Management Strategies with High Frequency Ventilation in Neonates Using the Infant Star 950 High Frequency Ventilator
Jonathan M. Klein, MD
Peer Review Status: Internally Peer Reviewed
Jonathan M. Klein, MD, Associate Professor of Pediatrics, University of Iowa
Infrasonics Infant Star Ventilator -- A flow interrupter which functions like an oscillator with a negative pressure phase generated by a Venturi effect. Normally used for premature infants < 2.5 kg. Theoretically delivers a tidal volume of 1.5 - 3.0 cc/kg in a 2 kg infant with normal compliance.
Initial HFV Settings
12-15 Hz (900 BPM) is the usual starting frequency in a premature infant with RDS (range used of 6 - 15 Hz). Changes in frequency are rarely made in the hour-to-hour management of ABGs. A frequency > 15 Hz may worsen ventilation.
- I.T.: The inspiration time for the High Frequency breath is fixed at 18 msec (0.018 sec), therefore the I:E ratio is dependent on the frequency. At 15 Hz, I:E is 1:3 while at 6 Hz I:E is 1:8.
- Decreased Frequency (6 - 12 Hz) is used:
- To treat air leaks: PIE, pneumothorax.
- To avoid hypocarbia from excessive ventilation when at minimum amplitude.
- To minimize inadvertent air trapping.
- Increased Frequency (from 6 Hz up to 15 Hz)
- To increase alveolar ventilation when the patient remains hypercarbic despite increasing amplitude.
- To improve oxygenation by increasing lung volume from decreased expiratory time (i.e., shorter I:E ratio).
A rough representation of the volume of gas generated by each high frequency pulse through the proportioning valves (maximum generated volume with all 10 valves open is 36 cc). THIS IS NOT THE TIDAL VOLUME DELIVERED!
- Tidal volume delivered is attenuated by the following: circuit tubing, humidifier (e.g., water level), ET tube diameter and length (FLOW is proportional to r4/L), the patient's airways and compliance. Thus, the theoretically delivered tidal volume is on the order of 1.5 cc/kg in a 2 kg infant.
- Initial Amplitude Settings: Range (approx 11 - 51 cm H2O)
- Adjust amplitude until you see vigorous chest wall vibrations (amp = 24 - 34 cm H2O) then titrate based on PaCO2 (e.g., RDS PaCO2:45 - 60).
- Alveolar Ventilation (Ve) on HFV is directly proportional to the Amplitude.
- Amplitude Drift: If the amplitude (a measured value) is drifting from ordered values, it is usually due to a change in the compliance of the system (i.e., the infant is improving, secretions in the ET tube or the water level in the humidifier is low).
- Management of PaCO2 (Ventilation on HFV):
- During HFOV: Alveolar Ventilation (Ve) = (Vt)2 x freq as compared to CMV where Ve = Vt x Rate
- Thus, PaCO2 is primarily regulated during HFV by changes in amplitude, not frequency! See table below for guidelines on adjusting the amplitude (minimal AMP change is 3 cm H2O).
- To change PaCO2 ± 2 - 4 mm Hg increase or decrease AMP by approx 3 cm H2O
- To change PaCO2 ± 5 - 9 mm Hg increase or decrease AMP by approx 6 cm H2O
- To change PaCO2 ± 10 - 14 mm Hg increase or decrease AMP by approx 9 cm H2O
PEEP or MAP
Oxygenation on HFV is directly proportional to MAP which is similar to CMV; however, with HFV almost all of the MAP is generated by PEEP. Thus, during HFV: MAP = PEEP.
- Initial PEEP settings: Initial PEEP should be equal to or slightly (1 cm) above the MAP on CMV. If starting immediately on HFV use a PEEP/MAP of 10-12 cm for RDS or 7-9 cm for more compliant cases (i.e. after surfactant replacement). When converting from CMV to HFV increase PEEP by 1 cm while decreasing rate by 5 bpm in order to keep MAP constant during the conversion. Keep decreasng rate and increasing PEEP until rate of CMV is 4 bpm (sighs) and MAP becomes equal to PEEP. It is very important to keep MAP constant during the conversion to HFV to avoid excessive atelectasis and concomitant loss of oxygenation.
- Follow CXR closely to assess for appropriate lung volume (approx 9 - 10 ribs)!
- Management of ABGs (Oxygenation): Oxygenation is directly proportional to PEEP or MAP
- Oxygenation inadequate -- if below optimal lung volume increase PEEP by 2 - 4 cm H2O (e.g., if FiO2 0.6 - 0.7 increase by 1-2 cm H2O, if FiO2 1.0 increase by 2-4 cm H2O), use sigh breaths or generate manual sighs by bagging.
- Sighs -- Conventional IMV breaths used for recruitment of alveoli to improve oxygenation without need for excessive PEEP. Normal settings: Rate = 1 - 4, I.T. = 0.4 - 0.6 sec., PIP = PEEP + 6 cm H2O (minimal adequate PIP).
- Warning: Oxygenation is directly proportional to PEEP (MAP) unless lung is overinflated. If hyperinflated, may need to decrease PEEP to improve oxygenation.
- Surfactant Replacement Therapy -- give surfactant then switch to HFV.
- Conversion to HFV:
- Set frequency to 15 Hz.
