High-frequency jet ventilation: theoretical considerations and clinical observations

High-Frequency Ventilation in Newborn Infants

In the past mechanical ventilation always mimicked the tidal volumes and ventilatory frequencies of normal breathing. Recently, there has been great interest in techniques that use rapid rates (60 to 3,000 per minute) and tidal volumes approximating dead space. These techniques are known collectively as high-frequency ventilation, although they differ in circuit design, use, potential complications, and mechanism of gas transport. High-frequency ventilation can be divided into four categories: (1) high-frequency positive pressure ventilation, (2) high-frequency jet ventilation, (3) high-frequency oscillatory ventilation and high-frequency flow interruption, and (4) high-frequency chest wall oscillation. In this review we discuss the similarities and differences of these high-frequency techniques, their clinical applications, and some physiological mechanisms involved in gas transport.

Bronchial-oesophageal fistula: a rare initial presentation of squamous cell carcinoma of the lung

A 61-year-old Caucasian man with hypertension and hepatitis C presented to the emergency department with 7 days of productive cough and low-grade fevers despite outpatient therapy with oral azithromycin. On initial evaluation, he was lethargic with peripheral cyanosis and oxygen saturation in the low 70 s on room air, necessitating endotracheal intubation. Chest imaging revealed diffuse bilateral infiltrates compatible with the diagnosis of acute respiratory distress syndrome. Patient subsequently developed profound hypoxaemia and on day 2 of admission, veno-veno extracorporeal membrane oxygenation (ECMO) was initiated. Bronchoscopy revealed a necrotic ulcer on the posterior wall of the left mainstem bronchus, compatible with a bronchial-oesophageal fistula, which was later confirmed by endoscopy, and stented. Histology revealed poorly differentiated squamous cell carcinoma of the lung. Despite stenting of the fistula and ECMO support, the patient expired 5 days after admission.

CT-guided needle lung biopsy is possible during apneic oxygenation: a case series

Background

It can be difficult to perform CT guided biopsy of small pulmonary nodules especially if the position is behind a costa or close to the diaphragm and respiratory movements may hamper the procedure. During apneic oxygenation with a pulmonary standstill these movements can be hindered.

Methods

Six patients with decreased lung function and suspicious lung nodules are presented. Under general anesthesia including a muscle relaxant and a cuffed tube in the trachea CT guided biopsy was prepared. Just before the biopsy the ventilation mode was switched to a continuous positive airway pressure of 5–10 cm H₂O, maintaining 100% oxygen delivery without ventilation. If the position of the lung nodule was inconvenient for biopsy the pressure was increased to up to 17 cm H₂O to expand the lungs to a better biopsy position. After retrieving the biopsy controlled ventilation was re-established and a finishing control CT-scan was performed. Blood gas analyses were performed with few minutes interval.

Results

All biopsies were diagnostic. All patients survived the procedure with no major complications, but 3 patients developed pneumothorax. The length of apneic oxygenation was median 10 minutes (8–10 minutes). No major changes in vital parameters were observed, and in all patients the peripheral oxygen saturation was 100% throughout the procedure. The arterial oxygen tension rose to very high values and the lowest pH was 7.18.

Conclusions

It is possible to perform lung biopsies in selected patients with decreased lung function during apneic oxygenation in at least 10 minutes in a safe way.

[Bilateral tension pneumothorax after retrieval of transtracheal jet ventilation catheter]

We report a bilateral tension pneumothorax which occurred in a 36-year-old man after high-frequency jet ventilation (HFJV) for panendoscopy. The patient had been treated with radiotherapy and chemotherapy two years ago for an oropharyngeal adenocarcinoma, and by surgery for a recurrence. The incident occurred after a cough episode triggered by the withdrawal of the Ravussin transtracheal catheter. We are discussing the risk factors and the mechanisms of pneumothorax during HFJV with special emphasis on trapping and lung fibrosis.

Transesophageal echocardiographic assessment of right heart hemodynamics during high-frequency jet ventilation

STUDY OBJECTIVE

To evaluate right ventricular dimensions and function by echocardiography in anesthetized patients during superimposed high-frequency jet ventilation (HFJV).

DESIGN

Prospective clinical study.

SETTING

University hospital operating room.

