High-Frequency Ventilation

Symposium on Respiratory Failure

High-Frequency Ventilation T. J. Gallagher, M.D.* M.D. *

High-frequency ventilation (HFV) represents the latest addition to the clinical armamentarium for the treatment of various respiratory problems. The technique was first developed by Swedish clinicians almost ahnost 20 years 22 Many questions regarding HFV reInain ago. 22 remain unanswered: How does it work? What systems are available? Which patients will respond best to HFV?

CHARACTERISTICS OF HIGH-FREQUENCY VENTILATION For adults, rates of mechanical ventilation greater than 60 breaths per min probably represent HFV. Current systems in use in the United States operate frequencies as high as 3000 breaths per min. Many authors now refer to these frequencies in hertz (Hz), which represents one cycle per second; therefore, a frequency of 10 Hz corresponds to 600 breaths per lllin. min. The characteristic features of HFV include its high rate and a tidal volume less than the anatomic deadspace. In a man weighing 70 kg, this implies a tidal volume less than 150 1nl, ml, in contrast to the 700 to 850 1nl ml required during more conventional fGrIns forms of ventilation. The third major feature of HFV is its brief inspiratory time (TI). J\1ost Most often in adults the TI is between 0.8 and 1.2 seconds. During HFV, the TI varies from 0.001 to 0.1 seconds. When frequency increases, failure to decrease TI results in 16 Functional residual capacity (FRC) increases inadvertent air trapping. trapping.16 even to the point of marked overventilation relative to perfusion. When positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) is required, this problem can sometimes be used to advantage to derive the desired baseline and mean airway pressure (PAW). system should Ininin1ally minimally interfere with cardiac A desirable ventilatory systen1 output yet maintain efficient ventilation. Overall, the rapid ventilator rate,

**Associate Associate

Professor, Departments of Anesthesiology and Surgery, University of Florida College of Medicine, Gainesville, Florida

America-Vo!' 67, No. 3, May 1983 Medical Clinics of North America-Vol.

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low tidal volulne, volume, and brief TI Tr cOlnbine combine during HFV to reduce peak inflation pressure (PIP) and PAW. The lower the PAW, the lower the intrapleural pressure. This decrease n1inimizes minimizes cardiovascular depression. detrimenCompared with conventional ventilatory modes, HFV has little detriInenperformance, a feature illustrating one of the major tal effect on cardiac perforlnance, advantages of HFV. Also, venous return improves. The decreased ainvay airway pressures can minimize increases in pulmonary artery pressure, the net result being a decreased right ventricular afterload. Pulmonary barotrauma develops during mechanical ventilation secondary to both PAW and large tidal volume. 66 While not yet established, the incidence of barotrauma during HFV, with both volume and pressure decreased, should theoretically be decreased. With most current modes of HFV, both oxygenation and carbon diox23 This appears to confirm the efficiency ide elimination are easily obtained. 23 of HFV. Although oxygenation can be maintained during apnea, carbon dioxide elimination remains virtually nil. There are three distinct modes of HFV, each having particular physiologic characteristics (Table 1). Particular clinical situations conceivably could require one mode rather than another. The nature of these variations makes their comparison difficult. Th~ The three major systems are: highfrequency jet ventilation (HFJY), (HFJV), high-frequency positive-pressure ventilation (HFPPV), and high-frequency oscillation (HFO).

HIGH-FREQUENCY JET VENTILATION HFJV is the most common system in the United States. A small bore, low compliance delivery circuit attaches to an even narrower 14- or 16gauge catheter, usually a 2-inch angiocath. This acts as an injector. This injector is then inserted through the stopper of the Portex 3-way swivel adapter used for fiberoptic bronchoscopy. The opposite port of the Portex adapter mates to the endotracheal tube while the third (at a 90° angle to the first two) is used to deliver a continuous flow of gas (20 liters per min). (Fr0 22) of the continuous flow source reThe fraction of inspired oxygen (FI0 mains identical to that of the injected gas. With inspiration, a jet of gas pulses through the angiocath, then accelerates while flowing through the narrowed lumen (Fig. 1). This creates Table 1.

