Intermittent positive pressure ventilation and high frequency ventilation in dogs with experimental bronchopleural fistulae

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.

Pleural Disease in the Intensive Care Unit

Pleural disease itself is an unusual cause for admission to the intensive care unit (ICU). Pleural complications of diseases and procedures in the ICU are common, however, and the impact on respiratory physiology is additive to that of the underlying cardiopulmonary disease. Pleural effusion and pneumothorax may be overlooked in the critically ill patient due to alterations in radiologic appearance in the supine patient. The development of a pneumothorax in a patient in the ICU represents a potentially life-threatening situation. This article reviews the etiologies, pathophysiology, and management of pleural effusion, pneumothorax, tension pneumothorax, and bronchopleural fistula in the critically ill patient. In addition, we review the potential complications of thoracentesis and chest tube thoracostomy, including re-expansion pulmonary edema.

Independent lung ventilation

Situations in which independent lung ventilation may be of use include massive hemoptysis, pulmonary alveolar proteinosis, risk of interbronchial aspiration, unilateral lung injury, single lung transplant, and BPF. Any decision to attempt independent lung ventilation should take into consideration the many technical difficulties associated with the procedure. They include difficulties in the placement of DLTs and monitoring tube position, the risk of tube displacement, and the risk of airway trauma. The clinician also must consider the costs in terms of available manpower and resources. Maintaining a patient on independent lung ventilation requires highly skilled nursing care, specialized monitoring devices, and readily available FOB. Even with these limitations, independent lung ventilation may be of use in certain clinical situations when standard methods have failed.

Lower airway injuries and anaesthesia

PURPOSE

The perioperative management of lower airway injuries is a difficult clinical problem. Since few reviews present the management of this injury from an anaesthetic perspective, we undertook a literature review of this topic.

SOURCES

A computerized search of the National Library of Medicine database using tracheal or bronchial injury as key words produced 140 English language citations. An eight-year chart review outlining our experience in an urban Canadian setting is also presented.

FINDINGS

The most frequent findings in patients with injury to the lower airway are dyspnoea and surgical emphysema. Other findings include cough, haemoptysis, sucking neck or chest wounds, mediastinal emphysema or pneumothorax. Endoscopy with a fiberoptic scope is the technique of choice for diagnosis, airway management and as a preparatory step in planning of the surgical repair. An airway technique employing direct vision is preferable to blind attempts during tracheal intubation. The use of a double lumen endobronchial tube or selective endobronchial intubation may be needed to achieve adequate pulmonary ventilation. A number of prospective randomized clinical trials comparing conventional mechanical ventilation with high frequency jet ventilation in patients with acute lung injury have demonstrated no difference in effectiveness of ventilation or oxygenation.

CONCLUSIONS

Patients with lower airway injuries usually present when they are least expected and are a challenge to manage. The clinical presentation of a lower airway injury may be overt or subtle. Resuscitation and anaesthetic management are directed towards control of the airway, maintenance of adequate pulmonary ventilation and management of blood loss.

Cardiorespiratory effects of conventional and high frequency ventilation in rabbits with bilateral pneumothoraces and surfactant depleted lungs

We compared high frequency ventilation (HFV) to conventional mechanical ventilation (CMV) under normoxic and normocapnic condition in surfactant depleted rabbits with bilateral pneumothoraces. We hypothesized that lower airway pressures would be required with HFV under these conditions. We applied CMV and HFV in 8 anaesthetized rabbits with a prototype ventilator at frequencies of 30, 100, 200, and 300 cycles/min. A positive end-expiratory pressure (PEEP) just below the pressure sufficient to open the air leak from the pneumothoraces was applied at all frequencies. Airway pressures, gas exchange, heart rate, and mean arterial pressure were recorded. Peak airway pressure decreased significantly from 2.50 to 2.10 kPa when the frequency of ventilation was increased from 30 to 300 cycles/min. There were no significant changes in mean airway pressure, PaO2, arterial pH, heart rate, and mean arterial pressure when HFV was compared to CMV. In conclusion, during HFV peak airway pressures measured at the mouth were decreased. Our ability to maintain adequate gas exchange in the face of ongoing pulmonary air leaks may reflect lower alveolar pressures.