PATIENTS

20 ASA physical status I patients undergoing elective minor otorhinolaryngological surgery, and undergoing conventional mechanical ventilation with subsequent superimposed HFJV.

INTERVENTIONS

Two-dimensional transesophageal echocardiography with a 5-MHz multiplane transducer to determine right ventricular dimensions and function from a mid-esophageal view. Insertion of a radial artery catheter for monitoring blood pressure and blood gases.

MEASUREMENTS AND MAIN RESULTS

Heart rate, mean arterial blood pressure, and right ventricular end-diastolic and end-systolic volumes determined by echocardiography, stroke volume, and ejection fraction. Measurements were performed after 10 minutes of conventional positive pressure ventilation (control) and after 10 minutes of subsequent superimposed HFJV at similar peak and positive end-expiratory airway pressures. Right ventricular systolic and diastolic volumes, stroke volume, and ejection fraction did not reveal statistical significant differences after transition to HFJV. Interventricular septum did not show any abnormalities in motion. In contrast, interatrial septum demonstrated momentary mid-systolic bows toward the left atrium in 9 of 17 patients (53%) during conventional ventilation, but in 15 of 17 patients (88%) during jet ventilation. Heart rate and mean arterial blood pressure remained unchanged, but arterial oxygen tension values were higher and arterial carbon dioxide tension values lower during HFJV.

CONCLUSION

Transesophageal echocardiographic evaluation of right heart hemodynamics did not show any significant difference after transition of ventilation to superimposed HFJV applying similar airway pressures. Furthermore, superimposed HFJV was safe and effective, it improved oxygenation, and it facilitated carbon dioxide elimination.

Effect of jet ventilation on heart failure: decreased afterload but negative response in left ventricular end-systolic pressure-volume function

OBJECTIVE

To examine the mechanism of cardiac assist with systolic jet ventilation, specifically effects on loading conditions and left ventricular pressure-volume function. Both systolic and diastolic jet ventilation were compared in the absence and presence of heart failure.

DESIGN

Prospective, two-factor, repeated-measures study.

SETTING

Animal laboratory.

SUBJECTS

Ten anesthetized, closed-chest dogs.

INTERVENTIONS

The measurement protocol consisted of two phases: a) apnea, randomized jet ventilation (systole- and diastole-synchronized); b) postjet ventilation apnea, before and after heart failure, induced with a propranolol-imipramine-plasma expansion treatment.

MEASUREMENT AND MAIN RESULTS

Systolic and diastolic jet ventilation was associated with mean airway pressures of approximately 7 mm Hg and intrapleural pressures of approximately 3 mm Hg in both heart conditions. In normal hearts, jet ventilation (either mode) decreased transmural left ventricular end-diastolic pressure by 40% to 60% (p < .05), left ventricular end-diastolic volume 25 +/- 8%, and stroke volume by 28% to 30%. Heart failure was associated with decreases (41 +/- 6%) in end-systolic pressure-volume function (i.e., pressure change/volume change or elastance), transmural left ventricular end-systolic pressure (22 +/- 3%), and stroke volume (16 +/- 4%), and increased transmural left ventricular end-diastolic pressure (139 +/- 6%). Application of jet ventilation (either mode) during heart failure did not affect stroke volume but significantly (p < .05) attenuated transmural left ventricular end-diastolic pressure by 30% to 40%, left ventricular end-diastolic volumes by 33 +/- 9%, and transmural left ventricular end-systolic pressure by 11% to 19% (p < .05). After jet ventilation, left ventricular elastance was decreased 36 +/- 8% in normal hearts and 35 +/- 11% in failing hearts. Stroke volume, however, returned to baseline levels because of increases in transmural left ventricular end-diastolic pressure in both heart conditions, and also in failing hearts, because transmural left ventricular end-systolic pressure remained decreased approximately 30% (p < .05).

CONCLUSIONS

Jet ventilation did not decrease stroke volume in failing hearts because of the afterload-reducing benefit (decreased transmural left ventricular end-systolic pressure) of increased intrapleural pressure in dilated ventricles. Moreover, jet ventilation did not have positive effects on myocardial function and had negative effects on left ventricular elastance in the postjet ventilation period in both normal and failing hearts. Cardiac assist by jet ventilation was not cycle specific, suggesting no selective benefit of jet ventilation over conventional positive-pressure ventilation during heart failure. These studies demonstrate a negative inotropy associated with jet ventilation that, during heart failure, may compromise the general benefit of positive-pressure-mediated increases in intrapleural pressure.

