Pulmonary Medicine

Pulmonary Medicine-Mechanical Ventilation ISSN 1930-6741

Article Editor

Ramesh Kaul ,MD, MS, FCCP

What is Inverse Ratio Ventilation (IRV) ?

This is the mode of ventilation where the inspiration phase of the cycle of ventilation is longer than expiration phase

What is Airway Pressure Release Ventilation (APRV) ? What is Bi-level and Proportional Assist Ventilation ?

This is the mode of Ventilation where ventilation cycles between PEEP and PLATEAU or positive hold of Inspiratory phase of ventilation, this can be structured to the need by making flow adjustability or slope of the flow.

We have to remember that increasing the oxygen delivery to the blood can be done by increasing ventilation, by rapidly removing air from the alveoli this will drop the CO2 levels and increase the oxygen.

Increasing the partial pressure of alveolar oxygen will increase the oxygen available for transfer thus blood oxygen will increase without effecting the CO2 and Ph.

The various ways by which patient's ventilation, pH and oxygenation can be adjusted in the blood is by changing the volume of minute ventilation. This includes adjusting pressure support which is given during inspiration and pressure support which is given during expiration is called PEEP or positive end expiratory pressure.

Volume control allows us to keep a steady volume during ventilation this makes it easier for us to keep a constant pH oxygenation and ventilation in a patient whose resistance and compliance can vary due to movement, positioning, effort, cough or relaxants/sedation.

If this fails to enhance oxygenation (without the patient incurring oxygen toxicity from high FiO2 levels), then it is necessary to raise the mean airway pressure to force gas in and out of the alveoli. The patient is put on a mode of inspiratory support – volume control, pressure control or pressure support. Most patients can be easily ventilated in this way. But what if we still have trouble with oxygenation?

We know that cyclical opening and closing of injured lung units damages them (particularly if tidal volumes are large. We would prefer if the patient could be ventilated at the top of the volume pressure curve, at high lung volumes, without phasic changes. This can be achieved using high frequency oscillation, but adult oscillators are not widely available. For the majority of patients, increasing mean airway pressure without increasing peak pressure means prolonging the inspiratory time in a pressure control mode. The longer the inspiratory time (Ti), the better the oxygenation benefit.

Once Inspiratory time (Ti) becomes longer than expiratory time (Te).

There is insufficient time for CO2 removal, and this tends to build up. respiratory acidosisWe know the patients tolerate very well, and we allow this to happen (permissive hypercapnia). This “inverse ratio” ventilation is very uncomfortable for patients, who generally need to be heavily sedated, often paralysed. In general, we prefer that our patients are awake and interacting with the ventilator – thus newer modes have been developed to enable patients to breath spontaneously on inverse ratio ventilation.

The method of achieving this uses a modification of ARPV (airway pressure release ventilation). In conventional ventilation, the baseline airway pressure is the PEEP or CPAP level, and ventilator cycling involves application of positive pressure to a higher airway pressure level: the purpose of cycling is CO2 removal.

The idea of ARPV is that the ventilator cycles between two different levels of CPAP – an upper pressure level and a lower level. The two levels are required to allow gas move in and out of the lung. The key element of ARPV is that the baseline airway pressure is the upper CPAP level, and the pressure is intermittently “released” to a lower level, thus eliminating waste gas.

Bi-level ventilation (bi-level CPAP)(6) or BIPAP (which is often confused with BiPAP, a form of non invasive ventilation), is ARPV with spontaneous breathing. A sophisticated valve has been developed which allows the patient to breath spontaneously at either CPAP/PEEP levels, and partial assistance (pressure support or automatic tube compensation) can be introduced to assist the spontaneous breaths. This mode appears to be extremely well tolerated, and heavy sedation is not required (it is in ARPV and IRV).
Any intensive care patient can be managed on this mode of ventilation. Bi-level can be used as conventional pressure controlled ventilation, or as Airway Pressure Release Ventilation. This involves ventilating the patient, essentially, at full tidal volumes. Usually an i.e. ratio of 8-9:1 is used (however - the short expiratory time is the key variable). This is generally well tolerated by patients - particularly if there is spontaneous breathing, which improves homodynamic performance and recruiting juxta-diaphragmatic tissues. For more information see here. For more information about ventilation strategy for APRV read here.

Bilevel Ventilation (inverse ratio) note that the patient is breathing spontaneously at the higher PEEP level without any evidence of dysynchrony.

Proportional assist ventilation in which the ventilator provides the percentage of work .

If there is change in compliance/elastance and resistance. The pressure delivered varies from breath to breath, due to changes in elastance, resistance and flow demand. Usually this is set to overcome 80% of the work of breathing: for example, the pressure required to overcome this may be 14cmH2O. So this mode is interactive, as the ventilator varies its output to maintain its proportion of the workload. A version of this is available in some Drager ventilators, and is called “proportional pressure support”.

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