Question

A term neonate (4 kg) was born with thick meconium following a prolonged and arduous labor. Apgar scores were 1 at 1 minute and 4 at 5 minutes of life. The neonate required full resuscitation measures at birth, was intubated and mechanically ventilated by t-piece resuscitator in the delivery suite, and transferred to the neonatal intensive care unit. He was ventilated in the SIMV mode on the following settings: PIP 26 cm H2O, PEEP 5 cm H2O, I-time 0.4 seconds, and a respiratory rate of 45 breaths per minute FI O2 0.80. The ABG revealed the following: pH 7.09, CO2 74 mm Hg, PaO2 35 mm Hg, bicarbonate 16 mEq/L, and base excess 8. The PIP was increased to 30 cm H2O, and 5 minutes after the increase to the PIP, the following was observed: SpO2 90% to 95% and the measured tidal volume 3 ml/kg. The measured MAP on the ventilator was 16 cm H2O, and tidal volume was 12 mL (3 mL/kg). The mode of ventilation was switched to HFOV. The initial oscillator settings were MAP 18 cm H2O, amplitude 38, frequency 10 Hz, and FiO2 75%. This continued for 8 hours, during which time the clinical condition and blood gases started to improve. The chest X-ray showed good lung expansion. The amplitude was reduced as chest wiggle was pronounced and CO2 started to decrease. Oxygenation also started to improve, so FiO2 was reduced to 40% on day 2. Over the next 48 hours, the MAP was slowly reduced by increments of 1 to 2 cm H2O until a MAP of 14 cm H2O. The oxygen requirement was only 35%. On day 6 of life, the neonate was to be extubated to nasal CPAP at 4 cm H2O and an FI O2 of 0.28.

1. What was the rationale for transitioning from conventional ventilation to HFOV?

2. How do you approach setting the mean airway pressure when transitioning from conventional ventilation to HFOV?

3. What ventilator parameters are used to increase or decrease CO2 on the HFOV?

4. What is an important aspect of setting the amplitude when transitioning to HFOV?

Answer 1

HFOV

It is a lung protective strategy used in acute lung injury. It is utilized as a rescue strategy when conventional mechanical ventilation failed. It has low tidal volume and constant mean airway pressure with high respiratory rates to provide beneficial effects on oxygenation and ventilation. It is safe and effective rescue mode of ventilation for treatment of ARDS. All patients who have ventilator induced lung injury or risk of developing ARDS would be connected with HFOV especially those who have failed conventional mechanical ventilation.

SETTING MEAN AIRWAY PRESSURE IN HFOV

HFOC ia a type of mechanical ventilation that uses constant mean airway pressure with pressure variations oscillating around the MAP at very high rates(upto 900 cycles per minute). In conventional ventilation large pressure changes create physiological tidal volumes and gas exchange. The large pressure changes and volumes associated with conventional ventilation can be implicated in ventilator induced lung injury and chronic lung disease.

INITIAL SETTINGS ON HFOV

Optimal lung volume strategy	Set MAP 2-3 cm H2O above the MAP on conventional ventilation MAP in 1-2 cm H2O until oxygenation improves Set frequency to 10 Hz
Low volume strategy	Set MAP equal to MAP on conventional ventilation Set frequency to 10 Hz Adjust amplitude to get adequate chest wall vibration

1. Obtain blood gas analysis and adjust settings
2. Obtain chest radiograph to assess inflation

Make adjustments once established on HFOV

Poor oxygenation	Over oxygenation	Under ventilation	Over ventilation
Increase FiO2	Decrease FiO2	Increase amplitude	Decrease amplitude
Increase MAP(1-2cm)	Decrease MAP(1-2cm)	Decrease frequency(1-2 Hz) if amplitude is maximal	Increase frequency(1-2Hz) if amplitude minimal

VENTILATOR PARAMETERS TO INCREASE OR DECREASE CO2 in HFOV

• Initially set at frequency of 10 Hz for term infants and 15 Hz for preterm, children between 6-10kg use 8 Hz and >10kg use 6Hz
• Set inspiratory time at 33%. It should not be increased more than 33% will leads to air trapping and barotrauma
• I:E ratio=1:2 for 3-5 Hz at 33% IT
• Set power at 2.5 if wt <2kg, 3 if wt <2.5kg, 4 if wt 2.5 -4kg, 5 if wt 4-5kg, 6 if wt <10kg, 7 if wt >20kg
• Check ABG every 15-20 min until PaCO2=40-60 and adjust power based on PaCO2 and chest wall vibration.
• In HFOV alveolar ventilation(Ve)=(TV)2f as compared to CMV where Ve=TV(R). Thus we adjust power to change tidal volume
• Change power 0.2-0.3 to change CO2=2-4mmhg, power 0.4-0.7 to change CO2=5-9mmhg, power 0.8-1.0 to change CO2=10-15mmhg
• If PaCO2 remains elevated at high power setting>8.0 decrease frequency by 2 Hz every 15-20 min until maximum tidal volume is reached
• Hand bagging should be avoided due to risk of barotrauma. if bagging should done means PIP should not exceed 8-10cm above the MAP and PEEP of 6-8cm should be maintained.
• Initial MAP SETTING-neonates 2-4cm above MAP on CMV, infants/children-4-8cm above MAP on CMV, if starting directly on HFOV use MAP of 8-10cm in neonates and 15-18cm in infants/children
• Obtain chest x-ray to assess lung volume. If lung is not hyperinflated increase MAP by 2-4cm every 20-30minuntil adequte oxygenation is achieved.
• If oxygenated adequately but lung is hyper inflated means decrease MAP BY 1-2 CM every 2-4 hrs until lung volumes returns to normal.

SETTING AMPLITUDE WHILE TRANSITIONING TO HFOV

1. During conventional ventilation amplitude is the difference between peak inspiratory pressure and positive and expiratory pressure. During HFOV amplitude is directly set at the ventilator.
2. As amplitude increases tidal volume also will increase