Modern ventilators use electromagnetic proportional solenoid valves controlled by microprocessors to control gas flow to patient

FIO2

FiO2 is the fraction of inspired oxygen. It is the percentatge of oxygen delivered on inspiration. Ranges from 21% to 100%.

The oxygen analysers In the inspiratory limb is used to ensure the accuracy of FiO2 delivered

Tidal volume (TV or VT) Measured volume with each breath

Normal tidal volume should be kept at

6 – 8 ml/kg of patients ideal body weight

Minute volume (MV)

Respiratory rate Number of breaths per minute

Inspiratory expiratory ratio

Peep Positive end expiratory pressure is the positive pressure applied at the end of

expiration during mechanical ventilation.

The use of PEEP improves gas exchange by recruiting collapsed alveoli and thereby improving functional residual capacity.

PEEP also helps in redistributing lung water from the alveolar space into the pulmonary circulation and improving ventilation perfusion mismatch.

All these contributes to reduction in dead space ventilation and thereby improving PaO2.

PEEP increases intra-thoracic pressure resulting in decreased venous return to the right heart and increasing the right ventricular after load (the force needed to eject blood from right ventricle) .

AUTO PEEP Auto PEEP is the actual pressure difference between the

higher pressure and the baseline pressure (set PEEP level or 0) at the end of full expiration

It is measured by pressing the expiratory hold button on the ventilator.

Peak airway pressure Pressure needed initially to assure the flow of gas

down the airways and open up the alveoli.

Depends upon resistance and compliance

PLATEAU PRESSURE Pressure required to hold the lungs and chest wall open when there is no air flow during

the inspiratory cycle

Measured at the end of a full inspiration during a breath hold (no air flow)

How to measure Plateau pressure1. Switch to volume controlled mode ventilation

2. Press the end-inspiratory hold button

3. The machine will display plateau pressure. It is automatically done by the ventilator by sensing the pressure which is required to keep the alveoli open at the end of inspiration when there is no airflow which will be generally 10-15cm H2O less than Peak airway pressure.

With decreased compliance, the plateau pressure will increase.

rESISTANCE Degree of easiness to move gases down the airways

High resistance --- difficult to move gases in and out

Low resistance --- easy to move gases in and out.

Measured using the difference between peak airway pressure and plateau pressure.

COMPLIANCE Degree of easiness to distend the alveoli, lungs and

chest wall.

High compliance --- easy to distend the alveoli

Low compliance ---- hard to distend the alveoli

Compliance is measured with plateau pressure

REDCRUITMENT Ventilation strategy whereby the non ventilated alveoli are opened by

progressively introducing higher pressures over a short period. The aim is reinflate atelectatic alveoli without distending the normal lung units.

Techniques

1. PEEP is increased in increments of 5cm H2O from a baseline PEEP to 35cm H2O reducing tidal volume to limit peak inspiratory pressure to 35cm H2O. CPAP is maintained for 30s.

2. Intermittent higher tidal volume in pressure controlled ventilation applied with escalating PEEP and constant driving pressure: peak pressure of 45cm H2O, I:E ratio of 1:2 and PEEP level of 16cm H2O for 2min.

3. Sustained inflation by application of CPAP at 30-40cm H20 for 40s.

Weaning Weaning is the process when the support of the ventilator is gradually

decreased to re-establish spontaneous breathing. Readiness for weaning is evaluated by assessing level of consciousness, adequacy of oxygen and ventilation, spontaneous breathing strength etc.

Anatomic dead space Anatomic Dead space is the portion of the respiratory

system not involved in gas exchange.

It is be divided into alveolar and airway dead space.

Airway dead space is also called tracheal dead space as it is the part of the respiratory system (mainly trachea) where tidal volume remains in the conducting passages at the end of inspiration and therefore does not participate in gas exchange.

Alveolar dead space is a measurement of the ventilated alveoli without adequate blood flow for gas exchange. It is negligible in healthy individuals.