Anaesthesia, 1976, Volume 31, pages 1245-1247 APPARATUS
Intermittent mandatory ventilation with ‘Manley’ ventilators
Intermittent mandatory ventilation (IMV) is a method of mechanical ventilation which allows spontaneous respiration between periodic mechanical hyperinflations.’ It evolved from paediatric practice where it has been used in the respiratory distress syndrome’ but is becoming popular in adult intensive therapy units. The technique may be used throughout the period of ventilator therapy’ or for weaning patients from mechanical ~ e n t i l a t i o n . ~The . ~ advantages claimed for the method are safe easy weaning, decreased duration of ventilation and decreased alkalosis. It has been suggested’ that controlled mechanical ventilation may lead to arterial oxygen desaturation while increasing oxygen consumption and that IMV, which leads to normocapnia, avoids this. I MV thus appears an attractive method of weaning patients from ventilators, particularly those machines without a patient trigger. After considering several ventilators we decided to adapt the Manley MPT, a gas-powered minute-volume divider, using standard components as far as possible.
Method No internal modifications to the ventilator were necessary. With these machines, when tidal volume and inflation pressure are fixed, decreasing the flow rate of gas to the ventilator increases the expiratory time. If gas is then supplied to the inspiratory limb of the patient Y-piece during expiratory time, the patient can breathe spontaneously. Twin, pressure-compensated, ball flowmeters were used for air and oxygen. This allowed the same air/oxygen mixture to be set for delivery to the ventilator and the inspiratory limb of the patient Y-piece proximal to the heated-water humidifier (Fig. 1). The gas supplied directly to the inspiratory limb passes through the inflating valve, from a ‘Laerdal’ emergency resuscitation bag, which has been modified to act as a oneway valve by the addition of an aluminium disc (4.1 cm in diameter, 0.162 cm thick H. J. Manson, MB, ChB, FFARCS, Senior Registrar and D. G . Ross, MB, ChB, FFARCS, Senior Registrar, Department of Anaesthetics, Aberdeen Royal Infirmary, Aberdeen.
1245
1246
H . J. Manson and D. G . Ross
Patient
I I Air
Fig. 1. The circuit for IMV.
with a central hole 1.6 cm in diameter) behind the flexible valve element; this closes the expiratory pathway. This valve closes during the inspiratory phase of the ventilator and prevents backflow. The supplementary gas flow is stored meanwhile in a 2 litre reservoir bag. A safety blow-off valve, adjustable from 2.9 to 6.9 kPa (30-70 cmH,O), was also included in the patient circuit to prevent undesirably high airway pressures should the mandatory inflation fall at the end-inspiratory point of a spontaneous breath. It was made up from the body and seat of a standard Heidbrink valve with the valve spindle and control knob from the pressure limiting valve of a Cyclator circle-circuit attachment and the spring from a Manley MPP patient safety valve. This apparatus used only one specially made part (the aluminium disc). The supplementary gas flow was introduced between the ventilator and humidifier so that while it was humidified, excessive moisture did not find its way into the valves.
Discussion
A full description of the method of employing IMV has been given by Downs and his colleagues.’ When weaning is begun, the ventilation rate is reduced stepwise, the patient’s blood-gases being checked between steps. The minute volume supplied to the patient Y-piece is kept constant, the gas flow added directly to the inspiratory limb being equal to the gas flow subtracted from the ventilator driving gas flow at each step, with the same oxygen concentration. Positive end-expiratory pressure can be used if required. The major considerations in the choice of ventilator to be adapted were expense and simplicity. Electrically powered machines (e.g. the Cape, East-Radcliffe and Engstrom) were excluded since, even if motor speed were reduced, their fixed inspiratory/expiratory ratios would allow unacceptable inspiratory times. Gas powered, minute-volume dividers have a wide range of inspiratory/expiratory ratios which is greater than that allowed by modern electronic ventilators. Other common machines in this class are the Howells ventilator and the early ‘Manley’ series.
Intermittent mandatory ventilation
1247
Summary A method is described whereby gas powered, minute volume divider, ventilators may be adapted to provide intermittent mandatory ventilation. Acknowledgments The authors’ thanks are due to Mr Gordon Bowie, Technician, Department of Anaesthetics, for technical assistance, and to D r D. D. Hart, Dr C. R. Dundas and Dr J. McG. Imray, Consultant Anaesthetists, Department of Anaesthetics, Aberdeen Royal Infirmary, for their encouragement. References 1. DOWNS, J.B., PERKINS,H.M. & MODELL,J.H. (1974) Intermittent mandatory ventilation. An
evaluation. Archives of Surgery, 109, 519. 2. KIRBY,R., ROBISON,E., SCHULZ,J. & DE LEMOS, R.A. (1972) Continuous flow ventilation as an alternative to assisted or controlled ventilation in infants. Anesthesia and Analgesia; Current Researches, 51, 871. 3 . DOWNS,J.B., KLEIN,F.F., DESAUTELS, D., MODELL,J.H. & KIRBY,R.R. (1973) Intermittent mandatory ventilation: A new approach to weaning patients from mechanical ventilators. Chest, 64,331. 4. DOWNS,J.B., PERKINS, H.M. & SUTTON,W.W. (1974) Successful weaning after five years of mechanical ventilation. Anesthesiology, 40,602.
