A NEW INFANT OSCILLATORY VENTILATOR

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British Journal of Anaesthesia1990; 64: 374-378 A NEW INFANT OSCILLATORY VENTILATOR M. K. CHAKRABARTI, A. HOLDCROFT, S. SAPSED-BYRNE AND J. G.

1 British Journal of Anaesthesia1990; 64: A NEW INFANT OSCILLATORY VENTILATOR M. K. CHAKRABARTI, A. HOLDCROFT, S. SAPSED-BYRNE AND J. G. WHITWAM SUMMARY A new, simple and inexpeive oscillatory ventilator is described in which a rotating jet mounted in the breathing duct generates cyclically positive and negative pressures in the airway with a sinusoidal flow waveform. Unlike conventional oscillatory ventilators it is free from restrictio to ipirator/ or expiratory gas flows and open to atmosphere at all times, making it intriically a safe system for ventilation. A prototype rotating jet oscillatory ventilator designed for application in infants was evaluated in rabbits (mean weight 3.8 kg). The positive peak and mean airway pressures were significantly less during oscillatory ventilation at 300 and 420 b.p.m. compared with normal and high frequency positive pressure ventilation at 30 and 300 b.p.m., respectively, while maintaining blood-gas teio within the normal range. An increase in the oscillatory frequency from 300 to 420 b.p.m. provided no further benefit in terms of airway pressure, tidal volume or blood-gas teio. KEY WORDS Ventilation: high frequency, oscillation, airway pressure. High frequency oscillatory ventilation (HFOV) has been used successfull y in the treatment of infants with respiratory failure from several causes [1-6]. Conventional oscillatory ventilators contain a piston or bellows pump which provides cyclically positive and negative pressures in the airway with a sinusoidal flow waveform. The respiratory fresh gas (RFG) is supplied as a continuous bias flow, usually at the proximal end of the tracheal tube. A narrow-bore outlet tube is used to discharge the expired gas from the patients along with the bias flow gas. It acts as a low pa s filter to impede the discharge of the high frequency ipiratory gas flow so that it is delivered to the lung. The tidal volume is adjusted normally by changing the excursion of the piston or bellows or by introducing a leak [5,6]. In all such systems, to avoid the accumulation of carbon dioxide from expired gas and to meet the peak ipiratory flow demand during spontaneous ipiration, the RFG must be high, which in turn may generate undesirably high continuous positive pressure in the airways (PEEP) because of the high resistance of the expiratory outlet. If the expiratory resistance is decreased to obviate the PEEP effect, most of the ipiratory tidal volume is lost. Some workers have used very high RFG with balanced suction at the expiratory outlet to counter this problem [7]. A new, simple and inexpeive ventilator is described which provides oscillatory ventilation especially in infants. METHODS Oscillatory ventilator The new oscillatory ventilator is essentiall y a rotating gas jet (J r ) placed dowtream in the widebore open limb of a T-piece breathing system and alternately directed upstream and dowtream (fig. 1). The warmed and humidified, low pressure RFG is supplied to the proximal end of the tracheal tube, which is connected to a standard three-way connector, at a flow rate of not less than 300 ml kg" 1 with a minimum of 2 litre min" 1. The rotating jet, the orifice of which has an internal diameter of 1.2 mm, is coupled to a variable speed electric motor and the oscillation frequency is set by controlling the speed of the motor. The driving M. K. CHAKRABARTI, B.SC, M.PHIL. ; A. HOLDCROFT, M.B., CH.B., F.F.A.R.C.S., M.D.J S. SAPSED-BYRNE, A.I.BIOL.J J. G. WHITWAM, M.B., CH.B., PH.D., F.R.c.p., F.F.A.R.C.S. ; Department of Anaesthetics, Royal Postgraduat e Medical School, Hammersmith Hospital, DuCane Road, London W12 ONN. Accepted for Publication: September 4, 1989.

