Weaning Strategies for HFOV

Sorasak Lochindarat, M.D. Queen Sirikit National Institute of Child Health

Specification of HFOV (Sensor Medics Model 3100 A)


Amplitude range 0-100

cmH2O




Frequency

Hz

3-15

(180-900 cycles/min)


Ti

33-50 % of total cycle time (I : E of 1:2 to 1:1)




MAP

3- 45

cmH2O




Bias gas flow

0-40

L/min

SensorMedics 3100B 

 

 



Electrically powered, electronically controlled piston-diaphragm oscillator Paw of 5 - 55 cmH2O Pressure Amplitude from 8 - 130 cmH2O Frequency of 3 - 15 Hz % Inspiratory Time 30% - 50% Flow rates from 0 - 60 LPM

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HFOV : decreasing in pressure amplitude from trachea to alveoli

Mechanism of gas transport during HFV 1. direct bulk flow (convection) 2. longitudinal (Taylor) dispersion 3. pendeluft 4. asymmetric velocity profiles 5. cardiogenic mixing 6. molecular diffusion

The benefit of HFOV in ARDS/ALS


High volume strategy will recruit the atelectatic lungs




Decreasing of tidal volume and pressure swing will reduce the lung injury and air leak syndrome




The different flow pattern of HFOV will improve ventilation-perfusion matching (New Horizons 1999;7:359)

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Ventilator Strategies - Goals •

Normalize lung volume

•

Minimize peak ventilator pressures

Physiological targets included: – Oxygen Saturation > 88% – Delay weaning mPaw until FiO2 < 50% – pH > 7.15 – PaCO2 in the range of 40 – 70 mmHg

Initial HFOV setting in DAD/ARDS


FiO2 1.0




Frequency 5-15 Hz




Inspiratory time (Ti) 33 % (1:2)




Paw 5 cmH2O > in mode CMV




Bias gas flow > 20 L/min




Pressure amplitude : adjust to provide adequate chest wall movement and/or PaCO2 40-70 (Start 10-15>PIP in CMV)

Ventilation made easy Oxygenation

Ventilation

• FiO2

• Amplitude

• MAP

• Frequency

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HFOV - Oxygenation Strategy •

Open lung strategy

•

Initial mPaw + 5, increase in 2-3 cmH2O increments Q 20 - 30 mins if FiO2 > 60% until max 45 cmH2O

•

%IT 33

•

Goal: FiO2 < 60% with SpO2 > 88%

HFOV - Ventilation Strategy •

∆ P: “chest wall vibration” - increase 10 cm H2O increments if rising PaCO2 to max ∆P of approximately 90 – 100 cmH2O

•

Hz 5-15 (could decrease to 3 Hz)

•

ET cuff leak if rising PaCO2

•

Goal: pH > 7.15 and PaCO2 40 - 70 mmHg

HFOV Effectively Decouples Oxygenation from Ventilation Vol

Vol Pressure

Vol

Pressure

Pressure

∆P

Paw

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Combining HFOV and recruitment maneuvers (RMs) in adults c ARDS


High MAP : to avoid atelectrauma and limiting volutrauma




avoiding both lung over-distention and under recruitment




RMs : sustained inflation c pressure 30 - 40 cmH2O for 30 - 40 sec. (Crit Care Med 2005; 33: 479-86)

* Recruitment Maneuvers Pre-oxygenated with FiO2 = 1.0 x 5 mins





Aborted immediately if hemodynamic compromise

Not performed if an active airleak present


Algorithms for HFOV setting in adult Weaning












Goal is FiO2 of 0.4, SpO2 of > 88%, mPaw 20-24 cm H2O before CV transition If required mPaw > 35 cm H2O, give equal priority to reducing mPaw and FiO2. Reduce mPaw 2-3 cm H2O every 4-6 hrs When mPaw approaches 20 cm H2O, transition to CV (e.g., TV 6 mL/kg; PEEP 10 cm H2O; Pplat < 30 cm H2O; I:E 1:1; rate 15-25 breaths/min or to APRV (e.g., Phi 20 cm H2O; Plo 0 cm H2O; Thi 4 secs, Tlo 0.8 sec) Conventional weaning; Progressing to spontaneous breathing trials

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Factors affecting gas exchange Oxygenation is depended on 1) - The recruitment and maintenance of lung volume related to MAP - high volume strategy 2) - FiO2 (Eur J Pediatr 1993;152:350)

Factors affecting gas exchange Ventilation in HFOV VA = f X (VT)2 VT ↑ : affect CO2 removal >>frequency VT ↑ is depended on Amplitude ↑ Frequency ↓ Ti ↑ (33% → 50%)

Frequency

Frequency controls the time allowed (distance) for the piston to move. Therefore, the lower the frequency , the greater the volume displaced, and the higher the frequency , the smaller the volume displaced.

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Ventilation in HFOV


If increasing the amplitude has no net affect on decreasing PaCO2 , consider decreasing the frequency by 1 Hz at a time




Cuff deflation may be necessary for CO2 elimination.




If minimal chest wiggle, and PCO2 are too low, consider increasing the frequency by 1 Hz

Frequency


Recommended guidelines for initial frequency setting : (may be disease-dependent) < 2000 gm 15 Hz 2-12 kg 10 Hz 13-20 kg 8 Hz 21-30 kg 7 Hz > 30 kg 5- 6 Hz

Frequency


Frequency MAY or MAY NOT have to be adjusted from the initial setting




Frequency is not weaned as is done with CMV




Decreasing Frequency in HFOV, will increase ventilatory support (VT)

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Inspiratory time (Ti)


Ti : Controls the time for movement of the piston : assists with CO2 elimination




Do not increase Ti for freq. 10-15 Hz, will cause gas trapping




Increase Ti in larger ped. patients as the third maneuver to control CO2 elimination

closed suction system

Transition back to Conventional Ventilation (CV)


FiO2 0.4




Paw 15-20 cm H2O




Clearing CXR and resolving disease process




Tolerance to circuit disconnect, turning and spontaneous respirations

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