- Increase amplitude over 1-3 min until you achieve vigorous chest wall vibrations which usually occurs at an amplitude of 24-34. However, if conventional rate is > 60, decrease rate to 40 and increase PEEP by 1 - 2 cm, before adjusting the amplitude. This will give the patient adequate expiratory time for the assessment of vibrations.
- Keep MAP constant during the conversion to HFV to avoid excessive atelectasis and concomitant loss of oxygenation.
- Use a stepwise process to set MAP: Thus, adjust MAP by decreasing conventional rate (by 5 bpm) while increasing PEEP (by 1 cm H2O) until conventional rate is 4 breaths per minute ("sighs") and the MAP becomes approximately equal to the PEEP. IT IS VERY IMPORTANT TO KEEP MAP CONSTANT DURING THE CONVERSION TO HFV TO PREVENT EXCESSIVE ATELECTASIS AND LOSS OF OXYGENATION. The goal being a MAP equal to or slightly (1 cm) above the previous MAP.
- Wean amplitude to keep PaCO2 45 - 60 mm Hg.
- Wean FiO2 until < 0.50 then PEEP, unless overinflated.
- The lower the FiO2, the more frequently the PEEP needs to be weaned to avoid overinflation. Minimal PEEP 3 - 6 cm H2O with FiO2 < 0.40 and appropriate lung inflation on CXR.
- In infants <1000 grams, once FiO2 < 0.40 and amplitude < 20, start to decrease frequency to minimize risk of inadvertent air trapping.
AIRLEAKS: Pulmonary Interstitial Emphysema (PIE) or Pneumothorax
- Minimize the number and intensity of IMV breaths. Thus decrease sighs (decrease PIP, decrease IT, decrease rate) or use no sighs, and set IMV rate to 0.
- Permissive Hypercarbia -- Decrease AMPLITUDE to keep PaCO2 55 - 70 mm Hg.
- Decrease Frequency -- Because of the fixed I.T. decreasing the frequency will increase the expiratory time, thus minimizing air trapping (e.g., at 10 Hz the I:E ratio is 1:5; at 6 Hz the I:E ratio is 1:8; at 4 Hz the I:E ratio is 1:13).
- Decrease PEEP -- Transiently tolerate increased FiO2 requirements (0.5 - 0.75) in order to heal severe PIE.
- The goal is to minimize barotrauma, volutrauma, and oxygen toxicity.
- Minimize AMPLITUDE to keep PaCO2 adequate (e.g., 50 - 70 mm Hg).
- Increase PEEP as necessary to keep FiO2 < 0.40 - 0.50 with minimum PIP and allow the patient to "self-wean by outgrowing the ventilator."
- Decrease PEEP by 1 cm H2O every 4-7 days once FiO2 remains < 0.40 - 0.45 after each change.
Once oxygenation is adequate and the patient is ready to be weaned, follow these steps:
- Only wean FiO2 until < 0.50, unless overinflated.
- Once FiO2 < 0.50, decrease PEEP by 1 cm H2O Q4 - 8h, if FiO2 < 0.30 - 0.35, decrease PEEP by 1 - 2 cm H2O Q2 - 4h to avoid overinflation.
- Also decrease PIP of Sigh breaths at the same time and by the same amount that you decrease the PEEP (e.g., PIP 16 and PEEP 10 to PIP 15 and PEEP 9).
- Minimal PEEP or MAP approximately 3 - 7 cm H2O with FiO2 < 0.40. At this point one can convert to conventional ventilation at low rates (approximately 15 - 20 bpm), or remain on HFV while the patient matures (anti-apnea settings) and grows, or extubate to NPCPAP if ready.
- Reduce AMPLITUDE by at least 3 cm H2O per change whenever PaCO2 decreases below threshold, until minimal AMPLITUDE (11-13) is reached.
- Always observe chest wall after a decrease in AMPLITUDE to confirm vibrations, if vibrations have ceased the AMPLITUDE is too low and should be readjusted to previous settings.
- If PaCO2 is still too low (< 35 mm Hg) on minimal amplitude, and the infant is not ready for extubation, decrease frequency to 10 Hz and then to 6 Hz to decrease alveolar ventilation.
Patients are usually ready for a trial of extubation with NPCPAP when they meet the following respiratory support criteria:
- RDS: PEEP or MAP < 7 - 8 cm H2O with FiO2 < 0.35 extubate to a NPCPAP of 6 - 8 cm H2O.
- BPD: PEEP or MAP < 10 - 12 cm H2O with FiO2 < 0.45 extubate to a NPCPAP of 8 - 10 cm H2O.
- Mechanical support required for ventilation is minimal (see table below).
|Weight||AMPLITUDE (cm H2O)|
Complications Associated with HFV
A. ATELECTASIS - increase PEEP, or increase the PIP, I.T., or rate of the sigh breaths (0-4).
B. SECRETIONS- Suction more frequently.
C. HYPOTENSION- decrease PEEP to decrease MAP to improve venous return if low BP is due to hyperinflation.
D. OVERINFLATION- decrease PEEP and decrease PIP if using sighs to decrease MAP.
E. APNEA- Increase amplitude or frequency, increase sighs to 4-6 BPM, or consider converting to conventional ventilation. HFV is not an optimal mode for the management of apnea.