High-frequency oscillatory ventilation: Mechanisms of gas exchange and lung mechanics

Measurement of gas transport kinetics in high-frequency oscillatory ventilation (HFOV) of the lung using hyperpolarized (3)He magnetic resonance imaging

PURPOSE

To protect the patient with acute respiratory distress syndrome from ventilator associated lung injury (VALI) high-frequency oscillatory ventilation (HFOV) is used. Clinical experience has proven that HFOV is an efficient therapy when conventional artificial ventilation is insufficient. However, the optimal settings of HFOV parameters, eg, tidal volumes, pressure amplitudes and frequency for maximal lung protection, and efficient gas exchange are not established unambiguously.

METHODS

In this work magnetic resonance imaging (MRI) with hyperpolarized (3)He was employed to visualize the redistribution of gas within the cadaver pig lung during HFOV. The saturated slice method was used to characterize fast gas kinetics.

RESULTS

The strong differences in kinetics were observed for HFOV-driven gas exchange in comparison with diffusive gas transport (apnea). The significant regional and HFOV frequency dependence was detected for washout and gas exchange within the lungs. Gas redistribution was much faster in posterior than in anterior parts of the lungs during HFOV, in contrast to minor differences with an opposite trend observed in apnea.

CONCLUSION

The method shows significant potential for visualization and quantification of gas redistribution under HFOV and may help in optimization of the parameters to improve the clinical effect of HFOV for patients.

High-frequency percussive ventilation revisited

High-frequency percussive ventilation (HFPV) has demonstrated a potential role as a rescue option for refractory acute respiratory distress syndrome and as a method for improving inhalation injury outcomes. Nevertheless, there is a lack of literature examining the practical application of HFPV theory toward either improving gas exchange or preventing possible ventilator-induced lung injury. This article will discuss the clinically pertinent aspects of HFPV, inclusive of high- and low-frequency ventilation.

Management of the crushed chest

Thoracic injuries are very common among trauma victims. This article reviews the current literature on the management of multiple aspects of the care of the patient with severe chest injury. The mechanics of chest injury are complex and varied. Chest wall injuries are the most common and noticeable manifestation of thoracic trauma. Overall morbidity and mortality are primarily determined by associated injuries. New ventilatory strategies permit oxygenation of the severely hypoxic patient. Acute pain management modalities offer the potential of decreasing associated pulmonary complications. Surgical chest wall fixation is clearly indicated in extreme cases of pulmonary herniation and chest wall disruption. There are potential benefits of surgical fixation in other settings, although further trials are needed.

Mechanical ventilation in trauma

PURPOSE OF REVIEW

The purpose of this review is to evaluate new concepts in mechanical ventilation in trauma. We begin with the keystone of physiology prior to embarking on a discussion of several new modes of mechanical ventilation. We will discuss the use of noninvasive ventilation as a mode to prevent intubation and then go on to airway pressure release ventilation, high-frequency oscillatory ventilation, and computer-based, closed loop ventilation.

RECENT FINDINGS

The importance of preventing further injury in mechanical ventilation lies at the heart of the introduction of several new strategies of mechanical ventilation. New modes of ventilation have been developed to provide lung recruitment and alveolar stabilization at the lowest possible pressure.

SUMMARY

The old modes of continuous positive airway pressure and bilevel positive airway pressure have been actively introduced in clinical practice in the case of trauma patients. Used with proper pain management protocols, there has been a decrease in the incidence of intubation in blunt thoracic trauma. Airway pressure release ventilation has been gaining a role in the management of thoracic injury and may lead to less incidence of physiologic trauma to mechanically ventilated patients. High-frequency oscillatory ventilation has been shown to be effective in patient care by its ability to open and recruit the lung in trauma patients and in those with acute respiratory distress syndrome but it may not have a role in patients with inhalational injury. Closed loop ventilation is a technology that may better control major pulmonary parameters and lead to more rapid titration from the ventilator to spontaneous breathing.

