British Journal of Anaesthesia 1990; 65: 262-267
COMPARISON OF TWO VENTILATORS USED WITH THE T-PIECE IN PAEDIATRIC ANAESTHESIA D. J. HATCH, M. K. CHAKRABARTI, J. G. WHITWAM, R. M. BINGHAM AND A. M. MACKERSIE
SUMMARY The Nuffield 200 ventilator was compared with a new valve less ventilator (CW 200) in 20 children undergoing general anaesthesia for paediatric surgery. The new ventilator incorporates design features which overcome the main disadvantages of the Nuffield 200 and make it an inherently safer machine. At identical ventilator settings it produced a significantly greater tidal volume with a reduction in end-tidal carbon dioxide partial pressure. This may have advantages in avoiding pulmonary barotrauma in children.
both in a group of children during general anaesthesia. MATERIAL AND METHODS
Clinical evaluation The study, which was approved by the Hospital Ethics Committee, was performed in 20 children aged 4 months to 8 yr (mean age 2.9 (SD 2.3) yr) and who weighed between 5.8 and 26.3 kg (mean weight 13.1 (5.5) kg), receiving general anaesthesia for a variety of surgical conditions (table I). Anaesthesia was induced with cyclopropane in oxygen or i.v. thiopentone and maintained with 70 % nitrous oxide in oxygen supplemented with KEY WORDS either 0.5-1 % halothane or i.v. fentanyl. Tracheal Anaesthesia: paediatric. Equipment: ventilators, T-piece. intubation was facilitated by suxamethonium 1-2 mg kg"1, after which neuromuscular block was maintained with atracurium (initial bolus Relatively few ventilators are convenient for use dose of 0.5 mg kg"1, followed by an infusion of with the T-piece in paediatric anaesthesia. T-Piece 1 1 occluders suffer from the drawback that the 8 ug kg" min" ) monitored by peripheral nerve ventilatory fresh gas flow (FGF) is used not only stimulation. to provide anaesthesia and oxygenation, but also TABLE I. Details of the 20 patients to inflate the lungs. This makes these ventilators relatively inflexible in use, limits the age range Age (yr) (mean (SD)) 2.9 (2.3) over which they are practicable and requires FGF Range 0.25-8.0 rates considerably in excess of those needed to Weight (kg) (mean (SD)) 13.1 (5.5) maintain normocapnia. Sex (M/F) 15/5 Type of operation Various modifications of adult ventilators have 7 Urological been described for use with the T-piece, the most 7 Plastic recent of which uses a fixed orifice leak built into 5 Abdominal the patient valve of the Nuffield 200 ventilator [1]. 1 Dental This is a more versatile machine which works well in practice. However, it does have serious theor- D. J. HATCH, M.B., B.S., F.F.A.R.C.S. ; R. M. BINGHAM, M.B., etical disadvantages, some of which make it B.S., F.F.A.R.C.S.; ANGELA M. MACKERSIE, B.SC., M.B., B.S., potentially dangerous in clinical use. The purpose F.F.A.R.C.S. ; Department of Anaesthestics, The Hospital for of this paper is to compare the performance of the Sick Children, Great Ormond Street, London WC1N 3JH. K. CHAKHABARTI, M.PHIL., B.SC.; J. G. WHTTWAM, M.B., recently described valveless all-purpose ventilator M. CH.B., PH.D., F.R.C.P., F.F.A.R.C.S.; Department of Anaesthetics, [2, 3] with that of the Nuffield 200 ventilator with Hammersmith Hospital, Du Cane Road, London W12 0HS. the Newton valve modification and to evaluate Accepted for Publication: January 19, 1990.
263
VENTILATORS FOR PAEDIATRIC ANAESTHESIA Ventilator
— Newton paediatric valve
.
^=—•
. j /
Infanta Wright respirometer
To aneroid manometer
Fresh gas flow FIG. 1. Diagram of apparatus used in the study, shown with the N 200 ventilator. Air oxygen or anaesthetics
Exhaust
FIG. 2. Diagrammatic representation of CW200 ventilator. Jl = Driving jet; J2 = positive endexpiratory pressure (PEEP) jet; T = trachea! rube. The ventilating frequency and I:E ratio are controlled by an electronic pulse generator operating a solenoid valve. The fresh gas flow is displayed continuously by a flow sensor, S2, (Magtrack) which is provided with an alarm for a minimum set flow. The expired tidal and minute volumes are measured by using another similar flow sensor, SI, and taking the difference in reading between Si and S2 during the expiratory phase.