(Hf]V) and Positive-Pressure Characteristics of High-Frequency Jet (HFJV) (HFPPV) Ventilation and High-Frequency Oscillation (HFO) HFJV

i) (min-I) Frequency (minFlow pattern Gas entrainment Flow characteristics FI0 FrO,2 Flow generator

100-200

Square wave Yes Turbulent Controlled Solenoid-pneumatic SolenOid-pneumatic

HFPPV

60-90

HFO

900-3000

Square \vave wave

Sinusoidal

Turbulent Controlled Pneumatic

Turbulent-Iaminar Turbulent-laminar Variable Piston

]\'0 No

No :\10

635 635

HIGH-FREQUENCY VENTILATION

HFJV SYSTEM FLOW CARTRIDGE

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10

0

0

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CONTINUOUS FLOW GENERATOR

Figure 1. During high-frequency ventilation, gas moves through the narrowed injector system, lumen and entrains gas from a continuous flow system.

an area of relative negative pressure at right angles to the angiocath and acts as a venturi,9 venturi. 9 The low flow gas entering at the third lumen of the airway, This increases the tidal volume acadapter is entrained into the airway. PA W or tually delivered to the patient without any apparent increase in PAW PIP. The high-pressure, pulsed flow of gas exerts its greatest impact in either the proximal airway or more likely the endotracheal tube, since minimal. Studies of anihigh-pressure injury to the trachea appears to be minilnal. mals ventilated for 24 hours with this technique did not demonstrate any untoward effects. effects.lI The kinetic energy of the gases is transferred to the motionless gases in the airway, which, in turn, transmit that energy further into the distal airway.I5 airway.!5 Since the energy moves from the outer edges of the jet toward its center, the profile of the gas flow resembles an elongated cone. The highest pressure and highest velocity are at the center and decrease progressively toward the periphery. It is unclear whether the locaimportance relative to gas exchange. tion of the catheter tip is of critical ilnportance Although studies show that carbon dioxide elimination improves as the impaired. The catheter tip approaches the carina,3 entrainment may be iInpaired. resultant decrease in tidal volume could affect ventilation. Further study clarifY this question. should clarify HFJV generators operate by means of fluidic, pneumatic, or solenoid valves. The most common, the solenoid valve, resembles a flow interrupter and controls frequency electronically. Since the opening time of the Tr can be adjusted. Most jet ventilators are valve can also be controlled, TI driven by an air-oxygen source at 50 pounds per square inch gauge pressure (psig). The drive pressure can be regulated to as low as 5 to 10 psig.

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The pressure helps to determine flow, tidal volume, and carbon dioxide elimination. Our studies indicate that the most efficient frequency for HFJV is 100 to 200 breaths per min coupled with a TI of 20 to 30 per cent of the entire cycle. 22 Factors important to selecting the most effective pattern of operation include minimal levels of PAW, PIP, and PaC0 22 as well as cardiac depression. It is hoped that maximal oxygenation develops. Inspiratory flow during HFJV resembles a square wave. Peak flow is reached early during inspiration and is maintained. Few data document the advantages or disadvantages of this pattern. The entrainment factor can complicate readings of PAW. The negative pressure necessary for entrainment affects pressures normally determined at the proximal airway. Recent evidence indicates that pressures are higher in the distallung units.

HIGH-FREQUENCY POSITIVE-PRESSURE VENTILATION HFPPV uses the same low compliance, nondistensible circuitry common to all high-frequency systems. The inspiratory limb attaches directly to the endotracheal tube without an injection catheter; therefore, entrainment does not occur. This was the original system used by Sjostrand, and it incorporated either an uncuffed endotracheal tube or a translaryngeal 22 Both provided for exhalation around, rather than through, the catheter. 22 tube lumen. With the development of rapidly responding exhalation valves, cuffed endotracheal tubes permit inspiration and expiration through the same lumen. Sjostrand described a "pneumatic value" in this original 22 The Coanda system that used frequencies of 60 to 100 breaths per min. 22 effect explained movement of gas further into the distal airways. With the cessation of positive pressure, gas exits through the same inspiratory lumen. Like HFJV, the flow pattern of HFPPV is described by a square wave. Flow generators used for HFJV can be easily adapted for HFPPV; the major modification is to the circuit, not to the ventilator per se. Venoperate at 60 to 150 breaths per min can be readily tilators that normally operate. modified for HFPPV. The only major requirement is a TI less than 30 per cent of the ventilatory cycle. The advantages of HFPPV include simple circuitry and easy adaptation to conventional machines. It should be noted that tidal volume or frequency is increased at the expense of increases in PAW and PIP.

