HFOV

The high frequency oscillatory ventilator (HFOV) has been around for quite some time now. It has been used on adults to treat ARDS, but its main implementation is on the neonate RDS (respiratory distress syndrome) population. 

HFOV has 5 main knobs. 

1. Amplitude 

2. I-Time %

3. Frequency (Hz)

4. Flow

5. Mean pressure

Amplitude - This knob can also be referred to as "delta P" or "driving pressure." This is the main knob adjusted for CO2 correction. Increasing the amplitude will increase the amount of volume delivered through each oscillation. As in conventional ventilation, increased volumes will decrease CO2 levels in the blood. So always remember that this is the first line of defense to reduce the PaCO2 on a blood gas. 

To ensure the patient has adequate volume, obtain a blood gas and adjust for CO2 accordingly. 

In addition, adjust the amplitude for chest wiggle. Chest wiggle ensures that there is an adequate volume being delivered. For a neonate, the chest should wiggle. For a pediatric, the chest down to the navel should wiggle. For the adult, the shoulders down to the mid-thigh should wiggle. 

I-Time % - A typical I:E ratio is 1:2. On the HFOV, a patient doesn't have a true I:E ratio as in conventional ventilation. The oscillations happen so fast that one can not physically count and determine the I:E ratio. But the oscillator will deliver an oscillation at the 1:2 ratio. 

So how do you set an I:E of 1:2? First, take the sum of the I:E (1+2). Then, divide 1 by the sum of the I:E (1/3). This comes out to 33%. Therefore, for an I:E of 1:2, set the I-Time % to 33%. 

Typically, this knob is rarely adjusted. I suppose if you wanted to dedicate more time to "exhalation" during the oscillation, you can change the I:E to 1:3. This translates to an I-Time % of 25% (1/4). 

Frequency - The frequency determines how many cycles ('breaths') the patient will receive. 1 hertz (Hz) delivers 60 cycles per minute. Depending on the size of a premature neonate, the Hz can be set as high as 15. This comes out to 900 cycles per minute!

Some wrongly think that increasing the Hz will have the same effect as in conventional ventilation when the RR is increased. However, this isn't the case. In HFOV, increasing the Hz will actually reduced the amplitude (volume) of each oscillation. If volume is reduced, PaCO2 increases. 

Here's what I mean: If you had a PaCO2 of 60mmHg, in conventional, you could increase the RR to reduce the CO2. In HFOV, if you increase the Hz, you'll increase the CO2. If anything, the Hz should be decreased because this will increase the volume, which will help correct a high CO2. 

The general rule of thumb is: The faster it oscillates, the smaller the volume. 

Therefore, the primary control to correct a high CO2 is to increase the amplitude. The secondary control would be to decrease Hz. 

Flow - Flow is set to allow the circuit to be pressurized. This ensures that there is no lapse from when the oscillator sends a volume to when the patient receives the volume. If the patient exhibits retractions, increase the flow. They're trying to 'pull' a breath. Increasing the flow ensures it reaches the patient quickly. 

What's a good initial setting? The smaller the patient, the lower the flow. The bigger the patient, the higher the flow. For a small premature neonate, a flow of 10-15L/m is acceptable. For an adult, 40L/min is acceptable. 

Mean Pressure - Mean Pressure inflates the lung to the set pressure and keeps it there. The lung does not deflate. This helps distend the alveoli, whereby recruitment is enhanced. This is the primary control to improve lung compliance and oxygenation. You can initially set it between 15-20cmH2O, and adjust it upwards to 30cmH2O for non-compliant, atelectatic lungs. 

Others suggest to set the mean pressure to the same, or slightly above, the plateau pressure the patient had on conventional ventilation. This is a good idea if the patient was previously on a conventional ventilator and the therapist had the ability to perform and record an accurate plateau pressure. This is achieved by performing an inspiratory hold for about a second. In other words, on a conventional ventilator, you can press the 'inspiratory hold' feature, and when the patient inhales, the ventilator will hold that breath and measure the pressure in the lung. That pressure is the plateau. 

When all is said and done, the HFOV looks intimidating but as you play with it, you'll discover it is pretty simple. It's been around for quite some time for a reason. As long as you maintain a safe mean pressure, HFOV is a safe method in treating patients with ARDS.