Dr Shri Alurkar
Does the thought of high frequency ventilation make you shake with fear? In this article, Shri Alurkar, one of the Nottingham PICU Consultants explains when to use it, what it does and how to turn the knobs.
High frequency oscillatory ventilation (HFOV) is an alternative method of mechanical ventilation which can help a patient out in specific circumstances, and can be used as a ‘lung protective strategy’ in the management of some severe lung conditions.
In short: when a child or infant can no longer move enough air in and out of their lungs to maintain gas homeostasis, for whatever reason, this is our cue to step in with some form of mechanical ventilation (see this earlier post for the basics).
All forms of mechanical ventilation are of course unphysiological, uncomfortable and potentially damaging to the lungs themselves. In severe lung disease invasive, high pressure ventilation can be very damaging, and actually end up worsening the lung condition. We still don’t know to what extent the different factors damage the lungs, but high pressures (barotrauma), shear stress (large tidal volumes) and high inspired oxygen levels are all culprits. HFOV is a way to reduce maximum pressure, reduce tidal volume and reopen those alveoli, improving oxygenation.
So… what is HFOV?
In conventional ventilation, large pressure changes (the difference between PEEP and PIP) recreate ‘physiological’ tidal volumes, while gas exchange depends on bulk convection (expired gas exchanged for inspired gas).
HFOV is different. HFOV uses a constant distending pressure (mean airway pressure [MAP]) with pressure variations oscillating around the MAP at very high rates – up to 900 cycles per minute. This creates small tidal volumes (less than the dead space of the lung and the circuit). HFOV relies on fancier mechanisms of gas exchange such as (take a deep breath, it’s a long list…) molecular diffusion, dispersion, turbulence, Pendelluft, cardiogenic mixing and collateral ventilation.
PHYSICS ALERT! – how gases are exchanged in HFOV
The rapidly oscillating pressures cause high gas velocities and turbulence – these molecular movements make gases better at mixing. With convective streaming, gas in the middle of the airways and gas next to the airway walls moves at different speeds depending on the phase of respiration.
The overall effect is that gas in the middle of the airway moves into the lungs and gas near the airway wall moves out of the lungs. This works even with small tidal volumes.
Where the gas flow comes to an airway bifurcation, the parabolic profile becomes skewed (asymmetric velocity), enhancing the effect as gas moves faster on the inner wall. The net effect is gas streams into the lung along the inner wall and out along the outer wall.
Radial mixing (or Taylor dispersion) is proposed to enhance gas mixing where there is laminar flow.
Collateral ventilation happens where alveoli communicate directly with other nearby alveoli.
Pendelluft (yes, German fans, ‘swinging air’) is when nearby lung units have different time constants and phase lags.
Cardiogenic mixing – the internal ‘wobble’ of heartbeats transmitted to the molecules of gas within the lungs causes gas mixing.
Figure 1 shows some important features of the oscillatory pressures as they are transmitted down the airway. Note how, when airway RESISTANCE rises, oscillations are damped down. This is really important – think about how airways can be narrowed by secretions, for example. Also note how compliant alveoli are, in a sense, protected from high pressure fluctuations whilst collapsed units are encouraged to expand by the undamped oscillations.
When to use HFOV
Conditions where HFOV is preferential:
- Persistent Pulmonary Hypertension of the Newborn [PPHN]
- Meconium Aspiration Syndrome [MAS]
- Air leak syndromes: pneumothorax, pulmonary interstitial emphysema [PIE]
- Severe RDS
- Pulmonary hypoplasia
Failure of conventional ventilation:
- Ventilation failure: Plateau pressures ≥ 30-35 cmH20 with expiratory tidal volumes of 5-7 ml/kg and a severe respiratory acidosis (pH< 7.1)
- In children and adults, any condition with oxygenation failure (for e.g. ARDS): defined as SpO2 < 90% and/or PaO2/FiO2 < 150, despite FiO2 > 60% and optimal PEEP
Or Oxygenation index (OI) > 15
[Oxygenation Index (OI) = [MAP x FiO2(%)] / [PaO2(kPa)x 7.5]
- Obstructive airway disease e.g. Asthma (although HFOV has been used successfully here)
- Traumatic brain injury
There used to be several oscillators around, but now mostly people are using the Sensormedics devices (see picture – look familiar?) Use the Sensormedics A (for < 20 kg patient weight) and B (> 20 kg).
You can think of HFOV like a vibrating CPAP machine.
The device delivers a constant flow of heated, humidified gas, providing flow rates (called Bias Flow) of 20 to 60 L/minute. This flow produces a constant mean airway pressure (MAP) like that of high-flow continuous positive-airway pressure (CPAP). An oscillating piston pump – a bit like the woofer of a loudspeaker – then vibrates the pressurized gas at a frequency that is generally set between 3 and 15 Hz (1 Hz = 60 cycles/minute). As the diaphragm moves forward and backward, some of the flow is pushed in and out of the lungs. The power setting (amplitude) on the ventilator controls the distance the diaphragm travels from its resting position. This oscillatory pressure amplitude, or delta P (∆P), is titrated to achieve desired CO2 elimination.
Frequency: High frequency ventilation rate (Hz = cycles per second, i.e. 10Hz = 10 cycles/sec = 600 cycles/min).
MAP: Mean airway pressure (cmH2O)
Amplitude: delta P or power is the variation around the MAP
Starting on HFOV
This is not easy. Obviously, if you have never done it before, this article is not going to be good enough! Make sure you are with someone who has lots of experience with HFOV.
