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HFV

Created: 19/2/2005

 

High Frequency Ventilation


HFV is a new technique of ventilation that uses respiratory rates that greatly exceed the rate of normal breathing. HFV is defined by the ‘high frequency’ (2.5-15 Hz) and low tidal volume (0.5-5 mL/kg). The tidal volume is barely greater than the dead space hence alternative mechanisms of gas transport are required to explain the effect of HFV.

There are three principal types of HFV: 

 High frequency positive pressure ventilation (HPPV, rate 60-150/minute) 
 High frequency jet ventilation (HFJV, rate 100-600)
 High frequency oscillatory ventilation (HFOV, rate 300-3000/minute)

Indications for use of high frequency ventilation are unclear but include: 

 Initial and subsequent ventilatory support in very low birth weight infants with respiratory distress syndrome.

 Air leak (pneumothorax, pulmonary interstitial emphysema). 

 Failure of conventional ventilation (pre-ECMO step) particularly in persistent pulmonary hypertension of the newborn, meconium aspiration syndrome, pneumonia, pulmonary haemorrhage. 

 To reduce the risk of volutrauma and barotrauma when conventional ventilator settings are very high. 

Advantages of HFOV

May allow gas exchange when conventional ventilation has failed. The advantage of high frequency oscillatory ventilation as compared to either conventional positive pressure or jet ventilation is its ability to promote gas exchange while using tidal volumes that are less than dead space. The ability of HFOV to maintain oxygenation and ventilation while using minimal tidal volumes allow clinicians to minimize barotrauma and thus reduce the morbidity associated with ventilator management of RDS. HFO ventilation is the delivery of small tidal volumes to the infant at fast frequencies. Both inspiration and expiration are active, therefore reducing the likelihood of gas trapping.

Disadvantages of HFOV

It is unclear which patients will respond and there is some risk involved in trialling HFOV. Switching ventilators on an unstable patient who is failing conventional ventilation may result in clinical deterioration. The high airway pressures often seen with high frequency ventilation can be transmitted to the heart (particularly with compliant lungs) and result in impaired cardiac output requiring inotropes and/or volume boluses. HFOV makes turning patients, taking x-rays, or performing ultrasounds more complex due to the heavy, non-flexible tubing. Stopping HFOV for suctioning or administering nebulized medications may negate its benefit.

Terminology

Amplitude
The maximum extent of a vibration or oscillation from a point of equilibrium.

Hertz
SI unit of frequency, equal to one cycle per second.

CMV
Conventional ventilation.

CPAP
Continuous positive airway pressure.

MAP
Mean airway pressure.

MAS
Meconium aspiration syndrome.

PFC/PPHN
Persistent fetal circulation also called persistent pulmonary hypertension caused by a sustained increase in pulmonary vascular resistance after birth, preventing the transition to a normal extrauterine circulatory pattern.

PTV
Patient triggered ventilation.

PVL
Perventricular leucomalacia. A decrease in cerebral blood flow causing anoxia resulting in necrosis of periventricular white matter.

RDS
Respiratory distress syndrome.

SIMV
Synchronous intermittent mandatory ventilation.


ArticleDate:20050219
SiteSection: Article
 
   
    
                                            
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