Mechanical ventilation was commenced using either the Nuffield 200 ventilator with a Newton valve (N 200) (fig. 1) or the all-purpose valveless ventilator (CW 200) (fig. 2), attached to the expiratory limb of a modified T-piece. The choice of initial ventilator for each patient was made by random selection. FGF was set according to weight (less than 10 kg: 31itremin~l; 10-15 kg:
4.5 litre min"1; more than 15 kg: 6 litre min"1) using flowmeters which were calibrated against those of known accuracy. Tidal volume (Vr) was adjusted to ensure adequate chest movement, with an inspiratory: expiratory (I:E) ratio of 1:2. The ventilatory frequency (/) was set at 14-30 b.p.m. according to the age of the child. The inspired oxygen concentration (FiOj) was
264
monitored with a fuel cell, peripheral oxygen saturation (5aOl) with a pulse oximeter, and endtidal carbon dioxide partial pressure (PEC0 ) with a Hewlett-Packard infra-red capnometer (model 472IDA with digital and paper readout), calibrated before each study. The capnometer cuvette was placed in the apparatus deadspace and the paediatric cuvette (deadspace 2 ml) was used in children weighing less than 10 kg. Airway pressure within the breathing system was measured using an aneroid manometer (0-7 kPa); its accuracy was checked against a mercury column manometer. Systemic arterial pressure (SAP) was measured every 3 min with an automated cuff (Dinamap) and the ECG and beat-by-beat heart rate (HR) recorded. A precordial stethoscope was applied routinely. Train-of-four responses to peripheral nerve stimulation (either theulnar or tibial nerves) were maintained at less than 25 % throughout the period of study. Measurements of / and PECO, from the capnometer, SaOjJ SAP, HR and peak inflation pressure (PIP) were made at 5-min intervals for 15 min after PEcOj had stabilized. The volume of gas leaving the ventilator during inspiration was measured also, using a Wright Infanta respirometer, which records VT accurately between 15 and 200 ml [4], placed in the distal limb of the T-piece as close as possible to the ventilator. After the 15-min measurement period a change was made to the alternate ventilator, which was adjusted to the same/, PIP and i: E ratio as the first machine. The measurements described above were then repeated at 5-min intervals for a further 15 min. In two subjects the surgical procedure was of sufficient duration to allow the patient to be returned to the first ventilator and for the initial measurements to be repeated. Laboratory evaluation The pressure, flow and volume wave forms developed by each ventilator were recorded in the laboratory under identical conditions. Each ventilator in turn was connected by a standard Jackson-Rees modification of Ayre's T-piece to a model lung with a compliance set at either 10 or 30 ml kPa"1 and a resistance set at either 10 or 20 kPa litre"1 s. The peak inflating pressure was set as close as possible to 2 kPa with / at 20 b.p.m. and an I:E ratio of 1:2. Pressure changes at the junction between the model lung and the T-piece were measured with a Validyne MP 45 (± 5 kPa)
BRITISH JOURNAL OF ANAESTHESIA pressure transducer, and air flow was measured using a Fleisch No. 0 pneumotachograph connected to a Validyne MP 45 (±0.2 kPa) differential pressure transducer. The flow signal was integrated electronically to give volume. Timebased pressure, flow and volume signals were stored on a Compaq Deskpro 110 computer and displayed on an Epson FX 850 printer. RESULTS
Clinical evaluation In all children the CW 200 produced a greater FT for a given airway pressure than the N 200, the smallest increase being 17 % and the largest being 173 % in individual patients, with an overall 19 % mean increase of 46 (19.6) ml, from 93 ml to 139 ml (table II) (P < 0.001). TABLE II. Tidal volumes produced by the CW 200 and N 200 ventilators at identical frequencies and peak airway pressures (I:E ratio 1:2)
Tidal volume (ml) Patient No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mean (SD) Range
CW200
N200
CW200-N200
55 170 110 305 55 140 280 200 175 180 160 155 100 125 85 60 60 85 105 170 139
33 115 90 220 33 120 200 150 125 110 120 120 50 53 40 23 22 50 50 140 93
22 55 20 85 22 20 80 50 50 70 40 35 50 72 45 37 38 35 55 30 46(19.6) 22-85
In all patients, PEC 0 , was less during ventilation with the CW200 than with the N 200, with differences ranging from 0.14 kPa (3%) to 0.93 kPa (19 %), and an overall mean difference of 0.55 kPa (11.5 %) (P < 0.001), (table III). No significant changes in HR, SAP or Sa^ were seen during the study period in any patient.
