Registered respiratory therapist helping the tiny humans breathe easier by night, cat mom by day.
The Pre-Brief
Often long periods of conventional mechanical ventilation, or positive pressure ventilation, can lead to lung injury- such as barotrauma, volutrauma, and atelectrauma. These types of ventilator-induced lung injuries (VILI) are especially important to prevent in our neonatal population with respiratory distress syndrome, whose premature lungs are prone to lung injuries. Reasons can include that they do not develop an adequate number of surfactant until around 35 weeks gestation. So, what type of lung-protective strategies do we use for neonatal lungs to prevent such lung injuries?
What is HFOV?
Think of a dog panting in the hot summer sun. Funny enough that is where inspiration struck for the idea of the high-frequency oscillatory ventilator, a softer mode of mechanical ventilation, often used in the NICU. Like a dog panting in the heat, the oscillator uses very rapid respiratory rates with a small tidal volume, often less than dead space. It uses an active exhalation and inspiration phase, which diffuses gas down to the alveoli. In the simplest terms, the oscillator is also referred to as “CPAP with a wiggle.” CPAP determines oxygenation which maintains optimal lung recruitment, and the wiggle controls alveolar ventilation with oscillating pressure. But why do we say a wiggle? Because the baby will literally be wiggling, and this is how we know that he or she is ventilating properly. Always look for your “chest wiggle factor” and for our neonatal population make sure he or she is wiggling from the nipple line to the umbilicus.
How do we ventilate on the oscillator?
There are three settings that control ventilation: the power, the amplitude (delta P), and the frequency.
Power and amplitude are the primary control of ventilation on the oscillator, which generates tidal volume. Power and amplitude are the size of the volume of gas produced by each high-frequency wave. However, what is the difference between power and amplitude? Power is dialed in on the power knob from 1-10 and is a set measurement, while the amplitude is a calculated number from 0-100% which reflects power and often fluctuates. Many institutions may only order amplitude, but be mindful of the power, because it is often a more reliable indicator of ventilation due to it being a set measurement. For power and/or amplitude, an initial starting setting should be determined by the chest wiggle factor, which determines ventilation; however, the power setting will normally start at around 2. Remember that when determining the correct setting, make sure your baby is wiggling from his or her nipple line to their umbilicus.
Frequency is the number of breaths per minute or how fast the high frequency wave goes in. It is measured in hertz (Hz), and 1 hertz equals 60 breaths per minute. Initial settings of the frequency are usually weight-based; a premature infant will usually start at 15Hz (900BPM) and a full-term infant will usually start at 12Hz (720BPM). Frequency is rarely changed once set, but if needed to blow off CO2, we can decrease the hertz, which can make a bigger change than only changing the amplitude.
What about oxygenation?
There are two settings that control oxygenation: MAP and FiO2.
Mean airway pressure, MAP, is the constant pressure throughout the respiratory cycle needed to maintain lung inflation. The initial setting should be two above the MAP on a conventional ventilator. If starting on HFOV right away, ten is a safe starting point. When setting your MAP, also set your MAP limit, which should be about 20% above your MAP. Also, set your maximum and minimum MAP alarms three above and below. After setting the MAP, an x-ray should be obtained to check for optimal lung expansion, which should be 8-9 ribs expanded.
The FiO2 is set on a blender on the HFOV from 21-100% and read out on an oxygen analyzer.
Any other settings?
The inspiratory time percent also needs to be set, which always should be set at 0.33, which is a 1:2 ratio. The bias flow also needs to be set, which is the continuous flow of gas throughout the circuit, which is set based on patient size normally 10-15 LPM for a neonate.
Conclusion
When working with neonatal lungs, the oscillator is a softer mode of lung ventilation, which can reduce ventilator-induced lung injury. The small tidal volumes the oscillator produces can reduce volutrauma. High peak pressures are not needed to create an adequate MAP to inflate the neonate’s lungs, instead, they maintain constant lung recruitment, reducing barotrauma, and atelectrauma. Remember to always look for your “chest wiggle factor” which determines if your neonate is ventilating properly which should be from the infant’s nipple line to their umbilicus.
Debrief
- HFOV is lung-protective ventilation for neonates.
- Watch for wiggle factor. Baby should wiggle from nipple line to umbilicus
- Power and Amplitude are ventilation controls.
- MAP and FIO2 are oxygenation controls.
References
https://usme.com/content/manuals/sensormedics-3100A-Operator-manual.pdf Sensormedics 3100A Operators Manual
https://uichildrens.org/high-frequency-oscillatory-ventilation-hfov-neonates-3100A-ventilator Jonathan M. Klein, MD Management strategies with high frequency oscillatory ventilation (HFOV) in neonates using the SensorMedics 3100A high frequency oscillatory ventilator
Amplitude is a measurement of the ∆P created by the settings you have placed on the oscillator, not a calculation of 0-100%.
The Power knob is your potentiometer control. 0-10 on power = 0-100% piston displacement.
Power is therefore a control. Amplitude is a variable dependent on lung dynamics, bias flow, MAP, and Power.