Comparison of high-frequency oscillation and tracheal gas insufflation versus standard high-frequency oscillation at two levels of tracheal pressure

PURPOSE

In acute respiratory distress syndrome (ARDS), combined high-frequency oscillation (HFO) and tracheal gas insufflation (TGI) may improve oxygenation through a TGI-induced increase in mean tracheal pressure (P(tr)). We compared standard HFO and HFO-TGI matched for P(tr), in order to determine whether TGI affects gas exchange independently from P (tr).

METHODS

We conducted a prospective, randomized, crossover, physiological study in a 37-bed intensive care unit. Twenty-two patients with early acute lung injury (ALI) or ARDS were enrolled. On day 1, patients were ventilated with HFO, without (60 min) and combined with TGI (60 min) in random order. HFO/HFO-TGI sessions were repeated in inverse order within 7 h. HFO/HFO-TGI mean airway pressure (P(aw)) was titrated to a P(tr) that was either equal to (low P(aw)) or 3 cmH(2)O higher than (high P(aw)) the P(tr) of the preceding conventional mechanical ventilation. On day 2, the protocol was repeated at the alternative P(tr) level relative to day 1.

RESULTS

Gas exchange and hemodynamics were determined before, during, and after HFO/HFO-TGI sessions. HFO-TGI-high P(aw) versus HFO-high P(aw) resulted in significantly higher PaO(2)/inspired O(2) fraction (FiO(2)) [mean +/- standard error of the mean (SEM): 281.6 +/- 15.1 versus 199.0 +/- 15.0 mmHg; mean increase: 42%; P < 0.001]. HFO-TGI-low P(aw), versus HFO-low P(aw), resulted in significantly higher PaO(2)/FiO(2) (222.8 +/- 14.6 versus 141.3 +/- 8.7 mmHg; mean increase: 58%; P < 0.001). PaCO(2) was significantly lower during HFO-TGI-high P(aw) versus HFO-high P(aw) (45.3 +/- 1.6 versus 53.7 +/- 1.9 mmHg; mean decrease: 16%; P = 0.037).

CONCLUSIONS

At the same P(tr) level, HFO-TGI results in superior gas exchange compared with HFO.

High-frequency oscillatory ventilation (HFOV) and airway pressure release ventilation (APRV): a practical guide

Despite advances in ventilator management, 31% to 38% of patients with adult respiratory distress syndrome (ARDS) will die, some from progressive respiratory failure. Inability to adequately oxygenate patients with severe ARDS has prompted extensive efforts to identify what are now known as alternative modes of ventilation including high-frequency oscillatory ventilation and airway pressure release ventilation. Both modalities are based on the principles of the open-lung concept and aim to improve oxygenation by keeping the lung uniformly inflated for an extended period of time. Although a mortality benefit has not been proven, some patients may benefit from these alternative modes of ventilation as rescue measures while the underlying process resolves. The purpose of this article is to review the evidence and mechanisms underlying each modality and to describe the fundamental steps in initiating, adjusting, and terminating these modes of ventilation.

Demand flow facilitates spontaneous breathing during high-frequency oscillatory ventilation in a pig model

OBJECTIVE

Maintenance breathing is advocated in mechanical ventilation, which is difficult for the high-frequency oscillatory (HFO) ventilation. To facilitate spontaneous breathing during HFO ventilation, a demand flow system (DFS) was designed. The aim of the present study was to evaluate the system.

DESIGN

Animal experiment.

SETTING

: University animal laboratory.

SUBJECTS

Eight pigs (47-64 kg).

INTERVENTIONS

Lung injury was induced by lung lavage with normal saline. After spontaneous breathing was restored HFO ventilation was applied, in runs of 30 minutes, with continuous fresh gas flow (CF) or the DFS operated in two different setups. Pressure to regulate the DFS was sampled directly at the Y-piece of the ventilator circuit (DFS) or between the endotracheal tube and measurement equipment at the proximal end of the endotracheal tube. In the end, animals were paralyzed. Breathing pattern, work of breathing, and gas exchange were evaluated.

