Ventilatory support is an integral part of 'intensive care management. As more and more critically ill .patients and patients with complicated, prolonged surgeries are being admitted in health institutions, so is the increasing need for ventilatory support. With the progress of civilization the increasing number of trauma cases due to motor vehicle accidents or otherwise are also putting a great burden on hospitals to provide ventilators as a life saving machine. To meet all these demands hospital provide sophisticated, imported ventilator with wide array of facilities. But these are costly and hence can only be availed by big health institutions. Although these ventilators are attractive but, are less in number than the actual requirement.

Previously our institution had evaluated the performance of Medisys system-I both in the laboratory and in clinical set up.' The same ventilator was evaluated further for providing prolonged ventilatory support.2 Recently, a newer version of the indigenous ventilator [Medivent Excel, Medisys with SIMV mode] has been launched to provide mechanical ventilation in the ICU set up. We carried out a study to find out the efficacy and usefulness of this ventilator by comparing with that of available standard ICU ventilator (Horus Taema, France).' After observing that this indigenous ventilator was performing satisfactorily in the controlled mode for short duration(six hours), we have embarked on the present study in which the indigenous ventilator was used as a sole ventilator to provide ventilation in ICU patients for prolonged period and till weaning.

MATERIALS AND METHODS
The indigenous ventilator (MEDIVENT EXCEL, MEDISYS) (Fig 1) is an electrically driven, time cycled, volume preset, pressure limited, constant flow generator. Compressed gas is not required for driving the ventilator. It provides 3 modes of ventilation-controlled (CMV), assist control mode ( ASSIST + CMV), and synchronized intermittent mandatory ventilation mode (SIMV). Oxygen flow of 2-4 Lmin-1 is required so as to deriver 40% O2 [ FiO2, 0.4]. It flows through a Venturi device and gets collected in a reservoir bag of 2 L capacity, from which the bellows extract the set amount of volume and delivers it to the patient through the circuit. With the help of Venturi device attached to the equipment, oxygen can be provided at three different concentrations – 40,50 and 100%. The front panel has an airway pressure dial and electronic display of adjustable variables like respiratory rate, tidal volume, inspriatory hold and I:E ratio. Alarms are provided for patient disconnection, high/low pressure. Safety pop-up valve device can be set up at 20, 40 and 60 cm H2O. The safety alarm system was checked using test lung (reservoir bag) and was within the specification range as quoted by the manufacturer. All the parameters can be changed by the use of a single knob. The range of parameters are provided in the table I. Ventilator allows the patient to breathe freely between mandatory breaths.

After obtaining approval from Institutional Ethics Committee and informed consent from next of kin, 20 add t critically ill patients of either sex admitted to our neurointensive care unit were included in the study. Patients with age less than 18 years, pre-existing pulmonary disease, high cervical spinal cord lesion were excluded from the study. All the patients were ventilated in the synchronized intermittent mandatory ventilation (SIMV) mode. The initial settings were: tidal volume (10ml/kg), respiratory rate (10-12/min), FiO2 (0.4), oxygen flow rate of 41/min. During the study period, continuous monitoring of physiological parameters like ECG, heart rate, NIBP, side stream capnometry, SpO2 (Dab Ohmeda AS/5, Helsinki, Finland) were carried out at hourly intervals. Simultaneously, FiO2

Delivered, tidal volume, respiratory rate, airway pressure, compliance and pressure-volume loop were monitored using spirometry module (D-lite AS/5, Datex Ohmeda, Finland) and end tidal gas analyzer. Patients were continuously observed for signs of neurological deterioration or improvement. When the patients maintained stable haemodynamics and neurological status at a particular respiratory rate blood gas analysis was done. If the patient maintained, an oxygen saturation >99%, PaO2> 100 mmHg and PaCO2, between 34 and 39 mmHg at the sat rate, then decision for weaning was entertained. Weaning was carried out by reducing respiratory rate at the rate of 2 breaths at a time. Oxygen saturation >99% and PaCO2 between 34 and 39 mmHg was maintained through out the weaning process. For each set respiratory rate, mean values were taken and arterial blood gas analysis was done before changing the respiratory rate. Once patient maintained stable haemodynamics and minute ventilation at lowest respiratory rate (4/min), 'T' piece trial was given. The study was terminated once the patient was successfully weaned off the ventilator, However the patients were followed up till they were discharged.