Regional tracheal blood flow during conventional and high-frequency jet ventilation in suckling pigs

OBJECTIVE

To determine whether intubation and ventilation with either conventional mechanical ventilation or high-frequency jet ventilation, using dry or humidified gas, could induce regional tracheal ischemia and serve as a basis for the tracheal necrosis observed clinically during ventilation.

DESIGN

Prospective, multiple group, controlled experimental study.

SETTING

Medical school research laboratory.

SUBJECTS

Twenty, 3- to 5-wk-old suckling pigs.

INTERVENTIONS

Anesthetized, closed-chest piglets were intubated and ventilated for 30 mins with conventional mechanical ventilation and then ventilated for 2 additional hrs with either conventional mechanical ventilation or high-frequency jet ventilation. Groups were also ventilated, using both modes of ventilation, with either 37 degrees C humidified gas or 25 degrees C dry gas.

MEASUREMENTS AND MAIN RESULTS

Blood flow groups were compared during spontaneous breathing, conventional mechanical ventilation, high-frequency jet ventilation and both ventilation modes, using 37 degrees C humidified or 22 degrees C dry inspired gas. Groups were compared, using an analysis of variance with a Newman-Keul's post-test. Regional tracheal blood flow was measured, using radioactive microspheres. Cardiac output and organ blood flows were also monitored. Tracheal blood flow increased 10.3-fold within 30 mins after intubation, but there were no significant differences in regional or total tracheal blood flow between conventional mechanical ventilation and high-frequency jet ventilation, using 37 degrees C humidified gas. Tracheal blood flow was increased further using high-frequency jet ventilation and 25 degrees C dry gas but not conventional mechanical ventilation with dry gas. Although ventilation reduced cardiac output by approximately 30%, there were no significant differences in organ distribution between modes of ventilation.

CONCLUSIONS

Acute tracheal hyperemia occurred with intubation and ventilation with both conventional mechanical ventilation and high-frequency jet ventilation but no differences were observed between ventilation modes. Hyperemia was further increased with cool, dry inspired gas, using high-frequency jet ventilation but not conventional mechanical ventilation. Although acute tracheal ischemia was not produced by high-frequency jet ventilation or conventional mechanical ventilation, factors which alter the balance between arterial supply and metabolic demand or induce inflammation may contribute to the tracheal necrosis reported during sustained ventilation.

High frequency ventilator therapy for newborns

High-frequency ventilation is a general term that refers to a family of ventilator techniques that utilize respirator rates greater than 60 breaths/minute and tidal volumes that are usually less than or equal to the anatomical dead space of the airways. These techniques include high frequency positive-pressure ventilation, high frequency jet ventilation, high frequency flow interruption, high frequency oscillatory ventilation, and high frequency chest wall oscillation. I review the proposed mechanisms of gas transport during high-frequency ventilation and the different ventilators capable of delivering this mode of ventilation. In addition, clinical studies involving infants treated with this new technology are reviewed, along with long-term patient follow-up and reported complications.

Medical management and therapy of bronchopleural fistulas in the mechanically ventilated patient

Bronchopleural fistulas are associated with high morbidity and mortality and are particularly challenging in the ventilated patient. Familiarity with both basic and more technical medical management techniques may lessen morbidity and improve survival. Prompt recognition of BPFs and appropriate placement of a chest tube with an adequate suction device are crucial to prevent potential tension pneumothorax and to drain an infected pleural space. The chest tube may be used therapeutically to decrease BPF air leak and to promote fistula repair. Appropriate conventional ventilator manipulations aimed at decreasing fistula air leak and maintaining adequate oxygenation and ventilation may fail and necessitate a trial of HFV. Definitive therapy by the bronchoscopic application of a sealing agent to occlude the fistula site can be used, particularly in the poor surgical candidate.