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.

Conventional versus high frequency jet ventilation with a bronchopleural fistula

A bronchopleural fistula (BPF) is an aberrant pathway through which inspired gas exits the lungs. A BPF may cause significant respiratory compromise, which in turn may result in the need of mechanical ventilation. The purpose of this study was to compare the efficacy of conventional positive pressure ventilation (CV) with high frequency jet ventilation (HFJV) using increasing increments of positive end expiratory pressure (PEEP) in the management of an induced BPF. A reproducible model of a BPF was surgically created in 10 mongrel dogs. Measurements of blood pressure (BP), cardiac output (CO), mean airway pressure (Maw), peak airway pressure (Paw), and fistula flow (FF) were carried out with the chest closed. Selective occlusion of the BPF allowed for blood gas stabilization at increased values of PEEP. Paired observations were performed at 0, 5, 10, 15, and 20 cm H2O of PEEP, while maintaining PaCO2 between 30 and 50 Torr. There was no difference in BP or CO between ventilation methods even though significantly lower Maw and Paw pressures were obtained using HFJV. While FF increased significantly with each increment of PEEP, there was no improvement in flows obtained using HFJV. This acute model of a BPF demonstrated that increasing PEEP dramatically increases FF irrespective of the method of ventilation.

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].

The performance of four pleural drainage systems in an animal model of bronchopleural fistula

In order to compare the performance of four pleural drainage units (PDU [Emerson Post-Operative Pump, Pleur-Evac, Sentinel Seal, Thora-Klex]), we created an animal model of bronchopleural fistula that simulated the type of air leak seen clinically (mean maximal flow = 5 L/min). The PDU were tested at 0 cm (water seal), -20 cm and -40 cmH2O suction. Compared to water seal, -20 cmH2O suction significantly increased the ability of all four PDU to evacuate air via the chest tube and abolished small differences in chest tube air leak seen among the PDU at water seal. An increase in suction to -40 cmH2O did not significantly alter flow via the chest tube. Previously demonstrated differences among the PDU in handling large air flows were not seen in this lower flow model of bronchopleural fistula. However, because of their higher resistance, use of the Sentinel Seal and of the Thora-Klex was technically impractical even at air leaks of 4 to 5 L/min.

High-frequency ventilation

Over the last six years high-frequency ventilation has been extensively evaluated both in the clinical and laboratory settings. It is now no longer the great mystery it once was, and it is now no longer believed (as many had hoped), that it will solve all the problems associated with mechanical pulmonary ventilation. Although the technique is safe and appears to cause no harm even in the long term, it has not yet been shown to offer any major advantages over conventional mechanical ventilation.

Pulmonary vascular response to high-frequency oscillatory ventilation in dogs: lack of effect of indomethacin

The role of prostaglandins (PG) in the pulmonary hypertension associated with high-frequency oscillatory ventilation (HFOV) was investigated in 7 pentobarbital-anesthetized dogs. The magnitude and time course of the mean pulmonary artery pressure (PAP) changes induced by HFOV (20 Hz) was quantified before and after PG-production blockade with 15 mg/kg indomethacin (INDO) i.v. It was found that HFOV before INDO induced a 27.2 +/- 5.6% increase in PAP above baseline and a 30.0 +/- 7.2% one hour after INDO administration. This difference was not statistically significant. The time course of the PAP changes induced by HFOV before and after blocking PG-production by INDO were also similar. Mean airway pressure, PaO2 and PaCO2 during HFOV, before and after INDO were not statistically different. It is concluded that the acute increase in PAP induced by HFOV is not due to PG release.

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.