HIGH-FREQUENCY OSCILLATION HFO differs radically from both HFJV and HFPPV. Unlike those systems, the same volume of gas moves continually back and forth in the airway. Oxygen is added at a rate to meet metabolic demands. Prototypical systems employed a diaphragm, which moved in and out depending on the electrical current, and the movement provided the oscillation. In small animals, carbon dioxide exchange and oxygenation are no problem with

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19 In humans and large animals, the diaphragm cannot move a this system. 19 large enough volume of gas, so that most investigators use a piston pump instead. With the addition of an offset cam, the piston stroke can be varied while the flow pattern remains sinusoidal. The current systems that deliver HFO are complex. Originally, carbon dioxide was removed by an in-line absorber. However, the absorber must be replaced quite often and increases the circuit resistance. All HFO systems now use a biased gas flow to remove carbon dioxide. This arrangement consists of a tube connected at 90° to the delivery circuit. 4 Resistance in this line is controlled by either a spring valve or the length of the tubing. The tidal volume delivered depends on the balance between the resistance in the patient's airway and that in the biased flow. As resistance to biased flow decreases, or airway resistance increases, more gas exits through that pathway and less gas reaches the patient. The high frequency and small tidal volume used in HFO preclude the use of spirometers and pneumotachographs to monitor and measure tidal volume. 13 Oxygen flow can be increased to increase the baseline airway pressure and to improve Pa0 22 • The range of frequency for HFO has not been defined. However, most systems usually operate in the range of 15 to 50 Hz. No single frequency is consistently optimal for either carbon dioxide elimination or oxygenation. 5

HUMIDIFICA nON HUMIDIFICATION Humidification is a major concern with all high-frequency systems. The typical wick or passover humidifiers have too much deadspace, which markedly decreases the tidal volume delivered to the patient. One simple system involves entrainment, as described for HFJV, in which the low flow, entrained gas passes first through a humidifier (Fig. 2). When added to the jet, the entrained gases more than adequately humidify the airway. drips saline in front of the jet. This proA second humidification system drip5 vides nebulization in addition to some humidification. However, if the drip is not regulated carefully, the patient can receive excessive amounts of fluid. flUid.

ENDOTRACHEAL TUBES Several different types of endotracheal tubes can be used for airway management during HFV. An uncuffed tube originally was placed so that gas could exit simultaneously as inspiration proceeded. Several different models of double-lumen tubes developed for HFV permit exhalation through one lumen simultaneous to inspiration through the other. Compared with a single-lumen tube, the double-lumen tube improves carbon dioxide elimination to some degree, which supports the concept of simultaneous bi-directional flow. A small catheter passed through the larynx has been used successfully with HFV during laryngeal surgery. By using the same concept, a percutaneous cricothyrotomy with a 14-gauge catheter

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coupled to HFV has demonstrated the value of this method to maintain gas exchange during complete airway obstruction. 17

APNEA Apnea often develops after the initiation of HFV.I6 HFV.16 This appears to be independent of PaC0 22 • Speculation on the cause of this phenomenon centers on some alteration of stretch receptors in both the lung and chest wall. Interestingly, vagotomy abolishes the apneic response. Not all patients or laboratory animals stop breathing during HFV. However, a markedly reduced respiratory rate without undue discomfort to the patient has been frequently reported. In our experience with HFJV, apnea has not occurred in either patients or laboratory animals.

