Pulmonary Medicine-Mechanical Ventilation ISSN 1930-6741
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?
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
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
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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