VENTILATORS FOR PAEDIATRIC ANAESTHESIA TABLE I I I . End-tidal carbon dioxide partial pressures (PB'co,) produced by the CW 200 and N 200 ventilators at identical frequencies and peak airway pressures (l.E ratio 1:2)
Patient No. 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 Mean (SD) Range
CW200
N200
N200-CW200
5.32 4.39 4.12 3.19 3.72 4.52 4.12 3.99 3.59 3.86 4.26 4.52 3.86 4.26 3.59 5.19 5.59 3.46 5.05 3.33 4.19
5.85 5.32 4.26 3.72 4.12 4.92 5.05 4.26 3.99 4.79 4.79 4.79 4.79 4.92 4.39 5.59 5.85 3.72 5.85 3.86 4.74
0.53 0.93 0.14 0.53 0.4 0.4 0.93 0.27 0.4 0.93 0.53 0.27 0.93 0.66 0.80 0.40 0.26 0.26 0.80 0.53
0.55 (0.26) 0.14-0.93
265
Laboratory evaluation
Whilst the pressure wave produced by the N 200 was similar to that of a pressure generating ventilator, the pressure wave produced by the CW 200 was almost square (fig. 3). Theflowand volume waves changed as expected for pressure generating ventilators when the compliance and resistance of the model lung were changed. DISCUSSION
Both the CW 200 and the N 200 ventilators are easy to use with the paediatric T-piece, as each can be attached to the expiratory limb instead of the manual inflation bag, acting very much as an "educated hand". As the driving gas used to inflate the lungs is independent of the ventilatory FGF, controlled rebreathing techniques may be used in infants and young children with FGF as low as 1000 + 200 ml kg"1, without causing hypercapnia [5] and with a reduction in heat and water vapour loss. The ability to control arterial partial pressure of carbon dioxide (PaCOj) by adjusting the FGF independently of the degree of lung inflation may be of benefit to patients in low
CW200
C R
3ml(cmH 2 O)' 1 1
10 kPa litre' «
1ml(cmH20)*1 10 kpa litre' 1 >
3ml (cm HjO)"1 .•1. 20kPe litre"*
FIG. 3. Pressure, flow and volume wave forms produced by the N 200 and CW 200 ventilators in vitro when ventilating a model lung at resistances (R) of 10 and 20 kPa litre"1 s and compliances (C) of 1 and 3 ml (cm H.O)"1.
266
output cardiac failure and in the avoidance of bronchopulmonary dysplasia [6, 7]. Although the NuiBeld 200 ventilator has proved satisfactory in practice, there are several disadvantages to the fixed orifice leak principle, which make it potentially very dangerous. Newton, Hillman and Varley [1] demonstrated that the modified Nuffield ventilator behaves as a pressure generator, and our in vitro measurements (fig. 3) confirmed this. As with any pressure generator, reduction in compliance or increase in resistance at the patient end of the system leads to a reduction in ventilation. This underventdlation may not be apparent by visual observation of the pressure monitor, and airways obstruction may not always be detected by the pressure alarm, as in small children the PIP is determined largely by the size of the leak through the Newton valve. Volume alarms are difficult to fit because of the continuous flow of the FGF passing through the expiratory limb of the T-piece, in addition to the expired volume. It is also possible to have apparently satisfactory ventilation with good chest movement despite significant rebreathing if the FGF is inadequate. Finally, the modified valve is suitable only for children up to about 15 kg, with an unmodified valve being required for larger children. The repeated changing of the valve required if children above and below this weight are to undergo ventilation is time consuming and may eventually cause damage to the valve. It is sometimes impossible to avoid the application of a small amount (0.1-0.2 kPa) of positive endexpiratory pressure (PEEP), especially in older children. Although the CW 200 has also been shown to have the characteristics of a pressure generator [2], again confirmed in this study (fig. 3), it incorporates design features which overcome the problems described above. It is suitable for use at all ages from birth to adult life, without modification. Expired volume is monitored by automatic subtraction of the ventilatory FGF flowing in expiration from the total expired volume, allowing tidal and minute volumes to be displayed continuously, with appropriate alarms, in addition to an FGF failure alarm. The expired volume signal could be used to provide feed-back information to the driving jet, which would enable the machine to deliver a servo-controlled volume. The PEEP is adjusted easily, and as it is a valveless ventilator it can be used for weaning children from ventilatory support through in-