CLINICAL INDICATIONS FOR HIGH-FREQUENCY VENTILATION At this time, the clinical indications for HFV remain to be delineated. The original indications for HFV, rigid bronchoscopy and laryngeal surgery, are still valid. Since lung movement and expansion are minimal during HFV, it may be advantageous during thoracic surgical procedures. The

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diminished movement may facilitate the operation by permitting the surgeon to work in a quieter field. With its small tidal volume and low airway pressure, HFV may lend itself well to the treatment of the idiopathic respiratory distress syndrome (IRDS) of the newborn. The high incidence of pulmonary barotrauma, bronchopulmonary dysplasia,2o dysplasia,20 and other related phenomena make HFV quite attractive for use with this disease. However, clinical experience relimited. mains liInited. A somewhat broader experience with adults suffering from acute respiratory distress syndrome (ARDS) treated with HFV has accrued;' accrued;? however, only one large study has been reported. A small number of isolated cases have demonstrated improved gas exchange during HFV compared with conventional therapy using mechanical ventilation and CPAP. However, in most of the case reports, HFV appeared to be neither better nor worse than conventional methods relative to gas exchange or outcome. A laboratory study using the oleic acid lung injury model demonstrated that with HFV, PAW had to be in the same range as with conventional venti12 The only siglation and CPAP before oxygenation improved significantly. 12 nificant difference was the PIP, which was lower with HFV. Nevertheless, this may be important since both a large tidal volume and a high PIP appear to be the major contributors to pulmonary barotrauma. In discussing the indications for HFV, the definition of "maximal" therapy or of "failure" with conventional techniques may vary markedly among clinicians. During the extracorporeal membrane oxygen study (EMCO),25 maximal therapy for ARDS included a PEEP level of 5 to 10 cm H 20. Nowadays, these values would be considered as only minimally therapeutic. HFV appears to work well in the management and treatment of bronchopleural fistulae. 88 Usually, with this condition, PaC0 22 increases, which requires that respiratory rate, tidal volume, and inflation pressure also be increased in order to maximize gas exchange and, particularly, PaC0 PaCO 2z.' These measures all tend to prevent the lesions from healing. However, the lower airway pressure achieved with HFV should provide a better environment for resolution of the injury. High airway pressure, by inhibiting venous return, can worsen an increased intracranial pressure. Since, with elevated ICP, the brain is on a steepened portion of the pressure/volume curve, small improvements in venous return can profoundly affect pressure. Here again, HFV can be beneficial (Fig. 3). Recent work in our laboratory indicates that, for ARDS, HFV can be most effective when combined with both conventional mechanical ventilation and CPAP. The large positive pressures appear to recruit alveoli that would otherwise not participate in gas exchange. Values for Pa0 PaO 2z and veCP AP and PAW levels less than those denous admixture are obtained at CPAP veloped during conventional support. Clinical observations with HFV often include a marked outpouring of broncheal secretions soon after the initiation of HFV. Since vigorous vibration of the chest occurs, HFV may provide SOlne some form of internal physiotherapy. Often, atelectasis improves with the use of HFV.

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Figure 3. A, Despite hyperventilation, intracranial pressure (ICP) is still elevated during conventional ventilation. B, During high frequency ventilation, there is a marked drop in = mean airway pressure; PEEP = intracranial pressure without any change in PaC0 PaCO,.2 • PAW PAw=mean PEEP= positive end-expiratory pressure.

THEORIES BEHIND HIGH-FREQUENCY VENTILATION There are still no definitive answers to the workings of HFV. Current monitoring technology is not yet up to the demands of this method. Devices such as the pneumotachograph are simply not fast enough to respond to the high frequencies. The high velocities prevent the use of various ventilation scanning techniques. It may become necessary to develop or borrow technologies not previously available in the biomedical sciences.

