Combined use of HFPPV with low-rate ventilation in traumatic respiratory insufficiency

Moderately high frequency ventilation with a conventional ventilator allows reduction of tidal volume without increasing mean airway pressure

Background

The aim of this study was to explore if positive-pressure ventilation delivered by a conventional ICU ventilator at a moderately high frequency (HFPPV) allows a safe reduction of tidal volume (V_T) below 6 mL/kg in a porcine model of severe acute respiratory distress syndrome (ARDS) and at a lower mean airway pressure than high-frequency oscillatory ventilation (HFOV).

Methods

This is a prospective study. In eight pigs (median weight 34 [29,36] kg), ARDS was induced by pulmonary lavage and injurious ventilation. The animals were ventilated with a randomized sequence of respiratory rates: 30, 60, 90, 120, 150, followed by HFOV at 5 Hz. At each step, V_T was adjusted to allow partial pressure of arterial carbon dioxide (PaCO₂) to stabilize between 57 and 63 mmHg. Data are shown as median [P25th,P75th].

Results

After lung injury, the PaO₂/FiO₂ (P/F) ratio was 92 [63,118] mmHg, pulmonary shunt 26 [17,31]%, and static compliance 11 [8,14] mL/cmH₂O. Positive end-expiratory pressure (PEEP) was 14 [10,17] cmH₂O. At 30 breaths/min, V_T was higher than 6 (7.5 [6.8,10.2]) mL/kg, but at all higher frequencies, V_T could be reduced and PaCO₂ maintained, leading to reductions in plateau pressures and driving pressures. For frequencies of 60 to 150/min, V_T progressively fell from 5.2 [5.1,5.9] to 3.8 [3.7,4.2] mL/kg (p < 0.001). There were no detrimental effects in terms of lung mechanics, auto-PEEP generation, hemodynamics, or gas exchange. Mean airway pressure was maintained constant and was increased only during HFOV.

Conclusions

During protective mechanical ventilation, HFPPV delivered by a conventional ventilator in a severe ARDS swine model safely allows further tidal volume reductions. This strategy also allowed decreasing airway pressures while maintaining stable PaCO₂ levels.

Mechanical ventilation strategies in massive chest trauma

Patients in extremis because of trauma-related massive chest injury require expedient evaluation and prompt intervention. The initial pathophysiology relates to the significant intrapulmonary shunting caused by disruption of pulmonary capillaries and extravasation into the alveolar spaces. Disproportionate or unilateral lung involvement needs measures more technical than general supportive care. Independent lung ventilation (mostly with unilateral lung involvement) and other strategies like inhaled nitric oxide, prone positioning, partial liquid ventilation, and extracorporeal membrane oxygenation (ECMO) have had good results. Intensivists confronted with this clinical subset may consider using these strategies as alternative/adjunctive options for optimizing respiratory and hemodynamic status in the supportive management of trauma-related acute lung injury (ALI) and adult respiratory distress syndrome (ARDS).

Superimposed high frequency ventilation with conventional mechanical ventilation

A 73-year-old man with ARDS-multiple organ failure due to Chlamydia psittaci, was successively supported with conventional respiratory techniques. After 48 hours of no clinical improvement, HFV was superimposed to CMV in order to combine the advantages of each one. Since improvement has been seen in all ventilatory parameters, this method is suggested as another mode of ventilation for patients with refractory hypoxia and hypercarbia who do not respond to conventional respiratory care.

Hemodynamic effects of high frequency ventilation superimposed on intermittent positive pressure ventilation in dogs

The hemodynamic effects of high frequency ventilation (HFV) superimposed on intermittent positive pressure ventilation (IPPV) in seven dogs before and after thrombin infusion were investigated. HFV was superimposed on a Servo 900 B ventilator by a Siemens Elema HFV prototype unit. Mean arterial blood pressure, heart rate, central venous pressure, pulmonary artery pressure, cardiac output, right and left ventricular pressures, pleural pressure, arterial blood gases, and right and left ventricular ejection fractions were recorded. Measurements were done during IPPV alone and during HFV superimposed on IPPV. The HFV frequencies were 5, 15, and 20 Hz at a constant minute volume of 5 1. When HFV was started, the IPPV minute volume was reduced to one third of the initial volume. No significant changes in the measured parameters were observed during the different ventilatory modes either before or after thrombin infusion which doubled the pulmonary vascular resistance. It is concluded that high frequency ventilation superimposed on IPPV might be a ventilatory mode that offers cardiovascular stability and reduces the risk of barotrauma.