BRITISH JOURNAL OF ANAESTHESIA termittent mandatory ventilation without the need for sensitive triggering devices, on to continuous positive airway pressure. If desired, the new ventilator can be used also at high frequencies of ventilation. The results of this study show that, when compared with the N 200 in the same patient at identical settings, the CW 200 ventilator gave a significantly greater FT for a given peak inflation pressure, with a consequently lesser PECO, m every patient. As volume and pressure were measured between the expiratory valve and the patient, this difference cannot be explained by the amount of gas passing through the fixed orifice leak of the N 200 ventilator. This was confirmed by the finding of a similar difference in older children, in whom the adult valve with no leak was used on the N 200 ventilator. Although the small amount of PEEP which is sometimes unavoidable with the N 200 may have been responsible for a slight reduction in tidal volume, the most likely explanation for these findings would appear to be the striking difference between the inspiratory pressure wave-forms produced by the two machines (fig. 3). A very important feature of the CW 200 is that FT is generated by the driving jet and not by the ventilatory FGF, which can be kept constant. At ventilatory frequencies up to 80 b.p.m., the inflation pressure profile is virtually a square wave, which implies that the PIP is applied throughout the inspiratory time. This is in contrast to the ramp function of the pressure wave produced by the N 200, and to the " thumb " devices with which the PIP is not reached until the ventilatory FGF generates a preset pressure in the system. This makes the CW 200 a much more efficient machine, particularly when inspiratory time is short and pulmonary compliance low, and may have important implications for the avoidance of bronchopulmonary dysplasia in infants with respiratory distress syndrome [6, 7]. Clinical studies have already shown improved results in very low birthweight babies with severe respiratory disease [8]. In conventional thumb devices, in order to increase the FT, FGF has to be greatly increased so that the preset pressure is reached earlier in the inspiratory phase, which in turn generates uncontrolled and possibly undesirable PEEP because of the resistance of the expiratory limb of the system. This study has demonstrated that the CW 200 ventilator is not only a safer, more flexible ventilator than the N 200 for use with the
VENTILATORS FOR PAEDIATRIC ANAESTHESIA paediatric T-piece, but that it is more efficient in terms of producing a significantly greater FT for a given peak inflation pressure under the conditions of the study. It is the first paediatric T-piece ventilator to provide an alarm responsive to FT in addition to the more conventional pressure alarms, and thus presents a major step forward in patient safety for ventilators used with the paediatric T-piece. Because there are no valves or other obstructions in the breathing duct, the patient can breathe freely at all times, allowing the provision of intermittent mandatory ventilation without complex systems for synchronization. PEEP can be applied deliberately as the frequency of ventilation is increased beyond the predicted time constant of the lungs.
2. 3.
4. 5.
6. 7.
REFERENCES 1. Newton NI, Hillman KM, Varley JG. Automatic ventilation with the Ayre's T-piece: a modification of the
8.
267
Nufficld 200 ventilator for neonatal and paediatric use. Anaesthesia 1981; 36: 22-36. Chakrabarti MK, Whitwam JG. A new valveless all purpose ventilator. British Journal of Anaesthesia 1983; 55: 1005-1015. Whitwam JG, Chakrabarti MK, Konarzewski WH, Askitopoulou H. A new valvelcss all-purpose ventilator: clinical evaluation. British Journal of Anaesthesia 1983; 55: 1017-1022. Hatch DJ, Williams GME. The Haloscale "Infanta" Wright rcspirometer. In vitro and in vivo assessment. British Journal of Anaesthesia 1988; 60: 232-238. Hatch DJ, Yates AP, Lindahl SGE. Flow requirements and rebreathing during mechanically controlled ventilation in a T-piece (Mapleson E) system. British Journal of Anaesthesia 1987; 59: 1533-1540. Taghizadeh A, Reynolds EOR. Pathogenesis of bronchopulmonary dysplasia following hyaline membrane disease. American Journal of Pathology 1976; 82: 241-264. Bancalari E. Bronchopulmonary dysplasia. In: Milner AD, Martin RJ, cds. Neonatal and Pediatric Respiratory Medicine, London: Butterworths, 1985: 54-80. Chan KN, Chakrabarti MK, Whitwam JG, Silverman M. Assessment of a new valveless ventilator. Archives of Disease in Childhood 1988; 63: 162-167.