Intermittent mandatory ventilation with ‘Manley’ ventilators
Intermittent mandatory ventilation (IMV) is a method of mechanical ventilation which allows spontaneous respiration between periodic mechanical hyperinflations.’ It evolved from paediatric practice where it has been used in the respiratory distress syndrome’ but is becoming popular in adult intensive therapy units. The technique may be used throughout the period of ventilator therapy’ or for weaning patients from mechanical ~ e n t i l a t i o n . ~The . ~ advantages claimed for the method are safe easy weaning, decreased duration of ventilation and decreased alkalosis. It has been suggested’ that controlled mechanical ventilation may lead to arterial oxygen desaturation while increasing oxygen consumption and that IMV, which leads to normocapnia, avoids this. I MV thus appears an attractive method of weaning patients from ventilators, particularly those machines without a patient trigger. After considering several ventilators we decided to adapt the Manley MPT, a gas-powered minute-volume divider, using standard components as far as possible.
Method No internal modifications to the ventilator were necessary. With these machines, when tidal volume and inflation pressure are fixed, decreasing the flow rate of gas to the ventilator increases the expiratory time. If gas is then supplied to the inspiratory limb of the patient Y-piece during expiratory time, the patient can breathe spontaneously. Twin, pressure-compensated, ball flowmeters were used for air and oxygen. This allowed the same air/oxygen mixture to be set for delivery to the ventilator and the inspiratory limb of the patient Y-piece proximal to the heated-water humidifier (Fig. 1). The gas supplied directly to the inspiratory limb passes through the inflating valve, from a ‘Laerdal’ emergency resuscitation bag, which has been modified to act as a oneway valve by the addition of an aluminium disc (4.1 cm in diameter, 0.162 cm thick H. J. Manson, MB, ChB, FFARCS, Senior Registrar and D. G . Ross, MB, ChB, FFARCS, Senior Registrar, Department of Anaesthetics, Aberdeen Royal Infirmary, Aberdeen.
1245
1246
H . J. Manson and D. G . Ross
Patient
I I Air
Fig. 1. The circuit for IMV.
with a central hole 1.6 cm in diameter) behind the flexible valve element; this closes the expiratory pathway. This valve closes during the inspiratory phase of the ventilator and prevents backflow. The supplementary gas flow is stored meanwhile in a 2 litre reservoir bag. A safety blow-off valve, adjustable from 2.9 to 6.9 kPa (30-70 cmH,O), was also included in the patient circuit to prevent undesirably high airway pressures should the mandatory inflation fall at the end-inspiratory point of a spontaneous breath. It was made up from the body and seat of a standard Heidbrink valve with the valve spindle and control knob from the pressure limiting valve of a Cyclator circle-circuit attachment and the spring from a Manley MPP patient safety valve. This apparatus used only one specially made part (the aluminium disc). The supplementary gas flow was introduced between the ventilator and humidifier so that while it was humidified, excessive moisture did not find its way into the valves.
Discussion
A full description of the method of employing IMV has been given by Downs and his colleagues.’ When weaning is begun, the ventilation rate is reduced stepwise, the patient’s blood-gases being checked between steps. The minute volume supplied to the patient Y-piece is kept constant, the gas flow added directly to the inspiratory limb being equal to the gas flow subtracted from the ventilator driving gas flow at each step, with the same oxygen concentration. Positive end-expiratory pressure can be used if required. The major considerations in the choice of ventilator to be adapted were expense and simplicity. Electrically powered machines (e.g. the Cape, East-Radcliffe and Engstrom) were excluded since, even if motor speed were reduced, their fixed inspiratory/expiratory ratios would allow unacceptable inspiratory times. Gas powered, minute-volume dividers have a wide range of inspiratory/expiratory ratios which is greater than that allowed by modern electronic ventilators. Other common machines in this class are the Howells ventilator and the early ‘Manley’ series.
Intermittent mandatory ventilation
1247
Summary A method is described whereby gas powered, minute volume divider, ventilators may be adapted to provide intermittent mandatory ventilation. Acknowledgments The authors’ thanks are due to Mr Gordon Bowie, Technician, Department of Anaesthetics, for technical assistance, and to D r D. D. Hart, Dr C. R. Dundas and Dr J. McG. Imray, Consultant Anaesthetists, Department of Anaesthetics, Aberdeen Royal Infirmary, for their encouragement. References 1. DOWNS, J.B., PERKINS,H.M. & MODELL,J.H. (1974) Intermittent mandatory ventilation. An
evaluation. Archives of Surgery, 109, 519. 2. KIRBY,R., ROBISON,E., SCHULZ,J. & DE LEMOS, R.A. (1972) Continuous flow ventilation as an alternative to assisted or controlled ventilation in infants. Anesthesia and Analgesia; Current Researches, 51, 871. 3 . DOWNS,J.B., KLEIN,F.F., DESAUTELS, D., MODELL,J.H. & KIRBY,R.R. (1973) Intermittent mandatory ventilation: A new approach to weaning patients from mechanical ventilators. Chest, 64,331. 4. DOWNS,J.B., PERKINS, H.M. & SUTTON,W.W. (1974) Successful weaning after five years of mechanical ventilation. Anesthesiology, 40,602.
