTIVE

To determine alterations in high-frequency oscillatory ventilation (HFOV) performance during clinical ventilator management.

DESIGN

Clinical investigation.

SETTING

Two level III intensive care nurseries in Wilmington, Delaware, and Philadelphia, Pennsylvania.

PATIENTS

Thirty infants 1.49 +/- 1.01 kg with respiratory distress receiving HFOV.

INTERVENTIONS

Due to the demonstrated benchtop load sensitivity of the HFOV (SensorMedics 3100), we hypothesized that measured tidal volume (Vt/kg) and high-frequency minute ventilation (HFMV) would vary inversely with respiratory rate adjustments and that ventilator performance will be affected with endotracheal tube (ETT) suctioning. Both Vt/kg and HFMV were recorded using a novel hot-wire anemometry technique at the time of ETT suctioning or changes in ventilator settings.

MEASUREMENTS AND MAIN RESULTS

During HFOV it was found that Vt/kg = 2.52 +/- 0.68 mL/kg and HFMV = 69 +/- 45 ([mL/kg]2 x Hz); effective ventilation was observed in the range of HFMV = 29-113 ([mL/kg]2 x Hz). HFMV decreased with an increase in breathing frequency. Although there was a significant increase in the mean Vt/kg after suctioning events, there was no difference in Vt/kg or HFMV after disconnection of the ETT alone. There were significant alterations in HFOV performance as a result of clinical adjustments in respiratory rate and suctioning. In addition, we found that measured Vt during clinically effective HFOV is at least equivalent to expected deadspace.

CONCLUSIONS

Measurement of tidal volume and HFMV may be clinically important in optimizing HFOV performance both during ETT suctioning and adjustments to breathing frequency.

Bench-to-bedside review: high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome

Mechanical ventilation is the cornerstone of therapy for patients with acute respiratory distress syndrome (ARDS). Paradoxically, mechanical ventilation can exacerbate lung damage – a phenomenon known as ventilator-induced lung injury. While new ventilation strategies have reduced the mortality rate in patients with ARDS, this mortality rate still remains high. High-frequency oscillatory ventilation (HFOV) is an unconventional form of ventilation that may improve oxygenation in patients with ARDS, while limiting further lung injury associated with high ventilatory pressures and volumes delivered during conventional ventilation. HFOV has been used for almost two decades in the neonatal population, but there is more limited experience with HFOV in the adult population. In adults, the majority of the published literature is in the form of small observational studies in which HFOV was used as 'rescue' therapy for patients with very severe ARDS who were failing conventional ventilation. Two prospective randomized controlled trials, however, while showing no mortality benefit, have suggested that HFOV, compared with conventional ventilation, is a safe and effective ventilation strategy for adults with ARDS. Several studies suggest that HFOV may improve outcomes if used early in the course of ARDS, or if used in certain populations. This review will summarize the evidence supporting the use of HFOV in adults with ARDS.

Effects of open endotracheal suction on lung volume in infants receiving HFOV

OBJECTIVE

To describe the pattern and magnitude of lung volume change during open endotracheal tube (ETT) suction in infants receiving high-frequency oscillatory ventilation (HFOV).

DESIGN

Prospective observational clinical study.

SETTING

Tertiary neonatal intensive care unit.

PATIENTS AND PARTICIPANTS

Seven intubated and muscle-relaxed newborn infants receiving HFOV.

INTERVENTIONS

Open ETT suction was performed for 6 s at -100 mmHg using a 6-F catheter passed to the ETT tip after disconnection from HFOV. The HFOV was then recommenced at the same settings as prior to ETT suction.

MEASUREMENTS AND RESULTS

Change in lung volume (DeltaV (L)) referenced to baseline lung volume before suction was measured with a calibrated respiratory inductive plethysmography recording from 30 s before until 60 s after ETT suction. In all infants ETT suction resulted in significant loss of lung volume. The mean DeltaV (L) during suctioning was -13 ml/kg (SD 4 ml/kg) (p<0.0001 vs. baseline, repeated-measures ANOVA), with a mean 76.5% (SD 14.1%) of this volume loss being related to circuit disconnection. After recommencing HFOV lung volume was rapidly regained with mean DeltaV (L) at 60 s being 1 ml/kg (SD 4 ml/kg) below baseline (p>0.05, Tukey post-test).