Effect of sternotomy and cardiopulmonary bypass on airway pressures and respiratory mechanics during high frequency ventilation

The airway pressures at ventilatory frequencies of 15, 60, 100, 120 and 150 breaths per minute were measured in eight adult patients undergoing coronary artery bypass grafting. Measurements were made perioperatively at four stages: precardiopulmonary bypass with the chest closed, precardiopulmonary bypass with the chest open, postcardiopulmonary bypass with the chest open and postcardiopulmonary bypass with the chest closed. In five patients thoracic compliance and airways resistance were also measured at these times. Neither sternotomy nor cardiopulmonary bypass made any significant difference to the airway pressures during normal and high frequency ventilation, nor were lung mechanics affected.

Ventilation and ventilators--an update

In the five years which have passed since the previous review, the literature has been concerned more with the ways in which ventilators may be applied to patients and the effects of differing patterns of ventilation than with the design philosophy of the ventilators themselves. This account should be read in conjunction with that of 1982 [1].

High frequency, high volume ventilation for right ventricular assist

During ventilation, the lungs may serve as accessory pumps for forward blood flow. We have noted significant hemodynamic improvement in several patients during short periods of vigorous hand ventilation with the Ambu bag and tried to reproduce the hemodynamic improvement in the management of four patients with profound low cardiac output syndrome secondary to right ventricular (RV) failure following open heart surgery. High frequency, high volume (HFHV) ventilation resulted in improved hemodynamics in these patients, as evidenced by pulsatile pulmonary blood flow and increased cardiac output. Right-sided stroke work was present and there was a fall in mean central venous pressure (CVP) and rise in mean pulmonary artery pressure (PAP).

Adult respiratory distress syndrome in pediatric patients. II. Management

Adult respiratory distress syndrome, a clinical syndrome of respiratory failure that follows many kinds of insults, often in patients with no previous pulmonary disease, occurs in pediatric patients. This group of disorders has a typical clinical, pathologic, and pathophysiologic course, the hallmark of which is injury to the alveolar-capillary membrane with increased permeability of the pulmonary vasculature and pulmonary edema. Resolution may occur at any stage, but most patients die and many develop chronic lung disease requiring respiratory support for weeks or months. Multiple organ system failure, secondary infection, and irreversible respiratory dysfunction are responsible for the poor outcome. The underlying mechanisms that relate injury to the development of pulmonary disease are unclear. In some cases there may be direct injury to the lung, but in others, such as septic shock, there are mediators that link the initial insult to the subsequent lung injury. The leukocyte may have a central role in this process, although this is uncertain. Therapeutic measures needed to support the patient, especially increased inspired oxygen, are additional factors in the progression of lung disease. Current therapy, as summarized in Table II, is primarily supportive. Efforts to treat ARDS after it is clinically apparent have been disappointing. The pathogenic mechanisms that lead to ARDS are probably well advanced by the time the syndrome is diagnosed on the basis of the usual clinical signs. Therefore an emphasis on understanding the mechanisms of lung injury so that specific markers can be used to predict which patients will develop ARDS, allowing intervention in the early stages of the process, may prove rewarding.

[High frequency ventilation. Study of the mechanisms of action]

Dried lungs and isolated bronchial trees dissected from large animals were submitted to high-frequency oscillation and jet-ventilation. The pattern of intrapulmonary pressure distribution and CO2 diffusion were measured through transalveolar chambers fixed to the perforated pleural surfaces and through airbags pasted on the isolated bronchial trees. Under oscillating conditions, the pressure profiles in different lung and bronchial compartments were inhomogeneous and frequency dependent; the pressure-wave amplitude was proportional to the oscillation frequency. On the other hand, the inhomogeneities found with jet-ventilation were mostly dependent on the airflow direction and position of the intratracheal cannula. Since these inhomogeneities were similar on dissected lungs as well as on isolated bronchial trees, it was concluded that they were essentially dependent on endobronchial aerodynamic effects. But the absence of the in vivo pulmonary and bronchial elastic recoil certainly modified the effects of these ventilation modes with respect to accepted clinical findings. Also results were shown to vary between individuals and within individuals, probably explaining the divergent results obtained by different authors.