MEASUREMENTS AND MAIN RESULTS

HFO ventilation with demand flow decreased breathing frequency and increased tidal volume compared with CF. Comparing HFO modes CF, DFS, and DFSPROX, total pressure-time product (PTP) was 66 cm H2O x sec x min (interquartile range 59-74), 64 cm H2O x sec x min (50-72), and 51 cm H2O x sec x min (41-63). Ventilator PTP was 36 cm H2O x sec x min (32-42), 8.6 cm H2O x sec x min (7.4-10), and 1 cm H2O x sec x min (-1.0 to 2.8). Oxygenation, evaluated by Pao2, was preserved when spontaneous breathing was maintained and deteriorated when pigs were paralyzed. Ventilation, evaluated by Paco2, improved with demand flow. Paco2 increased when using continuous flow and during muscular paralysis.

CONCLUSIONS

In moderately lung-injured anesthetized pigs during HFO ventilation, demand flow facilitated spontaneous breathing and augmented gas exchange. Demand flow decreased total breathing effort as quantified by PTP. Imposed work caused by the HFO ventilator appeared totally reduced by demand flow.

Nasal high-frequency ventilation for premature infants

AIM

To assess the use of nasal high-frequency ventilation (HFV) to provide noninvasive ventilatory support for very low birthweight (VLBW) infants.

STUDY DESIGN

VLBW infants, >7 days of age on nasal continuous positive airway pressure (CPAP), were placed on nasal HFV for 2 h using the Infant Star high-frequency ventilator (Mallinckrodt, Inc., St. Louis, MO, USA). Mean airway pressure was set to equal the previous level of CPAP, and amplitude was adjusted to obtain chest wall vibration. Capillary blood was sampled before starting HFV and after 2 h to determine change in pH and partial pressure of carbon dioxide (pCO(2)).

RESULTS

Fourteen subjects were studied, 10 males and 4 females. Gestational age was 26-30 weeks (median 27). Age at study was 18-147 days (median 30). Median birth weight was 955 g; median weight at study was 1605 g. Nasal CPAP pressure was 4-7 cm H(2)O (mean 5). Amplitude was 30-60 (median 50). After 2 h, PCO(2) (mean 45 torr) was significantly lower than initial PCO(2) (mean 50 torr) (p = 0.01), and pH had increased significantly (7.40 vs. 7.37, p = 0.04).

CONCLUSIONS

Nasal HFV is effective in decreasing pCO(2) in stable premature infants requiring nasal CPAP support. Long-term use of nasal HFV requires further study.

A brief report: the use of high-frequency oscillatory ventilation for severe pulmonary contusion

BACKGROUND

Severe pulmonary contusions are a common cause of acute respiratory distress syndrome (ARDS) and are associated with significant morbidity. High frequency oscillatory ventilation (HFOV) is a ventilatory mode that employs a lung protective strategy consistent with the ARDSNet low tidal volume ventilation strategy and may result in reduced morbidity. The objective of this report is to examine the impact of HFOV on blunt trauma patients with severe pulmonary contusions who failed or were at a high risk of failing conventional mechanical ventilation.

METHODS

We undertook a retrospective chart review of all patients at our institution who received HFOV for severe pulmonary contusions. Patients were placed on HFOV when their mean airway pressure (mP(aw)) surpassed 30 cm H2O and their FIO2 was greater than 0.6. Baseline demographic data including injury severity score (ISS), length of time requiring HFOV, total ventilator days, and inhospital mortality were collected. Serial determinations of oxygenation index (OI) and the PaO2/FIO2 ratio (P/F) were made up to 72 hours after initiation of HFOV. A linear mixed model was used to analyze the slope (beta) of the regression line of P/F versus time and that of OI versus time.

RESULTS

Seventeen patients were identified who underwent HFOV for ARDS due primarily to pulmonary contusions. Mean ISS was 36.6, mean APACHE II score was 21.7, and the mean time before initiation of HFOV was 2.0 days. P/F increased significantly after HFOV was initiated (beta = 12.1; 95% confidence interval 7.9 to 16.4, p < 0.001). OI significantly decreased after HFOV implementation (beta = -1.8; 95% confidence interval -2.3 to -1.3, p < 0.001). Mortality rate was 17.6%.

