High-Frequency Percussive Ventilation – Using the VDR-4

What is VDR?

VDR, Volumetric Diffusive Respiration, is a pneumatically driven pressure-limited and time-cycled ventilator that provides high-frequency percussive ventilation (HFPV).  It delivers biphasic oscillatory breaths, with the tidal volume being the product of the PIP setting and subtidal volumes produced by the oscillations. The VDR is an invention of Dr. Forrest Bird with the concept of mechanically ventilating the lungs with successive percussive subtidal volume deliveries.  

How does it work?

This type of ventilation uses both conventional and percussive ventilation (similar to IPV but much faster).  The phasitron is driven by a high-pressure gas supply delivering a high percussive rate of 200-900 beats/minute superimposed on a conventional pressure-controlled cycle usually set at a rate of 10-15 breaths/minute.  During inspiration, lung volumes are progressively increased by repetitively diminishing subtidal volume delivered until an oscillatory plateau is reached and maintained.  At end inspiration, the lung is allowed to empty passively until the expiratory baseline is achieved. This type of ventilation allows for airway clearance, lung recruitment, and rapid air exchange.  It can be used to manage patients with severe lung disease from the neonatal to the adult population. 

It’s all about the phasitron.

Unique to HFPV is the presence of a phasitron.  The phasitron is located at the end of the endotracheal tube.  The phasitron works off a sliding venturi principle which acts as both an inspiratory and expiratory valve. This allows for gas exchange and the delivery of rapid, high flow, mini-bursts into the lungs while simultaneously delivering therapeutic aerosols.  Due to the venturi effect, the flow delivered is converted to pressure and vice versa by adapting to lung resistance.  This allows for flow distribution to be optimized at the level of the airways and avoid preferential ventilation. 

The VDR can be used to treat the following conditions 

• Early non-focal ARDS
• Salvage therapy in ARDS
• Pneumonia
• Pre-ECMO patients 
• Thoracic trauma
• Smoke inhalation
• Cystic fibrosis 
• Alpha-1
• SMA
• ALS

Why would we use this?

The VDR would be used in patients to mobilize pulmonary airway congestion by secretion retention, mucosal and submucosal edema, and bronchial spasm.  But how?  The percussive high rate high-energy mini-bursts of subtidal volumes travel down the centers of the airways because that path is the path of least resistance.  Exhaled gas then travels in a countercurrent path along the inside walls of the airways.   Coupled with humidification, this flow pattern gives the VDR-4 the ability to mobilize pulmonary secretions from the airways.  In patients with high CO2, this can be useful due to bilateral uniform alveolar ventilation that enhances oxygen uptake and carbon dioxide elimination.  The high-frequency percussive endobronchial delivery of subtidal volumes to the lung is percussively pulsed to produce intrapulmonary mechanical gas mixing. This gas mixing serves to create intense endobronchial diffusion.  

Side note

Our concern as the respiratory therapist is protecting the patient’s lungs from damage, and using the VDR can do that if used correctly.   HFPV helps prevent baro/volu trauma to the lungs and improves gas exchange.  However, if not used properly, this can deliver three high-pressure breaths and cause a pneumothorax.   If you connect a patient before the machine is turned on, it can cause high pressures to a PIP of 65-70cmH2O. This machine needs to be started to pressurize before placing in line with the patient and ventilator.  It will alarm, but there isn’t a dump.  Always pay attention to your alarms. 

Conclusion

The concept of HFPV offers the potential advantage over conventional ventilation of providing improved oxygenation and ventilating at lower peak airway pressures along with improved secretion clearance.