HFOV (high frequency oscillatory ventilation) is indicated forventilatory support and treatment of respiratory failure andbarotrauma.The use of HFOV in the adult patient is an object of some debateand as a rule is discouraged. It is recognized however, thatsituations may arise where, short of ECMO, it is the onlymodality left available to the practitioner.Generally, it is felt that HFOV should not be used in patients witha body weight over 50 kilograms. Weights above this willgenerally require an amplitude pressure higher than the 3100Asmaximum output and lead to possible overheating, off-centerpositioning, and potential failure of the piston.Strategies in the use of HFOV in the larger patient are similar tothat used in the neonatal/pediatric population and are aimed atmaximizing oxygenation and adequate elimination of carbondioxide with the primary aim of incorporating lung protectivestrategy.
OXYGENATIONOxygenation will depend on the use of two parameters -the FiO2 and the mean airway pressure (MAP). Initial MAPsettings should be established at between 5-10 cmH20more than the mean on conventional ventilation and furtheradjusted according to the SPO2 and x-ray findings revealingatelectasis or hyperinflation.It is recommended that MAP not be weaned until theFiO2 is less than 60%.Occasionally, when the patient is on a high FiO2 (>70%) theMAP may have to be decreased to avoid hyperinflationand/or barotrauma and a relative degree of hypoxemia andhypercapnia may have to be accepted.
VENTILATIONThe primary control for ventilation is the delta P oramplitude pressure control.Increasing the amplitude should result in greater volumesand a decrease in PCO2.If this is unsuccessful, lowering the frequency, i.e.Hz/respiratory rate, can help. Remember 1Hz=60 BPM. Inthe adult, rates between 5-8 Hz are acceptable. Theminimum frequency provided by the 3100A is 3 Hz.A third method would be to increase the I-time from thestandard 33% up to a maximum of 50%. Even thoughallowing for less expiratory time, increasing the I-time willallow for the generation of a greater tidal volume. This mayalso, paradoxically, reduce PO2.It should be remembered that decreasing the frequencyis not a method of weaning as this, in fact, increasesventilation.
Though somewhat debatable, and not as problematic as injet ventilators, there may be some difficulties withhumidification, leading to inspissated secretions. Aconcomitant goal of medical therapy should be adequateperipheral hydration.While there are no positive contraindications to its use, sideeffects may include hyper/hypoventilation, IVH, BPD,necrotizing tracheal bronchitis, atelectasis, hypotension,pneumothorax, pneumopericardium, pneumomediastinum,pneumoperitoneum and PIE.
It is recommended that patients undergoing this therapy bemonitored with transcutaneous PO2 and PCO2 monitoringas well as venous and arterial blood gasses whennecessary. Mechanical ventilation will be implemented,maintained and adjusted by a RespiratoryTherapist. Decisions regarding changes in ventilationtherapy are a collaborative effort by the RespiratoryTherapist, Physician and Registered Nurse.SpecificationsBias flow 0 - 40 lpmMean airway pressure 3 - 45 cmH20Frequency 3 - 15 hz% inspiratory time 30 - 50%Power / Delta P 0 - 100 oscillator driver power
CALIBRATING PATENT CIRCUIT1. Turn on source gas and set Bias Flow to 20 1pm2. Set Mean Pressure Adjust and Mean Pressure Limit tomaximum (fully clockwise)3. Push in and hold RESET while observing MeanPressure digital readout4. Adjust Patient Circuit Calibration screw on right side ofcontrol module to achieve a mean pressure between 39and 43 cmH20.5. Release reset button
VENTILATOR PERFORMANCE CHECK1. Set frequency to 10 and % I-time to 33.2. Set Bias Flow to 20 lpm3. Depress RESET button long enough to allow MAP toincrease above 6 cmH204. With Mean Pressure Adjust control establish a meanpressure of between 19 and 21 cmH205. Depress START/STOP button to cause oscillator to run6. Increase POWER control setting to 6.0 and centerpiston with Piston Centering control7. With piston centered and a stable Delta P reading verifythat Delta P is between 56 and 75 and that MAP is between17 and 23.8. Depress START/STOP button to stop oscillator.9. The 3100A is now ready for patient use.
THERAPEUTIC STRATEGIESVentilation is largely governed by changes in Delta P.Increasing Delta P increases ventilation and vice versa. Ifmaximum Delta P will not provide sufficient ventilation, asecondary strategy is to decrease the frequency. This takesadvantage of the fact that less Delta P is attenuated by ETdiameter at lower frequencies. If PCO2 is still elevated, % I-time may be increased up to 50%.Oxygenation is largely a function of MAP. Typically webegin at a MAP 10% higher than that used in conventionalventilation. Attempt to wean FiO2 to less than 70% beforeweaning MAP
Managing Large Patients (>35 kg) on the 3100A OscillatorWhat is the FDAs limit on patient weight for use of the 3100A?While the pediatric prospective randomized controlled trial (RCT) waslimited to 35 kg, the FDAs Review Panel recognized that the range ofventilation with the 3100A was more limited by physiologicconsiderations rather than by absolute patient weight. As the MDDIGray Sheet reported in January 1995, "SensorMedics high frequencyventilator approval for pediatric use should not limit indications topatients who weigh less than 35 kilograms, FDAs Anesthesiology andRespiratory Therapy Device Panel agreed at a Jan 20 meeting."During the panel hearing for pediatric approval they noted that itrequires approximately 2 ml/kg to ventilate a patient with HFOV. With a6 mm endotracheal tube, at maximum power and at a frequency of 3Hz, approximately 180 ml could be delivered by the 3100A to a testsystem with a compliance of 20 ml/cmH2O. This equates to atheoretical 90 kg patient weight limit. As can be seen in Figure 1, alarger endotracheal tube (9 mm) will enable even larger volumes to bedelivered, raising the theoretical weight limit.