A comparative study of conventional versus high-frequency jet ventilation with relation to the incidence of postoperative morbidity in thoracic surgery

Sixty-five patients undergoing a thoracic procedure were randomly allocated to one of two groups. The first group received a conventional method of ventilation (double-lumen endobronchial tube and collapse of one lung) and the second, high-frequency jet ventilation (HFJV). The incidence of postoperative chest infections (clinical and bacteriological), the chest drain volumes, and the length of postoperative stay in the hospital were recorded. Results showed the jet ventilator group had a significantly reduced mean hospital stay (p less than 0.01), which could be attributed to a lower incidence of postoperative chest infections (p less than 0.001) and significantly improved arterial blood O2 tensions at 4 hours (p less than 0.05), 24 hours, and 7 days postoperatively (p less than 0.001).

Effect of alterations in frequency, inspiratory time, and airway pressure on gas exchange during high frequency jet ventilation in dogs with normal lungs

High frequency jet ventilation (HFJV) is becoming increasingly useful for providing respiratory support in patients with normal lungs during operative procedures, and also has been advocated as a technique for ventilating patients during cardiopulmonary resuscitation. We studied the effect of frequency, percent inspiratory time (I/E ratio), peak airway pressure, and airway pressure difference (peak-PEEP) during HFJV as operational variables on the efficacy of gas exchange in dogs with normal lungs. We observed that at a constant peak airway pressure and percent inspiratory time, PaCO2 generally increases as frequency rises above 100/min. In contrast, PaCO2 generally decreases as percent inspiratory time is reduced at a constant frequency and peak airway pressure. In addition, increasing peak airway pressure and airway pressure difference are associated with lower levels of PaCO2. Arterial oxygenation was adversely affected by frequencies above 300/min, but was otherwise not influenced by alterations in frequency, percent inspiratory time, or airway pressure.

Comparison of regional pulmonary perfusion in lobar pneumonia during high frequency and conventional mechanical ventilation in sheep

We compared the effects of high frequency jet ventilation (HFV), conventional ventilation (CMV), and spontaneous breathing (SB) on regional pulmonary blood flows (QLLL), standard cardiopulmonary measurements and the serum levels of the first generation metabolites of prostacyclin (6-keto-PGF1 alpha) and thromboxane A2 (TxB2) in established left lower lobe pseudomonas aeruginosa pneumonia in 11 sheep. Gram negative pneumonia resulted in significant increases in alveolar-arterial oxygen gradients [(A-a)DO2] and pulmonary shunt fractions (Qs/Qt), as well as a significant decrease in QLLL during SB. Significant differences in standard haemodynamics, (A-a)DO2, Qs/Qt, and QLLL were not observed when HFV was compared to CMV. However, serum levels of 6-keto-PGF1 alpha were elevated when the animals underwent HFV. We conclude that HFV is a safe and efficient method of oxygenation and ventilation in unilobar gram negative pneumonia and also results in a significant increase in the serum levels of 6-keto-PGF1 alpha when compared to CMV in sheep. The exact significance of the latter finding is the subject of current investigation.

High frequency ventilation

High frequency ventilation (HFV) presents a new respiratory therapy modality that has taught us much about the theories of gas transport in the lung. Both experimental and clinical applications are summarized. Although the future clinical role of HFV remains uncertain, pediatric applications and investigation continue at the forefront of this new technology.

Necrotizing tracheobronchitis: a complication of high-frequency ventilation

The tracheobronchial histopathologic findings in eight neonates who died after treatment with high-frequency jet ventilation (HFJV) were compared with those in eight similar infants who died after treatment with conventional mechanical ventilation. The HFJV and conventionally treated groups were matched as closely as possible for birth weight, gestational age, and duration of mechanical ventilation. A 4-point, nine-variable histologic scoring system was used to grade tissue changes in the trachea, carina, and mainstem bronchi. The patients who received HFJV had significantly more histologic damage in their tracheas, carinas, and right and left mainstem bronchi. At all levels of the airway examined, HFJV was associated with more inflammation, greater losses of ciliated epithelium, and more mucus within the lumen of the airway than was conventional mechanical ventilation.