641

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Normal ventilation combines convection and diffusion. Bulk flow occurs in the large airways and allows gas to move by convection to the peripheral exchange units. The gradients between alveolar gas and pulmonary capillary gas tensions are such that carbon dioxide diffuses from the capillaries into the alveoli, then moves toward the proximal airways. Oxygen moves in the opposite direction. Cardiogenic mixing acts as a secondary force in gas exchange. ll Heart movement activates the gas molecules in neighboring airways, which direct the diffusion gradients further into the terminal airway units. In effect, the gas profile is located more distal in the airway than explained solely by either simple diffusion or convection. Most explanations of how HFV works take these normal mechanisms into account, at least to some degree. flow represent one popular "theory." The aerodynamics of coaxial How more turbulent flow, While the large airways are usually associated with a n10re flow. 14 Laminar flow prothe smaller branches primarily develop laminar How. files have a parabolic-shaped edge. The gas molecules do not all move at files the same velocity. Those along the edges interact with the side walls and move slowly. The particles in the center encounter no such impedance. flow. Generators for The result is the smooth, parabolic profile of laminar How. flow characteristics. As gas moves into the HFO can operate with laminar How airway, the more centrally located molecules move faster than those along the edges. When flow reverses, the same phenomenon occurs. Particles in the center move faster; however, since they start to move from more peripheral points in the distal airway, cell particles are at the same point at the end of this stroke. If each molecule were tracked individually, it would appear that the gas molecules along the edge move back toward the proximal airways while those located centrally move into the gas exchange units (Fig. 4). The simultaneous, bi-directional flow accounts for both oxygenation and carbon dioxide elimination. This illustrates the coaxial How flow theory. Unfortunately, there is little proof that this pattern actually occurs, although at very high frequencies there is little delay between inspiration and expiration, which makes the postulated pattern plausible. Another popular explanation of HFV is the concept of enhanced diffusion. fusion.l10,0 ' 21, 24 Much of HFV is administered at velocities greater than 50 cm per sec. These velocities suggest Reynold's numbers in the range of

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Figure 5. The random momotion of the turbulent gas lecular lTIotion excites neighboring molecules so that the effect of the gas is exerted much more peripherally during enhanced diffusion than otherwise.

20,000. This is consistent with turbulent flow, in which all molecules move randomly. The greater the turbulence, the more the molecules move. The gas profile of turbulent flow resembles a blunted surface in contrast to the smooth parabola of laminar flow. Simplistically, enhanced diffusion implies that molecules already in motion excite neighboring particles, which are, then, also set into motion. The enhanced activity distributes the effect of the gases much more peripherally in the airway than can otherwise be accounted for (Fig. 5). This may help to explain the effectiveness of the small tidal volume. HFV.lS He ventilated Lehr has proposed a third mechanism to explain HFV.18 excised canine lungs at high frequencies and photographed them by using stroboscopic lighting. He observed that the lung inflated asymmetrically, with some units as much as 18000 out of phase. Grossly, this appeared to some alveoli, then be similar to a pendeluft type of movement. Gases fill son1e by retrograde or intra-alveolar flow expand other units as the originals deflate. Again, a much smaller volume of gas can accomplish what theoretically might require as much as twice that volume.

FUTURE DIRECTIONS What is the future of HFV and will it supplant more conventional techniques? The information presently available indicates that HFV will likely become a modest addition to the clinical armamentarium for managing specialized problems, such as bronchopleural fistula or the overly high PIP that sometimes develops during conventional methods. A A major goal of recent studies has been to identify those clinical situations for which HFV might be most appropriate. Clearly, much remains to be done. However, with technological progress, our understanding of HFV should increase dramatically and should reveal practical uses for HFV. As with any new technique, HFV will no doubt prove to have certain limitations and disadvantages. These need to be identified and clarified before widespread clinical application of the technique. In time and with continued investigation, H FV should have an appropriate place in clinical respiratory care. HFV AKNOWLEDGMENT

Table 1 and Figures 4 and 5 have been reprinted with permission from Volume 10, 1982 of the American Society of Anesthesiologists: Refresher Courses in Anesthesiology.

REFERENCES 1. Armengol, J. A., Man, P. S. F., Logus, J. W., et al.: Effects of high frequency oscillatory ventilation on canine tracheal mucous transport (abstract). Crit. Care Med., 9:192, 1981.