In summary, there is no upper weight limit for the 3100A.No special forms, IRB approval or informed consent isrequired for treating any child with an OI>13 with the3100A.What has been the largest patient managed with the3100A?While we are not aware of all large patients managed withthe 3100A, we do know of 2 patients weighing 110 and 113kg (242 and 249 lb.) who were adequately ventilated withthe 3100A. Weve started a database of large patients(Table 1) and the mean weight of the survivors is 64.4 kg(129 lb.)Table 1. 3100A Large Patient Registry
When should I consider using the 3100A?As with all candidates for the 3100A, the earlier, the better. When we went backthrough the pediatric RCT data to answer this question, we found that waitingmore than 72 hours on CMV raised the odds ratio for chronic lung disease insurvivors to 25.2. In both the pediatric RCT and our adult 3100B pilot rescuetrial, waiting more than 10 days to initiate HFOV was statistically specific forincreased mortality.The specific markers for use of the 3100A are:Gross Air LeaksARDS or Intractable RSV PneumoniaWith increasing FIO2 requirements (>60%).Use of PCIRV to recruit lung volume.Use of paralysis for patient management.
Clinical experience in both pediatric and adult applicationshas taught us that the ability of the 3100A to achievedesired levels of PCO2 in larger patients is tied closely tothe prior amount of conventional ventilation. When largerpatients are selected early for this therapy, CO2 eliminationis more easily accomplished.What are the recommended starting settings for use ofthe 3100A in large patients?FIO2 at 1.0MAP starting 4-8 cmH2O above that on CMV.Flowrate > 18 LPM, higher if required to meet MAP setting.Frequency starting at 6 Hz.Delta-P starting at a power setting of 4.0 and rapidlyincreasing it to achieve adequate chest movement.%I-Time set to 33%.
If CO2 retention persists at maximum settings, decreasingthe cuff pressure to allow gas to escape around the ETtube will move the point of fresh gas supply from the wyeconnector to the tip of the ET tube (Figure 2). This willreduce the deadspace and lower PaCO2. Note: The biasflow may have to be increased to compensate for the leakand maintain MAP.
What are the markers that the patient is failing on the3100A?Failure to oxygenate is defined by the inability to decreaseFIO2 by 10% within 24 hours. An OI < 42 at 24 hours ofHFOV is a good indicator of a positive response. An OI >42at 48 hours has been specific for oxygenation failure andnon-survival.Failure to improve or maintain adequate ventilation isdefined as the inability to maintain PaCO2 < 100 torr with apH > 7.25. It is extremely important to monitor PaCO2 inlarger patients.As reported in the large patient registry, most patients whosurvived had a significant oxygenation (OI) response withinthe first few hours of HFOV.
3100A High Frequency Oscillatory VentilationLarge Patient (>30 kg) Guideline Considerations andPatient ManagementInclusion Criteria: OI > 13 (OI = 100 x FiO2 x Paw / PaO2) in twoarterial blood gases within a six hour period. Examples: A patient with a PaO2 of 60 torr on an FIO2 of>.60 and a mean airway pressure > 13 cmH2O A patient with a PaO2 of 60 torr on an FIO2 of>.40 and a mean airway pressure > 20 cmH2O *** If pH <7.28 consider buffering the patient with THAM beforestarting.*** Assure adequate blood pressure*** Special attention to ventilatory requirements andPaCO2 should be emphasized in larger patients
Although patients with the following conditions havebeen adequately managed with the 3100A,consideration should be given to these factors prior toinstitution of HFOV therapy: Patient diagnosed with increased AirwayResistance. Elevated ICP Weight > 70 Kg Mean Arterial Pressure < 55 mm Hg Passive pulmonary blood flow dependency withnormal compliance
Clinical Experience with the 3100A Suggests that:1. There appears to be an inverse relationshipbetween prior days on CMV and ability to ventilate with the3100A. As the limitation in size of patient is usuallyconstrained by ventilation requirements, the longer the timeon CMV prior to institution of HFOV, the smaller the patientthat may be able to be managed.2. Patients managed for more than 72 hours on CMVfor ARDS prior to transfer to HFOV have a more than 25fold odds increase for developing chronic lung disease.3. Patients managed for more than 10 days on CMVfor ARDS prior to transfer to HFOV have a statisticallysignificant increased risk of mortality.4. Patients with an oxygenation index (OI) greaterthan 42 after 48 hours of HFOV have a significantlyincreased risk for non-survival and alternative therapeuticoptions should be considered._______________________________________________
Information to obtain and patient preparation beforeplacing the patient on HFOV1. If patient has a PA catheter, measure and recordcardiac output, PCWP, SVO22. CVP line -- CVP should be at least 8 mm Hg3. TcPCO2 monitor at 38-40 degrees C° to followtrend information4. Arterial line for MAP monitoring and ABG analysisInitial Set-Up1. Obtain HFOV Flowsheet2. Fill in conventional ventilator setting, blood gases,medications, hemodynamics3. Just prior to instituting HFOV, suction patient welland give one 10 second sustained recruitment inflation.4. All patients may require neuromuscular blockadeand sedation for initiation of HFOV.
Editor:Abdul fattah AbroNeonatal intensive care unit staffNurseKarachi sindh