2 NEW INFANT VENTILATOR 375 RFG- Air-oxygen or anaesthetics Gas Humidifier Outlet FIG. 1. Schemati c diagram of oscillatory ventilator. RFG = Respiratory fresh gas; Fl and F2 = bacterial niters; J r = rotating jet; J p = PEEP jet; PR1 and PR2 = pressure regulators for the driving pressure for J r and J p. gas to the rotating jet is supplied from a standard hospital pipe line (400 kpa, 60 lbf in 2 ) through a pressure or flow regulator (PR1). The outlet of the pressure regulator is connected to the rotating jet assembly. The gas flows into a cavity surrounding the jet shaft, which in turn connects to the jet nozzle through a 5-mm channel in the shaft. The outer body in which the jet shaft rotates is made of PTFE material so that no gas leaks between the shaft and the body; a high pressure gas seal could be used. However, as the gas flows continuously through a comparativel y low resistance path, gas leakage has not been a problem. The driving gas may be any dry gas, either the same air-oxygen mixture as the RFG, or air. The open limb of the T-piece system is a standard disposable infant ventilator tube (i.d. 10 mm; not less than 50 cm long). The outlet tube of the jet assembl y is 8-10 mm i.d. and cm long. When driving gas is supplied to the rotating jet it cyclically generates sinusoidal flow in the breathing duct, confirmed by Fourier analysis using a "waterfall" programme (Cambridge Electronic Design, CED 1401). A sinusoidal positivenegative pressure wave-form is generated with an ipiratory to expiratory (I:E) ratio of 1:1, which acts as a pneumatic piston to drive the RFG in and out of the lungs. The magnitude of the pressure which is generated and applied to the airways depends on the driving pressure, the gas flow, and the design of the jet assembly. The nominal driving gas coumption varies between 10 and 15 litre min" 1, depending on the airway pressure requirement. PEEP is created by a fixed jet (J p ) with an internal diameter of 0.8 mm placed dowtream of the rotating jet (fig. 1). A pressure or a flow regulator (PR2) controls the magnitude of externally applied PEEP, in a manner identical to that of a new infant ventilator described elsewhere [8]. At the outlet of the jet manifold (fig. 1), a wide bore tube (standard disposable tube of 22 mm i.d.) and a bacterial filter (F2) of negligible resistance to gas flows ( < 0.1 kpa at 1 litre s" 1 ) are used as a silencer to suppres jet noise. In clinical application, another similar filter (Fl) may be used between the breathing system and jet manifold to avoid bacterial contamination of the jet assembl y [8]. The jet assembl y can be sterilized in situ by nebulizing a chemical agent through the assembl y [8]. Alternatively, it may be removed and autoclaved or gas-sterilized. Although this particular oscillatory ventilator is designed for infants, the same principle could be used for adults, with suitable adjustment of the jet assembly. However, the gas coumptio are much greater.

3 376 Evaluationin rabbits The new ventilator was used to apply HFOV to seven rabbits (mean (SD) weight 3.8 (0.4) kg) with normal lungs. Comparison was made with high frequency ventilation (HFPPV) at 300 b.p.m. using a similar valvele s infant ventilator designed for both normal and high frequency ventilation [8,9]. Anaesthesi a was induced with thiopentone mg kg" 1 into an ear vein and maintained with a continuous infusion of 1 % thiopentone 6-9 ml h" 1 (20 mg kg" 1 h" 1 ). The femoral artery and vein were cannulated for arterial blood sampling and injection of drugs and fluids, respectively. The animals were paralysed with suxamethonium 10 mg i.v. in repeated bolus doses after tracheotomy had been performed with an uncuffed paediatric tracheal tube (4.5 mm i.d., length 10 cm). A snare was placed around the trachea to prevent gas leak and, initially, ventilation was controlled (IPPV) at 30 b.p.m. with an I: E ratio of 1:2 using the infant ventilator [8,9]. The pressure applied to the driving jet was adjusted to maintain the airway pressure at cmhjo and Pa COt at kpa. The arterial and airway pressures were measured by strain gauge traducers and displayed and recorded (M19 Devices, Welwyn Garden City, U.K.). Calibration was performed agait mercury and water colum. Airway pressure (Paw) was measured at the proximal end of the tracheal tube at the three-way connector. After the control samples and recordings were taken either HFPPV at 300 b.p.m. or HFOV at 300 and 420 b.p.m. were applied. The driving pressure of the rotating jet was adjusted to maintain a similar Pa co during HFOV as during HFPPV. At least 20 min was allowed after each change in mode or frequency of ventilation before bloodgas samples were taken and pressure measurements recorded. Arterial blood-gas samples were analysed immediately with a Radiometer bloodgas analyser. Tidal volumes (KT) were measured at each frequency of ventilation or oscillation with a pneumotachograp h system (Gould Medical) by placing the flow traducer between the tracheal tube and the three-way connector of the breathing duct. After the measuremen t of VT the flow traducer head was removed to avoid an increase in ventilation deadspace. The pneumotachograp h recording system used has a frequency respoe flat to 80 Hz and was calibrated before each measuremen t with a gas syringe. BRITISH JOURNAL OF ANAESTHESIA Statistical analysis was performed using twoway analysis of variance and paired t tests where appropriate. P < 0.05 was regarded as significant. RESULTS The flow waveform during HFOV at 300 b.p.m. in rabbits had a sinusoidal pattern creating sinusoidal positive and negative airway pressures as ipiration and expiration, respectivel y (fig. 2). The driving gas coumption was litre min" 1. Airway pressure waveform also was sinusoidal (fig. 2). The meanpeak airway pressure(paw) decreased significantly during HFPPV at 300 b.p.m. and HFOV at 300 and 420 b.p.m. compared with the control IPPV (table I), but differences in ipired peak Paw between HFOV and HFPPV were not significant. During HFOV, but not HFPPV, negative airway pressures were generated during the expiratory phase (table I). The mean airway pressuresduring HFOV decreased significantly compared with HFPPV and control IPPV (30 b.p.m.). A low PEEP of 0.2 kpa was observed during HFPPV, but not during HFOV (table I). Tidal volumewaveform produced by the ventilator is shown in figure 2. FT decreased significantly from a mean value of about 10 ml kg" 1 to 2 ml kg" 1 during HFPPV and HFOV compared ~ , 0 20J M M FIG. 2. Record of flow, airway pressure and volume characteristics of the new oscillatory ventilator. Oscillatory frequency = 420 b.p.m. Upward deflectio = ipiratory phase. 1s