Proximal high-frequency jet ventilation of the newborn

During a 14-month period 11 preterm infants were treated with high-frequency jet ventilation for severe respiratory failure that had been unresponsive to conventional mechanical ventilation. Primary indications included intractable air leaks (pulmonary interstitial emphysema, pneumothorax, or both), congenital diaphragmatic hernia, and progressive pneumonia. The Sechrist 990 HFV respirator, a solenoid-driven, pulse-generated high frequency jet ventilator, was used. This device delivers a volume of gas at a controlled pressure to a jet located in the patient connector proximal to a standard single lumen endotracheal tube. The ventilator was operated in tandem with a Sechrist IV-100B ventilator to provide a sigh effect and to improve humidification. Although only 3 of 11 patients survived, 9 displayed short-term benefits from HFJV with significant reduction in mean airway pressure and improvement in mean arterial blood pressure, and no concomitant adverse effects on oxygenation or ventilation. Preliminary results suggest short-term benefits of HFJV in infants who have severe respiratory failure that is unresponsive to conventional mechanical ventilation.

A double-crossover study comparing conventional ventilation with high frequency ventilation in a patient with tracheoesophageal fistula

Respiratory distress, from severe gastric aspiration pneumonitis and abdominal distention in the patient with tracheoesophageal fistula frequently requires mechanical ventilatory support. Bulk flow ventilation can lead to enlargement of the fistulous tract, elevation of gastric intraluminal pressures, raised airway pressures with hemodynamic instability, and retained secretions. We report a case of tracheoesophageal fistula, secondary to perforation of a squamous cell carcinoma of the esophagus, with temporary improvement in gas exchange on high frequency ventilation after failing on a conventional ventilator. The patient initially failed to improve on an Engstrom ventilator (Engstrom-Gambro, Inc., Barrington, IL) at 13 l/minute ventilation. Instituting high frequency jet ventilation with a VS 600 Jet Ventilator (Instrument Development Corporation, Pittsburgh, PA) at initial settings of 35 psi, rate 150, inspiratory time 40%, FiO2 0.8 and 12 cm H2O positive end expiratory pressure (PEEP), provided incremental improvement in gas exchange and oxygenation up to 26 cm H2O PEEP. However, in view of progressive multi-organ failure we terminated the jet ventilation after 48 h and returned the patient to conventional ventilation. We were unable to provide life-sustaining ventilation and oxygenation with either an Engstrom ventilator at 13 l/-minute ventilation or an MA-1 ventilator (Puritan-Bennett, Kansas City, MO) at a tidal volume of 800 cc and a ventilator rate of 30. Terminal respiratory failure occurred. Based on the period of improvement using high frequency jet ventilation, we believe this mode of ventilatory support is beneficial in the management of tracheoesophageal fistula.

High-frequency jet ventilation for tracheal surgery

The anaesthetic requirements for tracheal resection include a clear airway, adequate ventilation and good surgical access. Many techniques have been described, none of which is entirely satisfactory. This case reports on the use of high-frequency jet ventilation with 10Fg catheter combined with muscle relaxants and intravenous anaesthesia in a patient with tracheal stenosis.

Clinical applications of high-frequency jet ventilation

Six patients with acute respiratory failure were treated with high-frequency jet ventilation (HFJV): 3 because they developed barotrauma while on conventional mechanical ventilation (CMV), 2 because of sedative- or PEEP-induced hypotension on CMV, and 1 because of bronchopleural fistula. In all patients, except the one with bronchopleural fistula, who was treated from the start with HFJV, gas exchange before (while on CMV) and after institution of HFJV could be compared. In these five patients, including the two with acute respiratory failure not complicated by barotrauma, gas exchange was better during HFJV than during CMV for the same levels of FIO2 and PEEP. HFJV therefore seems the method of choice for ventilatory support, not only in patients with bronchopulmonary disruption, but also in patients with hemodynamic embarrassment during CMV.

Early prediction of outcome of respiratory failure. Comparison of high-frequency jet ventilation and volume-cycled ventilation