COMPARISON OF TWO VENTILATORS USED WITH THE T-PIECE IN PAEDIATRIC ANAESTHESIA D. J. HATCH, M. K. CHAKRABARTI, J. G. WHITWAM, R. M. BINGHAM AND A. M. MACKERSIE
SUMMARY The Nuffield 200 ventilator was compared with a new valve less ventilator (CW 200) in 20 children undergoing general anaesthesia for paediatric surgery. The new ventilator incorporates design features which overcome the main disadvantages of the Nuffield 200 and make it an inherently safer machine. At identical ventilator settings it produced a significantly greater tidal volume with a reduction in end-tidal carbon dioxide partial pressure. This may have advantages in avoiding pulmonary barotrauma in children.
both in a group of children during general anaesthesia. MATERIAL AND METHODS
Clinical evaluation The study, which was approved by the Hospital Ethics Committee, was performed in 20 children aged 4 months to 8 yr (mean age 2.9 (SD 2.3) yr) and who weighed between 5.8 and 26.3 kg (mean weight 13.1 (5.5) kg), receiving general anaesthesia for a variety of surgical conditions (table I). Anaesthesia was induced with cyclopropane in oxygen or i.v. thiopentone and maintained with 70 % nitrous oxide in oxygen supplemented with KEY WORDS either 0.5-1 % halothane or i.v. fentanyl. Tracheal Anaesthesia: paediatric. Equipment: ventilators, T-piece. intubation was facilitated by suxamethonium 1-2 mg kg"1, after which neuromuscular block was maintained with atracurium (initial bolus Relatively few ventilators are convenient for use dose of 0.5 mg kg"1, followed by an infusion of with the T-piece in paediatric anaesthesia. T-Piece 1 1 occluders suffer from the drawback that the 8 ug kg" min" ) monitored by peripheral nerve ventilatory fresh gas flow (FGF) is used not only stimulation. to provide anaesthesia and oxygenation, but also TABLE I. Details of the 20 patients to inflate the lungs. This makes these ventilators relatively inflexible in use, limits the age range Age (yr) (mean (SD)) 2.9 (2.3) over which they are practicable and requires FGF Range 0.25-8.0 rates considerably in excess of those needed to Weight (kg) (mean (SD)) 13.1 (5.5) maintain normocapnia. Sex (M/F) 15/5 Type of operation Various modifications of adult ventilators have 7 Urological been described for use with the T-piece, the most 7 Plastic recent of which uses a fixed orifice leak built into 5 Abdominal the patient valve of the Nuffield 200 ventilator [1]. 1 Dental This is a more versatile machine which works well in practice. However, it does have serious theor- D. J. HATCH, M.B., B.S., F.F.A.R.C.S. ; R. M. BINGHAM, M.B., etical disadvantages, some of which make it B.S., F.F.A.R.C.S.; ANGELA M. MACKERSIE, B.SC., M.B., B.S., potentially dangerous in clinical use. The purpose F.F.A.R.C.S. ; Department of Anaesthestics, The Hospital for of this paper is to compare the performance of the Sick Children, Great Ormond Street, London WC1N 3JH. K. CHAKHABARTI, M.PHIL., B.SC.; J. G. WHTTWAM, M.B., recently described valveless all-purpose ventilator M. CH.B., PH.D., F.R.C.P., F.F.A.R.C.S.; Department of Anaesthetics, [2, 3] with that of the Nuffield 200 ventilator with Hammersmith Hospital, Du Cane Road, London W12 0HS. the Newton valve modification and to evaluate Accepted for Publication: January 19, 1990.
263
VENTILATORS FOR PAEDIATRIC ANAESTHESIA Ventilator
— Newton paediatric valve
.
^=—•
. j /
Infanta Wright respirometer
To aneroid manometer
Fresh gas flow FIG. 1. Diagram of apparatus used in the study, shown with the N 200 ventilator. Air oxygen or anaesthetics
Exhaust
FIG. 2. Diagrammatic representation of CW200 ventilator. Jl = Driving jet; J2 = positive endexpiratory pressure (PEEP) jet; T = trachea! rube. The ventilating frequency and I:E ratio are controlled by an electronic pulse generator operating a solenoid valve. The fresh gas flow is displayed continuously by a flow sensor, S2, (Magtrack) which is provided with an alarm for a minimum set flow. The expired tidal and minute volumes are measured by using another similar flow sensor, SI, and taking the difference in reading between Si and S2 during the expiratory phase.