Set up varies depending on the size/age of the child you are dealing with. Before attaching to the patient, set up with a bias flow 15-40 depending on age (< 1 year old: 15-25 L/m, 1-8-year-old: 15-30 L/m, ≥8-year-old: 25-40 L/m)
The FiO2 to start with should be 1.0 (100%) and the MAP 3-4 cm H2O above the MAP you are on using conventional ventilation. The frequency can be set at 15Hz (pretermer), 12 (Term), 10 (infant and child), 8 (big child). Lastly amplitude should be at about 50 cmH2O, but be prepared to adjust immediately once you are on. Percent of inspiratory time is normally set at 33 % corresponding to an I: E ratio of 1:2. This is rarely changed.
Have to hand a syringe of saline to give 5-10 ml/kg as a bolus – starting HFOV often reduces venous return and affects BP. Some people attach directly to the oscillator, but it is probably better to oxygenate using hand ventilation, then clamp the ETT at inspiration, then attach to the HFOV. This avoids de-recruitment.
As soon as you are connected and the oscillator running, adjust the amplitude until the chest, abdomen and a bit of the thigh are wobbling. This seems to give a decent CO2. Over the next few minutes, watch the saturations like a hawk. If they are falling, despite 100% oxygen, it’s usually because the MAP isn’t high enough (but beware it might be a pneumothorax – been there), so adjust up by 2 cmH2O.
Hopefully all seems to be going well at the bedside. If so, you should be able to slowly reduce the FiO2 to give a SaO2 88-92%. Check a gas at 30 mins to see what the CO2 is doing, and a CXR at 4 hours to look for overexpansion.
How to fiddle – basic adjustments
Oxygenation is dependent on MAP and FiO2. MAP provides a constant distending pressure equivalent to CPAP. This inflates the lung to a constant and optimal lung volume maximising the area for gas exchange and preventing alveolar collapse in the expiratory phase.
In HFOV, oxygenation can be separated from ventilation as they are not as dependent on each other as in conventional ventilation. Ventilation or CO2 elimination is dependent on amplitude (and to a lesser degree, frequency).
What controls what?
|Poor Oxygenation||Over Oxygenation||Under Ventilation||Over Ventilation|
|Increase FiO2||Decrease FiO2||Increase amplitude||Decrease amplitude|
(1-2Hz) if amplitude Maximal
(1-2Hz) if amplitude Minimal
* Consider recruitment manoeuvres – discuss with consultant
** Changes in frequency are rare and should only be made in discussion with the consultant
Secretions are a big problem because of the underlying lung condition. Also, you have a child who is probably deeply sedated and muscle relaxed, which means their natural cough reflex is suppressed. Suction is your friend if there is diminished chest wall movement (chest wobble), rising CO2 and/or worsening oxygenation suggesting airway or ET tube obstruction, or (of course) if there are visible secretions in the airway. If you use an open-suction catheter system is used, the patient has to be disconnected from HFOV with the obvious problems of de-recruitment and a drop in MAP (eeeek). You’re then in a situation which you’ll need to increase the FiO2 and possibly have to do some recruitment manoeuvres to regain the lost lung volume.
Luckily, for this reason many units prefer to use a closed-system (inline) suction catheter, which you don’t have to disconnect. This should reduce the de-recruitment risk to some extent.
- Reduce FiO2 to <40% before weaning MAP (except when over-inflation is evident).
- Reduce MAP in 1-2cm H2O increments to around 15 cm H2O.
- Delta P should reduce as MAP is decreased.
- Do not wean the frequency.
- Once MAP is around 15 CM, change to conventional ventilation. This is the usual practice; even though it is possible to wean HFOV straight onto non-invasive CPAP.
Is there any evidence for HFOV?
HFOV was first trialled in neonates, and this remains the group for which there is most evidence.
Evidence for how effective it is in larger children is limited, mainly involving case series. In oversized children (known as ‘adults’), HFOV evidence is mixed – the two largest studies showing either no benefit or increased mortality. However, it remains an option as a rescue therapy.
Failure of HFOV
As you’ll know if you’ve done a PICU job, HFOV doesn’t always work. This might be because you’ve not set the device up appropriately, because there is another lung condition (esp pneumothorax, practically impossible to pick up on HFOV clinically), or because the lung disease is really (really) bad. Therefore, get a CXR, get your boss involved ASAP, switch back to conventional ventilation and contact the nearest ECMO centre.
Take home messages
HFOV is an advanced mode of ventilation which needs expert staff to initiate and maintain. It may help correcting refractory hypoxia or hypercarbia when nothing else works, but is not a miracle by any stretch and might not necessarily change the outcome.
As newer methods of conventional ventilation develop, we might see the need for HFOV reduce. However, currently, it is still useful as a rescue therapy.
|Advantages of HFOV
||Disadvantages of HFOV
|Decreases the incidence of ventilator induced lung injury||De-recruitment is a potential problem if the circuit is disconnected for any reason|
|Dissociation between oxygenation and carbon dioxide clearance||Requirement for heavy sedation and paralysis with their side effects|
|May help to correct refractory hypoxia and / or hypercarbia with respiratory acidosis||Higher risk of hemodynamic instability due to high mean airway pressure. Therefore, it is advisable to optimise this before commencing HFOV with fluid boluses or inotropes.|
|No statistical evidence of benefit in terms of reducing mortality|
|Clinical examination for respiratory sounds is difficult. Only finding is wobble|
|No feedback from the ventilator regarding lung volumes, compliance etc. Therefore, regular x-rays needed to look for and avoid hyper-expansion|
|Inline suction becomes necessary to prevent disconnection and de-recruitment. This type of suctioning is not as effective as conventional suctioning.|
Dr Shri Alurkar, PICU Consultant, Nottingham University Hospital
Editorial input from Dr Jonathan Round, PICU Consultant, St George’s Hospital, London