Data from a prospective randomized investigation comparing volume-cycled ventilation and high-frequency jet ventilation were reexamined to determine whether improvement of respiratory and hemodynamic function, as well as ultimate outcome (death or survival), could be predicted early in the course of the disease. End points were selected for the ratio of the arterial oxygen pressure over the fractional concentration of oxygen in the inspired gas (PaO2/FIO2), the arterial oxygen saturation (SaO2), the arterial carbon dioxide tension (PaCO2), and the cardiac index. Patients were assigned to "success" or "failure" groups, according to the values recorded for those end points 24 hours after institution of mechanical ventilation. Values obtained from initiation of mechanical ventilation to 16 hours later were divided into four time groups. Differences between patients who "succeeded" and "failed" were compared at each time interval. Ultimate outcome was also compared. The PaCO2 and cardiac index were poor predictors of survival. Early values did not foretell the progression of these variables. The PaO2/FIO2 and SaO2 effectively discriminated, at all time intervals, between patients who succeeded and failed on volume-cycled ventilation. On high-frequency jet ventilation, significant differences were evident only after eight hours of support. With both types of ventilator, patients who reached the end point of oxygenation at 24 hours survived in far greater numbers than those who did not. On the basis of this investigation, it appears justified to attempt high-frequency jet ventilation in patients who do not rapidly improve on volume-cycled ventilation. Institution of high-frequency jet ventilation as the initial support method may not be advisable, since failure does not become apparent for many hours.

High-frequency jet ventilation vs continuous positive airway pressure for postoperative respiratory support

In ten patients after recovery from general anesthesia for major vascular surgery, we compared continuous positive airway pressure (CPAP) and high-frequency jet ventilation (HFJV) at the same fractional concentration of oxygen in the inspired gas, both being superimposed on spontaneous breathing. The HFJV was delivered by a ventilator ( Acutronic VS-600) through the lateral lumen of a three-lumen endotracheal tube. Mean airway pressure was increased (3, 8, and 11 mm Hg) by adjusting the driving pressure during HFJV and by the setting of the expiratory valve during CPAP. During HFJV, the frequency (10 Hz) and inspiratory-expiratory ratio (0.25) were kept constant. Control values were measured when patients breathed spontaneously without positive end-expiratory pressure. Cardiac output and transmural filling pressures remained unchanged throughout the study. Neither the arterial oxygen pressure (PaO2) nor intrapulmonary shunt (Qs/Qt) changed significantly during CPAP. The 20- to 30-percent increase in PaO2 and the 5- to 10-percent decrease in Qs/Qt during HFJV when compared to control reflected a slightly but significantly (p less than 0.05) better oxygenation. The increase in arterial carbon dioxide tension was significantly greater during CPAP than during HFJV.

High-frequency jet ventilation. A prospective randomized evaluation

Three hundred nine patients were randomly allocated to two ventilatory protocols; 157 patients were supported with a volume-cycled ventilator (VCV) (Bear Medical BEAR 1) and 152 with a high-frequency jet ventilatory (HFJV) developed at our institution. The two ventilators were compared for safety, reliability, ease of use, and efficacy in maintaining gas exchange. On VCV, end points of therapy were: fractional concentration of oxygen in the inspired gas (FIo2) less than or equal to 0.40; arterial oxygen pressure (PaO2) greater than or equal to 70 mm Hg; cardiac index (CI) greater than or equal to 3.5 L/min/sq m; and spontaneous respiratory rate less than or equal to eight breaths per minute. On HFJV, end points were: FIo2 less than or equal to 0.45; arterial oxygen saturation greater than or equal to 0.90; and CI greater than or equal to 3.5 L/min/sq m. Spontaneous ventilation and pulmonary venous admixture reduction were the goals on VCV, with oxygen transport the goal on HFJV, Total duration of use of the ventilators was approximately 800 days with both types of devices; there were no technical failures, and the incidence of barotrauma was less than 5 percent. The end point of mechanical ventilation was reached by a significantly higher percentage of the patients randomized to HFJV. Patients who failed to reach the therapeutic goal within 24 hours were crossed over to the other form of support. Those crossed from VCV to HFJV improved more rapidly and in greater number than those crossed from HFJV to VCV. When survival and total duration of stay in the intensive care unit were considered, there was no difference between VCV and HFJV. Considering data on gas exchange, VCV provided a higher PaO2 at equivalent positive end-respiratory pressure than HFJV. Alveolar ventilation was slightly better on HFJV. Differences were statistically but not clinically significant. On HFJV, oxygenation and ventilation were maintained with lower peak inspiratory pressures and smaller tidal volumes than those required for VCV. This investigation proves that HFJV is a safe and reliable method to provide mechanical support which does not, at this time, offer obvious benefits over VCV.