CONCLUSIONS

Open ETT suction caused a significant but transient loss of lung volume in muscle-relaxed newborn infants receiving HFOV.

The Deflation Limb of the Pressure–Volume Relationship in Infants during High-Frequency Ventilation

RATIONALE

The importance of applying high-frequency oscillatory ventilation with a high lung volume strategy in infants is well established. Currently, a lack of reliable methods for assessing lung volume limits clinicians' ability to achieve the optimum volume range.

OBJECTIVES

To map the pressure-volume relationship of the lung during high-frequency oscillatory ventilation in infants, to determine at what point ventilation is being applied clinically, and to describe the relationship between airway pressure, lung volume, and oxygenation.

METHODS

In 12 infants, a partial inflation limb and the deflation limb of the pressure-volume relationship were mapped using a quasi-static lung volume optimization maneuver. This involved stepwise airway pressure increments to total lung capacity, followed by decrements until the closing pressure of the lung was identified.

MEASUREMENTS AND MAIN RESULTS

Lung volume and oxygen saturation were recorded at each airway pressure. Lung volume was measured using respiratory inductive plethysmography. A distinct deflation limb could be mapped in each infant. Overall, oxygenation and lung volume were improved by applying ventilation on the deflation limb. Maximal lung volume and oxygenation occurred on the deflation limb at a mean airway pressure of 3 and 5 cm H(2)O below the airway pressure approximating total lung capacity, respectively.

CONCLUSIONS

Using current ventilation strategies, all infants were being ventilated near the inflation limb. It is possible to delineate the deflation limb in infants receiving high-frequency oscillatory ventilation; in doing so, greater lung volume and oxygenation can be achieved, often at lower airway pressures.

Noninvasive assessment of lung volume: respiratory inductance plethysmography and electrical impedance tomography

OBJECTIVE

Respiratory inductance plethysmography (RIP) and electrical impedance tomography (EIT) are two monitoring techniques that have been used to assess lung volume noninvasively.

METHODS

RIP uses two elastic bands around the chest and abdomen to assess global changes in lung volume. In animal models, RIP has been shown to detect changes in lung mechanics during high-frequency oscillatory ventilation and has the potential to quantify lung volumes noninvasively. EIT measures regional impedance changes with 16 electrodes around the patient's chest, each of them injecting and receiving small currents. Impedance changes have been correlated with volume changes in animal models and in humans. In a recent animal model, EIT was shown to be capable of tracking lung volume changes during high-frequency oscillatory ventilation.

CONCLUSION

The promise of monitoring techniques such as RIP and EIT is that they will guide lung protective ventilation strategies and allow the clinician to optimize lung recruitment, maintain an open lung, and limit overdistension. EIT is the only bedside method that allows repeated, noninvasive measurements of regional lung volumes. In the future, it will be important to standardize the definitions of alveolar recruitment and ultimately demonstrate the superiority of EIT-guided ventilator management in providing lung protective ventilation.

Breath-to-breath analysis of abdominal and rib cage motion in surfactant-depleted piglets during high-frequency oscillatory ventilation

OBJECTIVE

To assess the value of monitoring abdominal and rib cage tidal displacement as an indicator of optimal mean airway pressure (Paw) during high-frequency oscillatory ventilation (HFOV).

DESIGN AND SETTING

Prospective observational study in a university research laboratory.

ANIMALS

Eight piglets weighing 12.0+/-0.5 kg, surfactant depleted by lung lavage.

INTERVENTIONS

Compliance of the respiratory system (C(rs)) was calculated from a quasistatic pressure volume loop. After initiation of HFOV lung volume was recruited by increasing Paw to 40 cmH(2)O. Then mean Paw was decreased in steps until PaO(2)/FIO(2) was below 100 mmHg. Proximal pressure amplitude remained constant.