CONCLUSIONS

The early use of HFOV appears to be safe and efficacious in blunt trauma patients sustaining pulmonary contusions, and results in a rapid improvement in OI and the P/F ratio.

Appropriate Ventilatory Settings for Thoracic Surgery: Intraoperative and Postoperative

Mechanical ventilation of patients undergoing thoracic surgery is often challenging. These patients frequently have significant underlying comorbidities, including cardiopulmonary disease, and often must undergo 1-lung ventilation. Perioperative respiratory complications are common and are multifactorial in etiology. Increasing evidence suggests that mechanical ventilation is associated with, and may even cause, lung damage in both sick and healthy patients. Gas exchange to provide acceptable end-organ oxygenation remains a primary goal but so too is minimization of risks for acute lung injury. Every ventilator strategy is associated with potential beneficial and adverse side effects. Understanding the impact of various ventilation strategies allows clinicians to provide optimal care for patients.

High frequency oscillatory ventilation for respiratory failure due to RSV bronchiolitis

OBJECTIVE

To describe the time course of high frequency oscillatory ventilation (HFOV) in respiratory syncytial virus (RSV) bronchiolitis.

DESIGN

Retrospective charts review.

SETTING

A tertiary paediatric intensive care unit.

PATIENTS AND PARTICIPANTS

Infants with respiratory failure due to RSV infection.

INTERVENTION

HFOV.

MEASUREMENTS AND RESULTS

Pattern of lung disease, ventilatory settings, blood gases, infant's vital parameters, sedation and analgesia during the periods of conventional mechanical ventilation (CMV, 6 infants), after initiation of HFOV (HFOVi, 9 infants), in the middle of its course (HFOVm), at the end (HFOVe) and after extubation (Post-Extub) were compared. All infants showed a predominant overexpanded lung pattern. Mean airway pressure was raised from a mean (SD) 12.5 (2.0) during CMV to 18.9 (2.7) cmH(2)O during HFOVi (P < 0.05), then decreased to 11.1(1.3) at HFOVe (P < 0.05). Mean FiO(2) was reduced from 0.68 (0.18) (CMV) to 0.59 (0.14) (HFOVi) then to 0.29 (0.06) (P < 0.05) at HFOVe and mean peak to peak pressure from 44.9 (12.4) cmH(2)O (HFOVi) to 21.1 (7.7) P < 0.05 (HFOVe) while mean (SD) PaCO(2) showed a trend to decrease from 72 (22) (CMV) to 47 (8) mmHg (HFVOe) and mean infants respiratory rate a trend to increase from 20 (11) (HFOVi) to 34 (14) (HFOVe) breaths/min. With usual doses of sedatives and opiates, no infant was paralysed and all were extubated to CPAP or supplemental oxygen after a mean of 120 h.

CONCLUSION

RSV induced respiratory failure with hypercapnia can be managed with HFOV using high mean airway pressure and large pressure swings while preserving spontaneous breathing.

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.

Determinants of airleak flow during humming V high-frequency oscillatory ventilation in an open-compartment ex-vivo model of airleak

High-frequency oscillatory ventilation (HFOV) using small tidal volumes and maintaining sufficient end-expiratory lung volume may be beneficial in the treatment of airleak. However, few published guidelines exist to advise clinicians on appropriate ventilator settings in this clinical scenario. The present experiment aimed to determine the effect of frequency, stroke volume (SV) and mean airway pressure (MAP) on airleak from an isolated lung model ventilated with a Humming V HFOV. We performed a crossover non-randomized experiment using the repeated measurement method to test the hypothesis that MAP is the major determinate for airleak. The lungs of 13 healthy juvenile New Zealand white rabbits were isolated and ventilated with high peak pressure to create airleak. The dataset obtained was analyzed using the generalized estimating equation method. We found that airleak flow did not change as frequency was raised from 13 to 17 Hz (P = 0.463) with MAP and SV kept constant. SV was positively correlated to the amount of change in airleak (P < 0.01, coefficients +/- SEM = 1.2 +/- 0.1 ml/min/ml). Leakage flow increased significantly from 275 +/- 168 ml/min to 1,721 +/- 552 ml/min as MAP was increased from 5 cm H(2)O to 30 cm H(2)O (P < 0.001, coefficients +/- SEM = 56.1 +/- 3.0 ml/min/cm H(2)O) while inspiratory flow increased less and amplitude pressure remained about the same. We concluded that MAP (lung volume) was the main independent factor for airleak, whilst SV (tidal volume) exerted a lesser effect. Within the operational range of the Humming V, frequency did not affect airleak.

