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Clinical Skills: High-frequency oscillatory ventilation (HFOV)

Notes

Transcript

Content Reviewers:

Hello! This video covers how to set up a patient on high frequency oscillatory ventilation. Normally you’d do everything on this list, but to keep things concise, this video will focus on the steps in blue and will also cover frequently asked questions, troubleshooting, tips, and a summary.

As a word of caution, we’re not covering every possible type of equipment on the market. Make sure you understand how your own equipment works and how it may affect your procedure.

Here is all the equipment you will need: a high frequency oscillatory ventilator, humidified oxygen source, an oxygen saturation monitor, a cardiac and end tidal or transcutaneous CO2 monitor, neuromuscular blocker medications, personal protective equipment, suction equipment, and a self-inflating bag. 

High frequency oscillatory ventilation, or HFOV, differs from conventional ventilation in that instead of delivering a set number of breaths at a certain pressure or volume, HFOV provides volumes equal to or less than the anatomical dead space using respiratory rates with a range of 3 to 15 Hertz which is also 3 to 15 cycles per second. For clarity, 3 to 15 cycles per second is equivalent to 180 to 900 breaths per minute! Whoah.

The lungs are partially inflated to maximize surface area for gas exchange, and the fast breaths allow for a large volume of gas exchange to occur. The fast, small breaths also help reverse and prevent atelectasis, improve CO2 elimination, and reduce the risk of barotrauma and volutrauma which may occur in conventional ventilation.

HFOV also can be employed in patients with acute lung injury, which is also called ALI, and acute respiratory distress syndrome, which is also called ARDS, but is not the first choice in these circumstances.

Calibration of the ventilator absolutely MUST be done before attaching the ventilator to a patient. Use a rubber stopper to block the circuit for calibration and performance procedures. The water trap stopcock should also be closed. If you find an issue (software- or hardware-related) with the machine while calibrating it - just don’t use that ventilator. Go find another machine.

First, adjust ‘bias flow’, which is the constant flow of gas to be delivered to provide oxygen and remove carbon dioxide; it is created by the movement of the piston diaphragm in the circuit. An initial bias flow is usually 20L/min, and it can be set to a rate up to 60 L/min. This flow works to generate the mean airway pressure, also called mPaw, or the average pressure in the airway at any given time. The mPaw is adjusted to provide gentle alveolar distension, to maintain a surface area appropriate for gas exchange.

Our mPaw is based on our patient's oxygenation requirements, as a higher mPaw improves oxygenation. Usually the initial mPaw is set to 5cm/H2O higher than the last mean airway pressure used on conventional ventilation (prior to switching to HFOV). This will usually be around 25 to 35 cm/H2O. Remember, we’re trying to obtain optimal lung volume and alveolar recruitment, which is gentle popping open of those alveoli, without over-distending the lungs. Lung volumes are slower to change with ventilator adjustments. 

As an example, let’s set our bias flow to 20 cm of H2O. 

Now we choose our ‘power control’. This will tell the piston or diaphragm how much power to push and pull the air with. It creates the amplitude number shown.  As you adjust your power setting, the circuit will calculate the fluctuations of air and translate them into a number we recognize as the amplitude.  

Amplitude is the key setting responsible for ventilation, or CO2 clearance. An increase in amplitude can enhance CO2 clearance by increasing the total tidal volume of the breath circuit. Again, tidal volumes change in small increments. But with a high respiratory rate, we increase the total volume of air moved in a minute, also called minute ventilation.

Usually amplitude is set initially at 90cm/H2O.  If amplitude is too low, the patient will be underventilated. And if the amplitude is too high, we can cause trauma to the lungs. A good amplitude is measured by observing a patient’s wiggle while attached to the HFOV. 

HFOV “wiggle” describes where on the patient’s body the vibrations from the HFOV machine can be seen, which helps us determine if the amplitude setting is set properly. Of course, we will continue to check the blood level of CO2 with arterial blood gases to titrate the amplitude, but wiggle is an important visual cue, as it is directly proportional to the amount of pressure transmitted to the lungs. Wiggle is also known as CWF, or “chest wiggle factor.”

In infants, the wiggle should be visible to the umbilicus; in children, as low as the hip bones; and in adolescents and adults, the wiggle should end mid-thigh.

As a quick tip: changes in the wiggle factor can alert you to changes in your patient's lung compliance or other problems like mucous plugs or endotracheal tube dislodgement.

Next, we’ll set the rate of oscillation for our patient. The fast rates of HFOV are created by a piston in the ventilator that pushes and pulls air, simulating small inspirations and expirations. These positive and negative pressure swings in cycles are measured in Hertz, where 1 Hertz equals 60 breaths/minute. 

Typical adult settings are 6-8 Hertz which equals 360-480 breaths/minute. Remember that too high a frequency won't allow for adequate gas exchange, so your CO2 will rise.  Inversely, too low of a frequency will allow for a longer gas exchange so your CO2 will drop.

Frequency is the second most important setting for ventilation and is a setting that is rarely changed once it’s established. Let’s set our frequency to 8 Hertz. 

Now let’s set our initial inspiratory time, also called Ti.  Normal adult respiration is 12-20 breaths per minute and the typical inspiratory time setting is 0.8-1.2 seconds. 

For ease of illustration, let's think about an inspiratory time of 2 seconds. 

If we make an inspiratory/expiratory ratio (also called I:E ratio) of 1:1, the expiratory time will also be 2 seconds and this will create 4 second breaths. 

For reference, a typical adult setting for inspiratory/expiratory ratio is 1:1.5 to 1:2. A trick to remember this ratio is to think of your own breathing! Expiration is longer than inspiration.

So, even though we won’t set a respiratory rate on the HFOV, we will set our Ti at the same 1:2 ratio as conventional ventilation to help prevent air trapping.

For the Fi02 we’ll start at 100% and we’ll wean to the lowest tolerated Fi02 as the patient adjusts to the HFOV. 

Now, any patient that requires HFOV also requires close monitoring.  Now that we have the ventilator set up and ready to go, be sure that your patient is connected to a cardio/respiratory monitor with an Sa02 and an end tidal or transcutaneous CO2 monitor before connecting to the circuit.

Okay! Now that we’re ready to connect to the patient, we first need to first suction our patient’s endotracheal tube to ensure it’s clear of secretions. Also, always make sure your patient is properly sedated and muscle relaxed. 

HFOV tubing is very stiff in comparison to conventional ventilation and has the potential to become very hot. Be sure to keep the tubing straight with no possibility of kinks and far away from your patient's skin, or cover the tube with a light cloth to protect them from burns. Also, always angle the tubing to drain any humidity build up away from the patient, directing it toward the water trap on the machine. 

Alright, now, connect the ventilator tubing securely to the end of the patient’s endotracheal tube. 

The last and most important thing to remember is to set your ventilator alarms. For patient safety, set your alarms tightly, just one or two points above and below our programmed settings.