MEASUREMENTS AND RESULTS

Abdominal and rib cage tidal displacement was determined using respiratory inductive plethysmography. During HFOV there was maximum in tidal volume (Vt) in seven of eight piglets. At maximal mean Paw abdominal and rib cage displacement were in phase. Phase difference between abdominal and rib cage displacement increased to a maximum of 178+/-28 degrees at minimum mean Paw. A minimum in abdominal displacement and a maximum of Vt was found near the optimal mean Paw, defined as the lowest mean Paw where shunt fraction is below 0.1.

CONCLUSIONS

During HFOV abdominal and rib cage displacement displayed mean Paw dependent asynchrony. Maximal Vt and minimal abdominal displacement coincided with optimal C(rs), oxygenation, and ventilation, suggesting potential clinical relevance of monitoring Vt and abdominal displacement during HFOV.

High frequency oscillatory ventilation in acute respiratory failure

High frequency oscillatory ventilation (HFOV) has emerged over the past 20 years as a safe and effective means of mechanical ventilatory support in patients with acute respiratory failure. During HFOV, lung recruitment is maintained by application of a relatively high mean airway pressure with superimposed pressure oscillations at a frequency of 3 to 15Hz, creating adequate ventilation using tidal volumes less than or equal to the patient's dead space volume. The physiologic rationale for the application of HFOV in the clinical arena comes from its ability to preserve end-expiratory lung volume while avoiding parenchymal overdistension at end-inspiration and theoretically limiting the potential for ventilator-associated lung injury. Data in the neonatal population suggests significant benefits in pulmonary outcomes when HFOV is applied with a recruitment strategy in preterm infants with respiratory distress syndrome (RDS). Use of HFOV in the paediatric and adult populations has not as yet been associated with significant improvements in clinically important outcome measures.

Imbalances in Regional Lung Ventilation

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right-left ventilation imbalances (bias = 0% and limits of agreement = -10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r(2) > or = 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Regional lung volume during high-frequency oscillatory ventilation by electrical impedance tomography*

OBJECTIVE

To investigate the value of electrical impedance tomography for the assessment of regional lung mechanics during high-frequency oscillatory ventilation (HFOV).

DESIGN

Prospective, interventional animal study.

SETTING

University research laboratory.

SUBJECTS

Eight pigs with lavage-induced lung injury.

INTERVENTIONS

Electrical impedance tomography measurements were performed during a pressure-volume maneuver and during a recruitment-derecruitment maneuver on HFOV by stepwise variation of continuous distending pressure (CDP).

MEASUREMENTS AND MAIN RESULTS

Lung volume was estimated by calibrated strain-gauge plethysmography; regional lung volume changes were assessed by electrical impedance tomography in various regions of interest. We found that inflation during the pressure-volume maneuver was distributed nonhomogeneously, whereas deflation was homogeneous. During HFOV, no major regional differences were found during either inflation or deflation. The upper inflection point on the deflation limb was at a slightly higher continuous distending pressure (26+/-3 cm H2O) than the minimal physiologic shunt fraction (at continuous distending pressure of 23+/-7), where there were hemodynamic signs of overdistension. Maximal compliance on the deflation limb (at continuous distending pressure of 13+/-3 cm H2O) agreed well with the minimal continuous distending pressure, where shunt fraction was just below 0.1 (14+/-2 cm H2O).

CONCLUSIONS

HFOV has a homogenizing effect on lung volume distribution. Regional lung volume distribution can be assessed using electrical impedance tomography. However, thoracic fluid accumulation may complicate its interpretation.

High-frequency oscillatory ventilation for acute respiratory distress syndrome in adult patients

INTRODUCTION

High-frequency oscillatory ventilation (HFOV) using an open-lung strategy has been demonstrated to improve oxygenation in neonatal and pediatric respiratory failure, without increasing barotrauma. Animal studies using small (<4 mm) endotracheal tubes have shown reduced histopathologic evidence of lung injury and inflammatory mediator release, suggesting reduced ventilator-induced lung injury.