High-frequency oscillatory ventilation for adult patients with ARDS

High-frequency oscillatory ventilation (HFOV) is characterized by the rapid delivery of small tidal volumes (Vts) of gas and the application of high mean airway pressures (mPaws). These characteristics make HFOV conceptually attractive as an ideal lung-protective ventilatory mode for the management of ARDS, as the high mPaws prevent cyclical derecruitment of the lung and the small Vts limit alveolar overdistension. In this review, we will summarize the literature describing the use of HFOV in adult patients with ARDS. In addition, we will discuss recent experimental studies of HFOV that have advanced our understanding of its mechanical properties. We identified 2 randomized controlled trials (RCTs) and 12 case series evaluating HFOV in adults with ARDS. In these studies, HFOV appears to be safe and consistently improves oxygenation when used as a rescue mode of ventilation in patients with severe ARDS. The two RCTs comparing HFOV to conventional ventilation revealed encouraging results but failed to show a mortality benefit of HFOV over conventional ventilation. Further research is needed to identify optimal patient selection, technique, the actual Vt delivered, and the role of combining HFOV with other interventions, such as recruitment maneuvers, prone positioning, and nitric oxide.

Acute effects of combined high-frequency oscillation and tracheal gas insufflation in severe acute respiratory distress syndrome

OBJECTIVE

In acute respiratory distress syndrome (ARDS), high-frequency oscillation (HFO) improves oxygenation relative to conventional mechanical ventilation (CMV). Alveolar ventilation is improved by adding tracheal gas insufflation (TGI) to CMV. We hypothesized that combined HFO and TGI (HFO-TGI) might result in improved gas exchange relative to both standard HFO and CMV according to the ARDS Network protocol.

DESIGN

Prospective, randomized, crossover study.

SETTING

A 30-bed university intensive care unit.

PATIENTS

A total of 14 patients with early (<72 hrs in duration), severe (PaO2/FiO2 of <150 mm Hg and prerecruitment oxygenation index of 22.8 +/- 1.9 [mean +/- SEM]), primary ARDS.

INTERVENTIONS

Patients were ventilated with HFO without (60 mins) and combined with TGI (6.1 +/- 0.1 L/min, 60 mins) in random order. HFO sessions were repeated in inverse order within 24 hrs. HFO sessions were preceded and followed by ARDS Network CMV. Four recruitment maneuvers were performed during the study period. During HFO sessions, mean airway pressure was set at 1 cm H2O above the point of maximal curvature of the respiratory system expiratory pressure-volume curve.

MEASUREMENTS AND MAIN RESULTS

Gas exchange and hemodynamics were determined before, during, and after HFO sessions. HFO-TGI improved PaO2/FiO2 relative to HFO and CMV (174.5 +/- 10.4 vs. 136.0 +/- 10.0 and 105.0 +/- 3.7 mm Hg, respectively, p < .05 for both) and oxygenation index relative to HFO (17.1 +/- 1.3 vs. 22.3 +/- 1.7, respectively p < .05). PaO2/FiO2 returned to baseline within 3 hrs after HFO. During HFO-TGI, shunt fraction and mixed venous oxygen saturation improved relative to CMV (0.36 +/- 0.01 vs. 0.45 +/- 0.01 and 77.8% +/- 1.2% vs. 71.8% +/- 1.3%, respectively, p < .05 for both). PaCO2 and hemodynamics were unaffected by HFO sessions. Respiratory mechanics remained unchanged throughout the study period.