Mechanical ventilation was commenced using either the Nuffield 200 ventilator with a Newton valve (N 200) (fig. 1) or the all-purpose valveless ventilator (CW 200) (fig. 2), attached to the expiratory limb of a modified T-piece. The choice of initial ventilator for each patient was made by random selection. FGF was set according to weight (less than 10 kg: 31itremin~l; 10-15 kg:
4.5 litre min"1; more than 15 kg: 6 litre min"1) using flowmeters which were calibrated against those of known accuracy. Tidal volume (Vr) was adjusted to ensure adequate chest movement, with an inspiratory: expiratory (I:E) ratio of 1:2. The ventilatory frequency (/) was set at 14-30 b.p.m. according to the age of the child. The inspired oxygen concentration (FiOj) was
264
monitored with a fuel cell, peripheral oxygen saturation (5aOl) with a pulse oximeter, and endtidal carbon dioxide partial pressure (PEC0 ) with a Hewlett-Packard infra-red capnometer (model 472IDA with digital and paper readout), calibrated before each study. The capnometer cuvette was placed in the apparatus deadspace and the paediatric cuvette (deadspace 2 ml) was used in children weighing less than 10 kg. Airway pressure within the breathing system was measured using an aneroid manometer (0-7 kPa); its accuracy was checked against a mercury column manometer. Systemic arterial pressure (SAP) was measured every 3 min with an automated cuff (Dinamap) and the ECG and beat-by-beat heart rate (HR) recorded. A precordial stethoscope was applied routinely. Train-of-four responses to peripheral nerve stimulation (either theulnar or tibial nerves) were maintained at less than 25 % throughout the period of study. Measurements of / and PECO, from the capnometer, SaOjJ SAP, HR and peak inflation pressure (PIP) were made at 5-min intervals for 15 min after PEcOj had stabilized. The volume of gas leaving the ventilator during inspiration was measured also, using a Wright Infanta respirometer, which records VT accurately between 15 and 200 ml [4], placed in the distal limb of the T-piece as close as possible to the ventilator. After the 15-min measurement period a change was made to the alternate ventilator, which was adjusted to the same/, PIP and i: E ratio as the first machine. The measurements described above were then repeated at 5-min intervals for a further 15 min. In two subjects the surgical procedure was of sufficient duration to allow the patient to be returned to the first ventilator and for the initial measurements to be repeated. Laboratory evaluation The pressure, flow and volume wave forms developed by each ventilator were recorded in the laboratory under identical conditions. Each ventilator in turn was connected by a standard Jackson-Rees modification of Ayre's T-piece to a model lung with a compliance set at either 10 or 30 ml kPa"1 and a resistance set at either 10 or 20 kPa litre"1 s. The peak inflating pressure was set as close as possible to 2 kPa with / at 20 b.p.m. and an I:E ratio of 1:2. Pressure changes at the junction between the model lung and the T-piece were measured with a Validyne MP 45 (± 5 kPa)
BRITISH JOURNAL OF ANAESTHESIA pressure transducer, and air flow was measured using a Fleisch No. 0 pneumotachograph connected to a Validyne MP 45 (±0.2 kPa) differential pressure transducer. The flow signal was integrated electronically to give volume. Timebased pressure, flow and volume signals were stored on a Compaq Deskpro 110 computer and displayed on an Epson FX 850 printer. RESULTS
Clinical evaluation In all children the CW 200 produced a greater FT for a given airway pressure than the N 200, the smallest increase being 17 % and the largest being 173 % in individual patients, with an overall 19 % mean increase of 46 (19.6) ml, from 93 ml to 139 ml (table II) (P < 0.001). TABLE II. Tidal volumes produced by the CW 200 and N 200 ventilators at identical frequencies and peak airway pressures (I:E ratio 1:2)
Tidal volume (ml) Patient No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mean (SD) Range
CW200
N200
CW200-N200
55 170 110 305 55 140 280 200 175 180 160 155 100 125 85 60 60 85 105 170 139
33 115 90 220 33 120 200 150 125 110 120 120 50 53 40 23 22 50 50 140 93
22 55 20 85 22 20 80 50 50 70 40 35 50 72 45 37 38 35 55 30 46(19.6) 22-85
In all patients, PEC 0 , was less during ventilation with the CW200 than with the N 200, with differences ranging from 0.14 kPa (3%) to 0.93 kPa (19 %), and an overall mean difference of 0.55 kPa (11.5 %) (P < 0.001), (table III). No significant changes in HR, SAP or Sa^ were seen during the study period in any patient.