CLINICAL STUDIES

During the last decade, case reports and observational studies of HFOV in patients failing conventional ventilation strategies have suggested improved oxygenation in adult patients with severe acute respiratory distress syndrome. These reports have also suggested that early (2 days) initiation of HFOV is more likely to result in survival than delayed initiation (>7 days). A recently published randomized, controlled trial in acute respiratory distress syndrome patients (n = 148) comparing HFOV with a pressure-control ventilation strategy (Pao(2)/Fio(2) ratio of <or=200 mm Hg on positive end-expiratory pressure of >10 cm H(2)O) demonstrated early (<16 hrs) improvement in Pao(2)/Fio(2) (p =.008) in the HFOV group but no significant difference in oxygenation index between the two groups during the initial 72 hrs of treatment. Thirty-day mortality was 37% in the HFOV group and 52% in the conventional ventilation group (p =.102). There was no significant difference between treatment groups in the prevalence of barotrauma, hemodynamic instability, or mucus plugging. This study suggests that HFOV is as effective and safe as the conventional strategy to which it was compared.

CLINICAL APPLICATION

For clinical use in adults, a trial of HFOV may be considered when Fio(2) requirements exceed 60% and mean airway pressure is approaching 20 cm H(2)O or higher (or, alternatively, positive end-expiratory pressure of >15 cm H(2)O). It is currently unknown whether initiating HFOV at a lower severity threshold would result in reduced ventilator-associated lung injury or mortality.

FUTURE DIRECTIONS

Future studies should compare different algorithms of applying HFOV to determine the optimal techniques for achieving oxygenation and ventilation, while minimizing ventilator-associated lung injury. The potential role of adjunctive therapies used with HFOV (e.g., prone ventilation, inhaled nitric oxide, aerosolized vasodilators, liquid ventilation) will require further research.

Comparison of loss in lung volume with open versus in-line catheter endotracheal suctioning

OBJECTIVE

Disconnecting the endotracheal tube from the ventilator causes significant loss in lung volume, which is further exacerbated by suctioning. In-line catheter suction systems have putative benefits over open catheter suction by maintaining positive pressure, thereby minimizing hypoxemia and hemodynamic instability. However, there is a theoretical risk of generating large negative airway pressures and auto-cycling of the ventilator with in-line catheter suction systems. We studied the effects on lung volume with both these techniques.

DESIGN

Open, randomized, crossover, clinical trial.

SETTING

Pediatric critical care unit.

PATIENTS

Fourteen paralyzed patients, age 6 days to 13 yrs.

INTERVENTIONS

Each patient, acting as his or her own control, was suctioned with an in-line catheter suction system and open catheter suction. Each suction maneuver was standardized. Changes in lung volume were measured by inductance plethysmography. Heart rate, blood pressure, and oxygen saturation were continuously monitored.

MEASUREMENTS AND MAIN RESULTS

Total lung volume loss was greater with open catheter suction compared with in-line catheter suction systems (p = .008). The most significant amount of lung volume loss associated with open catheter suction appears to be related to ventilator disconnection, rather than actual suctioning. Patients with decreased pulmonary compliance (< 0.8 mL/cm H2O/kg) demonstrated a greater loss in lung volume, both absolute and relative, as a result of ventilator disconnection (p = .038 and .006, respectively). Patients suctioned with open catheter suction desaturated to a greater extent than patients suctioned with in-line catheter suction (p = .026). There was evidence of ventilator triggering during the actual suction maneuver in all patients during in-line catheter suctions.

CONCLUSIONS

The most significant loss in lung volume during suctioning occurs primarily during ventilator disconnection. Hence, open catheter suction results in greater lung volume loss when compared with in-line catheter suction. We suggest that in-line catheter suction is preferable, especially in patients with significant lung disease and who require high positive end-expiratory pressures, to avoid alveolar derecruitment and exacerbating hypoxemia during endotracheal tube suctioning.