CONCLUSIONS

In early onset, primary, severe ARDS, short-term HFO-TGI improves oxygenation relative to standard HFO and ARDS Network CMV.

Hochfrequenz-Oszillations-Ventilation

The concept of lung protective ventilation strategies is based on the limitation of the inspiratory pressure and the reduction of the tidal volume, in order to minimize the extent of breathing cycle-dependent damaging mechanisms from mechanical ventilation. This concept is coupled with various procedures for optimization of the end-expiratory lung volume in acute lung failure in order to improve the compromized oxygenation. In this situation high-frequency oscillatory ventilation (HFOV) has achieved a renaissance. Theoretically this procedure offers advantages which differentiates it from conventional ventilation procedures. The system allows the use of a constant higher mean airway pressure, a reduction of the peak pressure and the use of a tidal volume in the dead-space area. Very little data exist with respect to the application of this procedure in adult patients. For the clinical use of HFOV as a secondary procedure in adult patients suffering from acute lung failure it could be demonstrated that it is a safe and effective method of treatment. The effect of HFVO on the morbidity and mortality outcome, however, still needs to be characterized.

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.

Effect of a lung recruitment maneuver by high-frequency oscillatory ventilation in experimental acute lung injury on organ blood flow in pigs

Introduction

The objective was to study the effects of a lung recruitment procedure by stepwise increases of mean airway pressure upon organ blood flow and hemodynamics during high-frequency oscillatory ventilation (HFOV) versus pressure-controlled ventilation (PCV) in experimental lung injury.

Methods

Lung damage was induced by repeated lung lavages in seven anesthetized pigs (23–26 kg). In randomized order, HFOV and PCV were performed with a fixed sequence of mean airway pressure increases (20, 25, and 30 mbar every 30 minutes). The transpulmonary pressure, systemic hemodynamics, intracranial pressure, cerebral perfusion pressure, organ blood flow (fluorescent microspheres), arterial and mixed venous blood gases, and calculated pulmonary shunt were determined at each mean airway pressure setting.

Results

The transpulmonary pressure increased during lung recruitment (HFOV, from 15 ± 3 mbar to 22 ± 2 mbar, P < 0.05; PCV, from 15 ± 3 mbar to 23 ± 2 mbar, P < 0.05), and high airway pressures resulted in elevated left ventricular end-diastolic pressure (HFOV, from 3 ± 1 mmHg to 6 ± 3 mmHg, P < 0.05; PCV, from 2 ± 1 mmHg to 7 ± 3 mmHg, P < 0.05), pulmonary artery occlusion pressure (HFOV, from 12 ± 2 mmHg to 16 ± 2 mmHg, P < 0.05; PCV, from 13 ± 2 mmHg to 15 ± 2 mmHg, P < 0.05), and intracranial pressure (HFOV, from 14 ± 2 mmHg to 16 ± 2 mmHg, P < 0.05; PCV, from 15 ± 3 mmHg to 17 ± 2 mmHg, P < 0.05). Simultaneously, the mean arterial pressure (HFOV, from 89 ± 7 mmHg to 79 ± 9 mmHg, P < 0.05; PCV, from 91 ± 8 mmHg to 81 ± 8 mmHg, P < 0.05), cardiac output (HFOV, from 3.9 ± 0.4 l/minute to 3.5 ± 0.3 l/minute, P < 0.05; PCV, from 3.8 ± 0.6 l/minute to 3.4 ± 0.3 l/minute, P < 0.05), and stroke volume (HFOV, from 32 ± 7 ml to 28 ± 5 ml, P < 0.05; PCV, from 31 ± 2 ml to 26 ± 4 ml, P < 0.05) decreased. Blood flows to the heart, brain, kidneys and jejunum were maintained. Oxygenation improved and the pulmonary shunt fraction decreased below 10% (HFOV, P < 0.05; PCV, P < 0.05). We detected no differences between HFOV and PCV at comparable transpulmonary pressures.