4 NEW INFANT VENTILATOR 377 TABLE I. Ventilatory and blood-gasdata {mean {SEM)) urith different modesof ventilation in rabbits (n = 7). IPPV = Intermittent positive pressureventilation {control); HFPPV = high frequency positive pressureventilation; HFOV = high frequency oscillatory ventilation. *P < 0.05, paired t test comparedwith IPPV; ^analysis of variance IPPV 30 b.p.m. HFPPV 300b.p.m. HFOV 300 b.p.m. HFOV 420 b.p.m. Peak airway pressure (kpa) lpiratory Expiratory Mean airway pressure (kpa) Tidal volume (ml) /X-o, (kpa) Pao.CkPa) ph (arterial) Mean arterial pressure (mm Hg) 1.3(0.17) (0.07) 41 (5.6) 4.2 (0.27) 54.3 (3.3) (0.052) 113(8) 0.9 (0.20)* + 0.2(0.05) 0.50 (0.09) 8(1.4)* 4.0 (0.41) 57.3 (3.6) (0.019) 114(12) 0.6(0.16)* -0.4(1.0)* 0.17(0.04)* 7.5(1.2)* 4.1 (0.51) 55.2 (3.6) (0.019) 113(10) 0.5(0.10)* -0.4(0.09)* 0.16(0.04)* 7.1 (1.8)* 4.5 (0.81) 51.4(3.8) (0.025) 110(9) with control IPPV, without a change in Pa COi (table I). There was no significant difference in VT between HFPPV and HFOV at 300 and 420 b.p.m. Ventilation was maintained adequatel y throughout, as reflected in Pa COf values (table I). However, there was a smal increase in Pa^ during HFOV at 420 b.p.m. compared with 300 b.p.m., although the driving pressure was kept cotant. There were no significant differences in ph, Pao or mean arterial pressure (MAP) during the study period (table I). DISCUSSION The phenomenon of gas trapping in the lungs during high frequency ventilation (HFV) because of short expiratory time compared with the time cotant of the pulmonary system is well known [8]. An active expiratory phase during HFOV may overcome or minimize the problem of an undesirable increase in lung volume during HFV, and hence maintain a lesser mean airway and alveolar pressure. Oscillatory ventilators normally comprise either a reciprocating piston or bellows and the ventilation system is almost closed, with high impedance to expiratory flow [5 7]. The new oscillatory ventilator described here, although providing very similar ipiratory and expiratory sinusoidal flow patter (fig. 2) is open to the atmosphere at all times, with a negligible resistance to both ipiratory and expiratory flow; therefore, spontaneous respiration is unimpeded, even when the RFG is only 2 litre min" 1, which is less than the peak ipiratory flow rate of an infant. The rotating jet could also be applied directly to the airway, in which case the driving gas would become the respiratory fresh gas; however, this configuration would increase the apparatus deadspace because of the connecting tubes. In this study the frequency of oscillatio was maintained at 300 and 420 b.p.m. Rossing and colleagues [7] demotrated that, at frequencies greater than 300 b.p.m., no further improvement in elimination of carbon dioxide could be achieved, even with a cotant tidal volume. The present study confirmed this observation. A desirable feature of this oscillatory ventilator is the absence of resistance during expiration, thereby preventing uncontrolled PEEP and gas trapping in the lungs. During HFPPV at a similar frequency of ventilation, 0.2 kpa PEEP was observed. A trapped gas volume of 2 litre has been observed in man after application of an oscillatory ventilator at frequencies less than 10 Hz for only 30 s [10]. If PEEP is required in this new system, it may be applied by using the second fixed jet. Although the blood-gas teio during HFPPV and IPPV were very similar to HFOV at 300 b.p.m., the mean airway pressures were significantly lower during HFOV, which has been shown to be beneficial to infants with persistent pulmonary hyperteion [6]. A multicentre trial [11] of oscillatory ventilation in critically ill pre-term infants has not shown any benefit compared with conventional positive pressure ventilation. However, some of the earlier studies [1-6], which led to the multicentre trial, were more encouraging. The differences between the studies may be explained by lack of uniformity of lung pathology, the ventilation frequency used and the characteristics of the ventilators used. This new jet driven oscillatory ventilation