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2. Banner, M. J., Gallagher, T. J., and Caruthers-Banner, T.: Determining ideal frequency and inspiratory time for high frequency jet ventilation in dog (abstract). Anesth. Analg., 62:249, 1983. 3. Baum, M., Benzer, H., Geyer, A., et al.: Gas exchange and flow properties during high frequency jet ventilation (abstract). Crit. Care Med., 9: 159, 1981. 9:159, 4. Bohn, D. J., Miyasaka, K., Marchak, B. E., et al.: Ventilation by high-frequency oscillation. J. Appl. App!. Physiol., Physio!., 48:710-716, 1980. 5. Butler, W. J., Bohn, D. J., Bryan, A. C., et al.: Ventilation by high-frequency oscillation in humans. Anesth. Analg., 59:577-584, 1980. 6. Caldwell, E. J., Powell, R. D., Jr., and Mullooly, J. P.: Interstitial emphysema: A study Am. Rev. Resof physiologic factors involved in experimental induction of the lesion. Anl. pir. Dis., 102:516-525, 1970. 7. Carlon, G. C., Kahn, R. C., Howland, W. S., et al.: Clinical experience with high frequency jet ventilation. Crit. Care Med., 9:1-6, 1981. 8. Carlon, G. C., Ray, C., Jr., Klain, M., et al.: High-frequency positive-pressure ventilation in management of a patient with bronchopleural fistula. Anesthesiology, 52:160162, 1980. 9. Duffin, J.: Physics for Anaesthetists. Springfield, Charles C Thomas, 1976. 10. Fredberg, J. J.: Augmented diffusion in the airways can support pulmonary gas exchange. App!. Physiol., Physio!., 49:232-238, 1980. J. Appl. 11. Fukuchi, Y., Roussos, C. S., Macklem, P. T., et al.: Convection, diffusion and cardioPhysio!., genic mixing of inspired gas in the lung; an experimental approach. Respir. Physiol., 26:77-90, 1976. 12. Gallagher, T. J., and Banner, M. J.: High frequency positive pressure ventilation for oleic acid induced lung injury (abstract). Crit. Care Med., 8:232, 1980. 13. Goldstein, D., Slutsky, A. S., Ingram, R. H., et al.: CO 22 elimination by high frequency ventilation (4 to 10 Hz) in normal subjects. Am. Rev. Respir. Dis., 123:251-255, 1981. 14. Haselton, F. R., and Scherer, P. W.: Bronchial bifurcations and respiratory mass trans208:69-71, 1980. port. Science, 208:69--71, 15. Hill, D. H.: Physics Applied to Anaesthesia. Edition 3. Boston, Butterworths, 1976. 16. Jonzon, A., Oberg, P. A., Sedin, G., et al.: High-frequency positive-pressure ventilation insuffiation. Acta Anaesthesio!. by endotracheal insufflation. Anaesthesiol. Scand., 15(Supp!. 15(Suppl. 43), 1971, 43 pp. 17. Klain, M., and Smith, R. B.: High frequency percutaneous transtracheal jet ventilation. Crit. Care Med., 5:280-287, 1977. 18. Lehr, J.: Circulating currents during high-frequency ventilation (abstract). Fed. Proc., 39:576, 1980. a!.: Carbon dioxide clearance during high 19. Ngeow, Y. K., Mitzner, W., Permutt, S., et al.: frequency oscillation (abstract). Crit. Care Med., 9:164, 1981. 20. Philip, A. G.: Oxygen plus pressure plus 'time: The etiology of bronchopulmonary dysplasia. Pediatrics, 55:44-50, 1975. 21. Scherer, P. W., Shendalman, L. H., Greene, N. M., et al.: Measurement of axial diffuApp!. Physiol., Physio!., 38:719-723, 38:719-723,1975. sivities in a model of the bronchial airways. J. Appl. 1975. 22. Sjostrand, U.: Pneumatic systems facilitating treatment of respiratory insufficiency with Anaesthesio!. alternative use of IPPV/PEEP, HFPPV/PEEP, CPPB or CPAP. Acta Anaesthesiol. Scand., 21(Suppl. 21(Supp!. 64):123-147, 1977. 23. Sjostrand, U.: High-frequency positive-pressure ventilation (HFPPV): A review. Crit. Care Med., 8:345-364, 1980. 24. Taylor, G. T.: The dispersion of matter in turbulent flow through a pipe. Proc. R. Soc. Lond. [Sect. A],223:446-468, 1954. 25. Zapol, W. M., Snider, M. T., and Schneider, R. c.: C.: Extracorporeal membrane oxygenation for acute respiratory failure. Anesthesiology, 46:272-285, 1977. Department of Anesthesiology University of Florida College of Medicine Box J-254, J. Hillis Miller Health Center Gainesville, Florida 32610