Conclusion

A typical recruitment procedure at the initiation of HFOV improved oxygenation but also decreased systemic hemodynamics at high transpulmonary pressures when no changes of vasoactive drugs and fluid management were performed. Blood flow to the organs was not affected during lung recruitment. These effects were independent of the ventilator mode applied.

Unloading work of breathing during high-frequency oscillatory ventilation: a bench study

Introduction

With the 3100B high-frequency oscillatory ventilator (SensorMedics, Yorba Linda, CA, USA), patients' spontaneous breathing efforts result in a high level of imposed work of breathing (WOB). Therefore, spontaneous breathing often has to be suppressed during high-frequency oscillatory ventilation (HFOV). A demand-flow system was designed to reduce imposed WOB.

Methods

An external gas flow controller (demand-flow system) accommodates the ventilator fresh gas flow during spontaneous breathing simulation. A control algorithm detects breathing effort and regulates the demand-flow valve. The effectiveness of this system has been evaluated in a bench test. The Campbell diagram and pressure time product (PTP) are used to quantify the imposed workload.

Results

Using the demand-flow system, imposed WOB is considerably reduced. The demand-flow system reduces inspiratory imposed WOB by 30% to 56% and inspiratory imposed PTP by 38% to 59% compared to continuous fresh gas flow. Expiratory imposed WOB was decreased as well by 12% to 49%. In simulations of shallow to normal breathing for an adult, imposed WOB is 0.5 J l^-1 at maximum. Fluctuations in mean airway pressure on account of spontaneous breathing are markedly reduced.

Conclusion

The use of the demand-flow system during HFOV results in a reduction of both imposed WOB and fluctuation in mean airway pressure. The level of imposed WOB was reduced to the physiological range of WOB. Potentially, this makes maintenance of spontaneous breathing during HFOV possible and easier in a clinical setting. Early initiation of HFOV seems more possible with this system and the possibility of weaning of patients directly on a high-frequency oscillatory ventilator is not excluded either.

High-frequency percussive ventilation improves oxygenation in trauma patients with acute respiratory distress syndrome: a retrospective review

BACKGROUND

High-frequency percussive ventilation (HFPV), a hybrid of conventional mechanical ventilation and high-frequency oscillatory ventilation, has been used to salvage patients with persistent hypoxemia on conventional mechanical ventilation. We hypothesized that oxygenation would improve in injured patients with severe hypoxemia who were converted to HFPV after initial management with conventional ventilation.

METHODS

Chart review identified patients with acute respiratory distress syndrome (ARDS) managed with HFPV. Oxygenation parameters (oxygenation index, OI; Pao(2)/Fio(2) ratio, P/F) and mean airway pressures (mPaw) were recorded at baseline and at 1 to 4, 8 to 12, and 12 to 24 hours after initiation of HFPV. Values at baseline and each time point after conversion to HFPV were compared by using analysis of variance or Kruskal-Wallis tests.

RESULTS

Twelve patients, over 24 months, were reviewed. Baseline measurements were OI: 42.2 +/- 33, P/F: 70 +/- 31, (median +/- interquartile range), and mPaw: 29 +/- 8 (mean +/- standard deviation) cm H(2)O. After initiation of HFPV, mPaw did not differ from baseline. There was an improvement in OI (P = .01) from baseline at 12 to 24 hours after initiation of HFPV and in P/F at 12 to 24 hours (P = .002) and 8 to 12 hours (P = .001) after initiation of HFPV.

CONCLUSIONS

HFPV may improve oxygenation in patients with ARDS without a concomitant increase in mPaw. A randomized trial of HFPV versus conventional ventilation in trauma patients is needed.

High-frequency oscillatory ventilation in an infant with cystic fibrosis and bronchiolitis

Infants with cystic fibrosis (CF) may develop severe respiratory compromise related to viral lower respiratory tract infections due to impaired mucous clearance and plugging of small airways. Consequently air trapping may lead to lung hyperinflation, impaired gas exchange, and respiratory failure. We describe the case of an infant with newly diagnosed CF who developed severe hypercarbic respiratory failure in the setting of viral bronchiolitis successfully treated with high-frequency oscillatory ventilation (HFOV).