High Frequency Ventilation (HFV)
Jonathan M. Klein, MD
Peer Review Status: Internally Peer Reviewed
The use of surfactant replacement therapy has helped to decrease neonatal mortality from respiratory distress syndrome (RDS), but the incidence of pulmonary interstitial emphysema (PIE) and bronchopulmonary dysplasia (BPD) in ventilated neonates (700-1350 grams) is still relatively high (PIE 20-25%, BPD 15-19%; U.S. Exosurf Pediatric Study Group 1990). Thus new therapies involving alternative methods of managing respiratory failure are currently being utilized. One of these new therapies is high frequency ventilation.
HIGH FREQUENCY VENTILATION (HFV)
HFV is a new technique of ventilation that uses respiratory rates that greatly exceed the rate of normal breathing. There are three principal types of HFV:
- High frequency positive pressure ventilation (HPPV, rate 60-150/minute);
- High frequency jet ventilation (HFJV, rate 100-600);
- High frequency oscillatory ventilation (HFOV, rate 300-3000/minute).
The advantage of high frequency oscillatory ventilation as compared to either conventional positive pressure or jet ventilation is its ability to promote gas exchange while using tidal volumes that are less than dead space. The ability of HFOV to maintain oxygenation and ventilation while using minimal tidal volumes allow us to minimize barotrauma and thus reduce the morbidity associated with ventilator management of RDS
INFRASONICS INFANT STAR
High-Frequency Ventilator: We are currently using the Infrasonics Infant Star ventilator at a frequency of 15 Hz (900 breaths/minute) in premature infants who develop PIE while on conventional mechanical ventilation. The Infant Star is a flow interrupter, not a true oscillator, but its physiological effects and advantages are similar to those of true oscillators. While on Infant Star, one observes rapid vibration of the infant's chest wall instead of the normal chest wall excursion that is seen with conventional ventilation.
The Infant Star is used for the treatment of pulmonary air leaks, primarily pulmonary interstitial emphysema (PIE) and pneumothorax. HFV with the Infant Star allows gas exchange to occur even while the lung is atelectatic, thus the size of the air leak is diminished, allowing for more rapid resolution of air leak syndromes. Thus, by decreasing the severity of PIE, HFV should allow us to minimize the mortality and morbidity (BPD) associated with barotrauma.
|HFPPV||60-150||> dead space|
|HFJV||100-600||> dead space|
|HFOV||300-3000||< dead space|
During conventional mechanical ventilation or spontaneous respiration, gas exchange occurs because of bulk transport (convective flow) of the O2 and CO2 molecules from the central or conducting airways to the peripheral airways. The volume of inhaled gas must exceed the volume of dead space.
GAS EXCHANGE DURING HFV
Theories on why ventilation can still occur when using tidal volumes that are less that dead space:
- Augmented Diffusion;
- Bulk Axial Flow;
- Interregional Gas Mixing (Pendelluft);
- Axial and Radial Augmented Dispersion (Taylor Dispersion);
- Convective Dispersion.
INDICATIONS FOR HFV
- BAROTRAUMA - pulmonary airleaks.
- PULMONARY INTERSTITIAL EMPHYSEMA (PIE)
- Respiratory failure unresponsive to conventional ventilation (compassionate use).
(Infrasonics INFANT STAR High-Frequency Ventilator) - Consult with Staff Neonatologist before instituting high frequency ventilation.
- FREQUENCY: 15 Hz (900 "breaths per minute")
- AMPLITUDE: a rough representation of the volume of gas flow in each high frequency pulse or "breath." Adjust the amplitude until you achieve vigorous chest wall vibrations, usually occurs at an amplitude of 20-30. If conventional rate is greater than 60, decrease rate to 40 and increase PEEP by 1 to 2 cm, before adjusting the amplitude. This will give the patient adequate expiratory time for the assessment of vibrations.
- MAP: Adjust 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 (1-3 cm) below the previous MAP.
- IMV RATE (sighs): The conventional or "sigh" breaths should be similar to the previous settings in terms of PIP, however the inspiratory time should be 0.4 - 0.6 seconds.
- PEAK PRESSURE (sighs): The PIP is usually set at a pressure equal to MAP +6 cm.
BLOOD GAS MANAGEMENT
- Inadequate oxygenation (low PO2): Manage by increasing the FiO2, increasing the MAP by increasing the PEEP (i.e. PO2 is directly proportional to MAP or by decreasing atelectasis by manually ventilating the infant with an anesthesia bag and then adjusting the "sigh" breaths by increasing either the rate, inspiratory time or PIP of the conventional breaths).
- IMPORTANT: If oxygenation is lost during weaning when Peepwas decreased, manually "bag" the infant back up to restore lung volumes and reset Peep at 2-3 cm above the previous value. Once adequate oxygenation has been reestablished weaning can begin again, but proceed more slowly with changes in Peep.
- Inadequate ventilation (high PCO2): Manage by increasing the AMPLITUDE (i.e., PCO2 is inversely proportional to AMPLITUDE).
COMPLICATIONS OF HFOV
- ATELECTASIS: treat by increasing the rate or PIP of the conventional breaths ("sighs");
- INCREASED MOBILIZATION OF SECRETIONS: treat by increasing frequency of suctioning of ETT as needed;
- HYPOTENSION: treat by lowering MAP by decreasing PEEP, if other methods such as volume and positive inotropic agents have been inadequate.
- Reduce the amplitude of the oscillations by 3 units per change (Q1-2h) until the PCO2 rises. After a change in AMPLITUDE, always observe the chest wall to confirm that it is still vibrating, if vibrations have ceased the AMPLITUDE is too low and thus should be reset at the previous setting. A minimal AMPLITUDE tends to occur around 12-14 units.
- Once oxygenation is adequate (FIO2 less than 0.70) slowly lower the MAP by decreasing the PEEP by 1 cm H2O per change (Q4-8h). Minimal HFOV settings tend to be reached around a MAP of 7 cm with an O2 requirement that is less than 40%. At this point depending on the patient, you can remain on the HFOV while the patient grows, you can convert the patient back to convention ventilation at a low respiratory rate (usually 15-20 bpm), or you can extubate the patient to Nasal CPAP.
- Management Strategies with High Frequency Ventilation in Neonates Using the SensorMedics 3100A High Frequency Oscillatory Ventilator
- Management Strategies with High Frequency Ventilation in Neonates Using the Infant Star 950 High Frequency Ventilator
- Management Strategies with High Frequency Jet Ventilation
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