VENTILATORS FOR PAEDIATRIC ANAESTHESIA TABLE I I I . End-tidal carbon dioxide partial pressures (PB'co,) produced by the CW 200 and N 200 ventilators at identical frequencies and peak airway pressures (l.E ratio 1:2)
Patient No. 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 Mean (SD) Range
CW200
N200
N200-CW200
5.32 4.39 4.12 3.19 3.72 4.52 4.12 3.99 3.59 3.86 4.26 4.52 3.86 4.26 3.59 5.19 5.59 3.46 5.05 3.33 4.19
5.85 5.32 4.26 3.72 4.12 4.92 5.05 4.26 3.99 4.79 4.79 4.79 4.79 4.92 4.39 5.59 5.85 3.72 5.85 3.86 4.74
0.53 0.93 0.14 0.53 0.4 0.4 0.93 0.27 0.4 0.93 0.53 0.27 0.93 0.66 0.80 0.40 0.26 0.26 0.80 0.53
0.55 (0.26) 0.14-0.93
265
Laboratory evaluation
Whilst the pressure wave produced by the N 200 was similar to that of a pressure generating ventilator, the pressure wave produced by the CW 200 was almost square (fig. 3). Theflowand volume waves changed as expected for pressure generating ventilators when the compliance and resistance of the model lung were changed. DISCUSSION
Both the CW 200 and the N 200 ventilators are easy to use with the paediatric T-piece, as each can be attached to the expiratory limb instead of the manual inflation bag, acting very much as an "educated hand". As the driving gas used to inflate the lungs is independent of the ventilatory FGF, controlled rebreathing techniques may be used in infants and young children with FGF as low as 1000 + 200 ml kg"1, without causing hypercapnia [5] and with a reduction in heat and water vapour loss. The ability to control arterial partial pressure of carbon dioxide (PaCOj) by adjusting the FGF independently of the degree of lung inflation may be of benefit to patients in low
CW200
C R
3ml(cmH 2 O)' 1 1
10 kPa litre' «
1ml(cmH20)*1 10 kpa litre' 1 >
3ml (cm HjO)"1 .•1. 20kPe litre"*
FIG. 3. Pressure, flow and volume wave forms produced by the N 200 and CW 200 ventilators in vitro when ventilating a model lung at resistances (R) of 10 and 20 kPa litre"1 s and compliances (C) of 1 and 3 ml (cm H.O)"1.
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output cardiac failure and in the avoidance of bronchopulmonary dysplasia [6, 7]. Although the NuiBeld 200 ventilator has proved satisfactory in practice, there are several disadvantages to the fixed orifice leak principle, which make it potentially very dangerous. Newton, Hillman and Varley [1] demonstrated that the modified Nuffield ventilator behaves as a pressure generator, and our in vitro measurements (fig. 3) confirmed this. As with any pressure generator, reduction in compliance or increase in resistance at the patient end of the system leads to a reduction in ventilation. This underventdlation may not be apparent by visual observation of the pressure monitor, and airways obstruction may not always be detected by the pressure alarm, as in small children the PIP is determined largely by the size of the leak through the Newton valve. Volume alarms are difficult to fit because of the continuous flow of the FGF passing through the expiratory limb of the T-piece, in addition to the expired volume. It is also possible to have apparently satisfactory ventilation with good chest movement despite significant rebreathing if the FGF is inadequate. Finally, the modified valve is suitable only for children up to about 15 kg, with an unmodified valve being required for larger children. The repeated changing of the valve required if children above and below this weight are to undergo ventilation is time consuming and may eventually cause damage to the valve. It is sometimes impossible to avoid the application of a small amount (0.1-0.2 kPa) of positive endexpiratory pressure (PEEP), especially in older children. Although the CW 200 has also been shown to have the characteristics of a pressure generator [2], again confirmed in this study (fig. 3), it incorporates design features which overcome the problems described above. It is suitable for use at all ages from birth to adult life, without modification. Expired volume is monitored by automatic subtraction of the ventilatory FGF flowing in expiration from the total expired volume, allowing tidal and minute volumes to be displayed continuously, with appropriate alarms, in addition to an FGF failure alarm. The expired volume signal could be used to provide feed-back information to the driving jet, which would enable the machine to deliver a servo-controlled volume. The PEEP is adjusted easily, and as it is a valveless ventilator it can be used for weaning children from ventilatory support through in-