RESULTS
Out of the 20 patients included in the study, 17 were males. The mean age was 40.69±12.26 years and the average weight was 66.52±10.25 kg. The mean duration of ventilation was 75±16 hours. The mean Glasgow coma score was 8±1.2. Majority of the patients presented with acute subdural haematoma (15), whereas there were two patients of spontaneous cerebellar haematoma, two patients of traumatic frontal contusion and one patient of posttraumatic hydrocephalus (table 2). out of the 15 patients with acute subdural haematoma, 12 patients were operated upon. One patient of cerebellar haematoma and the patient of posttraumatic hydrocephalus also underwent surgery.

DISCUSSION
Synchronised intermittent mandatory ventilation (SIMV) was designed to present he stacking of breaths. In the early 1970s, Siemens servo ventilator incorporated this mode and enabled the mandatory breath lo be synchronized with a spontaneous breath.3 To allow the patient to take spontaneous breaths between the mandatory breaths, the machine creates a regular time window which is the only period during which the ventilator will trigger in response to a spontaneous breath. The duration and frequency of this window is usually controlled by the mandatory and SIMV frequency controls. The duration of the window can be adjusted by the mandatory frequency control and by SIMV frequency control.

The major problem, however, with this mode of ventilation is that the mandatory breath rate may not provide adequate ventilation if the patient becomes apnoeic and so fails to trigger synchronized breaths. Careful monitoring of respiratory rate and expired minute volume is therefore required. Similarly, if the patient does not have enough respiratory drive he will continue to inspire low tidal volumes resulting in rapid respiratory rate, respiratory fatigue and delayed weaning.

Nonetheless, most intensivists use the SIMV mode as the primary mode for ventilatory support and as well as for weaning. Hence, as a continuation of our efforts to evaluate the indigenous ventilator we have designed the present study of assessing the performance of the ventilator in the SIMV mode. This was an observational study in which the performance of the indigenous ventilator was assessed with the help of spirometer attached to our critical care monitor (Datex AS/5). The spirometry provided bath inspiratory and expiratory measurements of tidal volume, respiratory rate, airway pressure, compliance. Resistance and pressure-volume loop for every breath. These values can be stored in the monitor and can be retrieved, later for assessment.

The present study showed that the indigenous ventilator functioned properly at declining respiratory rate. Weaning was successful in all the patients after a SIMV rate of 4 per minute. All the patients maintained satisfactory haemodynamics during the course of weaning. As the rate was gradually decreased all the patients maintained adequate minute ventilation which as reflected on adequate EtCO2. Over and above, the blood gas parameters remained within acceptable range till weaning. Similar observations were also recorded pertaining to compliance, airway pressure and inspired oxygen fraction during weaning. Neurological and neurosurgical patients of ten require ventilatory support for a prolonged period. In such situations, a ventilator with volume preset time cycling device with all weaning modes is ideal. The ventilator must be sturdy and provide ventilation without any problems for months and months. Endurance test of the ventilator is tested when it is used in patients with neurosurgical 'ailments, This fact also prompted us to evaluate this indigenous ventilator in our Neuro ICU.

The present study was not without limitations. It was conducted on a small number of haemodynamically stable patients with normal lungs. Therefore its performance in abnormal lung conditions need to be evaluated. All the patients in the study group were adults. Further studies are required to assess the ventilator in the paediatric age group.

This ventilator can be improved further with the help of certain modifications like

1. Provision of varying concentration of oxygen to patients.
2. Inbuilt humidifier, positive end expiratory pressure device PEEP and provision for nebulisation must be provided.
3. Variable trigger and display of expiratory parameters.
4. Provision of pressure support mode and,
5. Battery back-up are also eventual.

CONCLUTION
The indigenous ventilator's SIMV mode can safely be used for weaning the patients off the ventilator.

REFERENCES

1. Kaul HL, Dash HH, Arora SK, Gode GR. Evaluation of an indigenous anaesthesia ventilator. lndian J. Anaesth 1984; 32: 369
2. Mohanty B, Bhutra S, Chauhan RS, Dash HH. is MEDISYS system I ventilator effective in providing IPPV in severe head trauma patients. Indian J. Anaesth 2001; 45:24-29,
3. Ghosh I, Radhakrishnan M, Rath GP, Bithal PK , Dash HH: Comparison of indigenous volume preset ventilator with servo-controlled ventilator. (Presented as free paper at ISACON, Bhopal 2004. (abstract no.54)
4. Sykes K: Respiratory support: techniques for minimising mean airway pressure. In: Sykes K, Young JD, Hahn CEW, Adams AP eds. Respiratory support in intensive care. 2nd Edition. London : BMJ books, 2000;134-63.