378 BRITISH JOURNAL OF ANAESTHESIA system using a relatively low respiratory fresh gas flow upstream in a wide bore breathing tube free of restrictio and valves prevents an increase in airway

5 378 BRITISH JOURNAL OF ANAESTHESIA system using a relatively low respiratory fresh gas flow upstream in a wide bore breathing tube free of restrictio and valves prevents an increase in airway pressures. The preliminary results suggest that it may overcome some problems of earlier ventilators and is worthy of further investigation. REFERENCES 1. Bohn DJ, Miyasaka K, Marchak BE, Thompson WK, Froese AB, Bryan AC. Ventilation by high-frequency oscillation. Journal of Applied Physiology1980; 48: Frantz ID in, Werthammer J, Stark AR. High frequency ventilation in premature infants with lung disease: adequate gas exchange at low trachcal pressure. Pediatrics 1983; 71: 483-^ Marchak BE, Thompson WK, Duffty P, Miyaki T, Bryan MH, Bryan AC, Froese AB. Treatment of RDS by high frequency oscillatory ventilation: A preliminary report. Journal of Pediatrics 1981;99: Boynton BR, Mannino FL, Davis RF, Koptic RJ, Friederichsen G. Combined high frequency oscillatory ventilation and intermittent mandatory ventilation in critically ill neonatcs. Journal of Pediatrics1984; 105: Froese AB, Butler PO, Fletcher WA, Byford LJ. Highfrequency oscillatory ventilation in premature infants with respiratory failure: a preliminary report. Anesthesiaand Analgesia 1987; 66: Kohelet D, Perlman M, Kripalani I, Hanna G, Koren G. High-frequency oscillation in the rescue of infants with persistent pulmonary hyperteion. Critical Care Medicine 1988; 16: Rossing TH, Slutsky AS, Lehr JL, Drinker PA, Kamm R, Drazen JM. Tidal volume and frequency dependence of carbon dioxide elimination by high frequency ventilation. New England Journal of Mediane 1981; 305: Chakrabart i MK, Whitwam JG. A new infant ventilator for normal and high-frequency ventilation: influence of tracheal tube on distal airway pressure during high frequency ventilation. Critical Care Medicine1988; 16: Chan KN, Chakrabart i MK, Whitwam JG, Silverman M. Assessmen t of new valveles infant ventilator. Archivesof Diseasesof Childhood 1988; 63: Saari AF, Rossing TH, Solway J, Drazen JM. Lung inflation during high frequency ventilation. American Reviewof RespiratoryDiseases1984; 129: The HIFI Study Group. High-frequency oscillatory ventilation compared with conventional mechanical ventilation in the treatment of respiratory failure in preterm infants. New England Journal of Medicine1989; 320:

How does HFOV work? John F Mills MBBS, FRACP, M Med Sc, PhD Neonatologist Royal Children s Hospital. Synopsis

How does HFOV work? John F Mills MBBS, FRACP, M Med Sc, PhD Neonatologist Royal Children s Hospital Synopsis Definition of an oscillator Historical perspective Differences between HFOV and CMV Determinants