BRITISH JOURNAL OF ANAESTHESIA termittent mandatory ventilation without the need for sensitive triggering devices, on to continuous positive airway pressure. If desired, the new ventilator can be used also at high frequencies of ventilation. The results of this study show that, when compared with the N 200 in the same patient at identical settings, the CW 200 ventilator gave a significantly greater FT for a given peak inflation pressure, with a consequently lesser PECO, m every patient. As volume and pressure were measured between the expiratory valve and the patient, this difference cannot be explained by the amount of gas passing through the fixed orifice leak of the N 200 ventilator. This was confirmed by the finding of a similar difference in older children, in whom the adult valve with no leak was used on the N 200 ventilator. Although the small amount of PEEP which is sometimes unavoidable with the N 200 may have been responsible for a slight reduction in tidal volume, the most likely explanation for these findings would appear to be the striking difference between the inspiratory pressure wave-forms produced by the two machines (fig. 3). A very important feature of the CW 200 is that FT is generated by the driving jet and not by the ventilatory FGF, which can be kept constant. At ventilatory frequencies up to 80 b.p.m., the inflation pressure profile is virtually a square wave, which implies that the PIP is applied throughout the inspiratory time. This is in contrast to the ramp function of the pressure wave produced by the N 200, and to the " thumb " devices with which the PIP is not reached until the ventilatory FGF generates a preset pressure in the system. This makes the CW 200 a much more efficient machine, particularly when inspiratory time is short and pulmonary compliance low, and may have important implications for the avoidance of bronchopulmonary dysplasia in infants with respiratory distress syndrome [6, 7]. Clinical studies have already shown improved results in very low birthweight babies with severe respiratory disease [8]. In conventional thumb devices, in order to increase the FT, FGF has to be greatly increased so that the preset pressure is reached earlier in the inspiratory phase, which in turn generates uncontrolled and possibly undesirable PEEP because of the resistance of the expiratory limb of the system. This study has demonstrated that the CW 200 ventilator is not only a safer, more flexible ventilator than the N 200 for use with the
VENTILATORS FOR PAEDIATRIC ANAESTHESIA paediatric T-piece, but that it is more efficient in terms of producing a significantly greater FT for a given peak inflation pressure under the conditions of the study. It is the first paediatric T-piece ventilator to provide an alarm responsive to FT in addition to the more conventional pressure alarms, and thus presents a major step forward in patient safety for ventilators used with the paediatric T-piece. Because there are no valves or other obstructions in the breathing duct, the patient can breathe freely at all times, allowing the provision of intermittent mandatory ventilation without complex systems for synchronization. PEEP can be applied deliberately as the frequency of ventilation is increased beyond the predicted time constant of the lungs.
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REFERENCES 1. Newton NI, Hillman KM, Varley JG. Automatic ventilation with the Ayre's T-piece: a modification of the
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Nufficld 200 ventilator for neonatal and paediatric use. Anaesthesia 1981; 36: 22-36. Chakrabarti MK, Whitwam JG. A new valveless all purpose ventilator. British Journal of Anaesthesia 1983; 55: 1005-1015. Whitwam JG, Chakrabarti MK, Konarzewski WH, Askitopoulou H. A new valvelcss all-purpose ventilator: clinical evaluation. British Journal of Anaesthesia 1983; 55: 1017-1022. Hatch DJ, Williams GME. The Haloscale "Infanta" Wright rcspirometer. In vitro and in vivo assessment. British Journal of Anaesthesia 1988; 60: 232-238. Hatch DJ, Yates AP, Lindahl SGE. Flow requirements and rebreathing during mechanically controlled ventilation in a T-piece (Mapleson E) system. British Journal of Anaesthesia 1987; 59: 1533-1540. Taghizadeh A, Reynolds EOR. Pathogenesis of bronchopulmonary dysplasia following hyaline membrane disease. American Journal of Pathology 1976; 82: 241-264. Bancalari E. Bronchopulmonary dysplasia. In: Milner AD, Martin RJ, cds. Neonatal and Pediatric Respiratory Medicine, London: Butterworths, 1985: 54-80. Chan KN, Chakrabarti MK, Whitwam JG, Silverman M. Assessment of a new valveless ventilator. Archives of Disease in Childhood 1988; 63: 162-167.
