Inverse ratio ventilation (IRV) is an alternative strategy for mechanical ventilation that reverses the classical inspiratory/expiratory scheme. This is achieved by modifying the inspiratory to expiratory (I:E) ratio, typically with the intention to increase oxygenation by increasing the mean airway pressure (MAP). Discussion of IRV requires an understanding of basic ventilator management which can be reviewed in a separate article. Here we discuss additional terms necessary to the utilization of IRV.

I:E Ratio

The I:E ratio denotes the proportions of each breath cycle devoted to the inspiratory and expiratory phases. The duration of each phase will depend on this ratio in conjunction with the overall respiratory rate. The total time of a respiratory cycle is determined by dividing 60 seconds by the respiratory rate. Inspiratory time and expiratory time are then determined by portioning the respiratory cycle based on the set ratio. For instance, a patient with a respiratory rate of 10 breaths per minute will have a breath cycle lasting 6 seconds. A typical I:E ratio for most situations would be 1:2, if we apply this ratio to the patient above, the 6-second breath cycle will break down to 2 seconds of inspiration and 4 seconds of expiration. Increasing the I:E ratio to 1:3 will result in 1.5 seconds of inspiration and 4.5 seconds of expiration. Thus, a "higher" I:E ratio results in less inspiratory time and more expiratory time in the same length of the breath cycle.

Standard Volume Control and Pressure Control ventilation modes typically use I:E ratios of 1:2, or as high as 1:3 or 1:4 in certain populations. In these cases, the expiratory phase is set longer than the inspiratory phase to more closely mimic normal physiologic breathing. Inverse Ratio Ventilation instead uses I:E ratios of 2:1, 3:1, 4:1, and so on, sometimes as high as 10:1, with inspiratory times that exceed expiratory times.

Mean Airway Pressure

Mean Airway Pressure (referred to as MAP in this article) is the pressure measured at the opening of the airway, averaged over the complete respiratory cycle. The primary determinate of MAP are PEEP, Inspiratory Pressure, and time spent on each phase. In standard mechanical ventilation, MAP can be estimated by assuming that the pressure at the airway is approximately equal to the PEEP during expiration and approximately equal to the Inspiratory pressure during inspiration. MAP can then be calculated by multiplying the fraction of a cycle spent of inspiration by the inspiratory pressure and adding this to the fraction of a cycle spent on expiration multiplied by the PEEP.

For instance, in a patient mechanically ventilated using a PEEP of 5, inspiratory pressure of 20, and I:E ratio of 1:2. The patient will have a base pressure at the airway of 5, but for one-third of a respiratory cycle (I:E ratio of 1:2 means that one-third of the cycle is spent on inspiration) this will increase to 20. We then calculate 5 x 2/3 + 20 x 1/3 = 10.

MAP correlates with mean alveolar pressure and thus transpulmonary pressure. Though there are multiple factors involved, increased transpulmonary pressure results in increased gas exchange, notionally improving oxygenation. The primary conceit of IRV is to increase mean airway pressure by increasing the time spent on the higher pressure portion of the cycle. This allows the increase of MAP while minimizing risk for pulmonary injury relative to other aggressive oxygenation strategies which often rely on high PEEP or inspiratory pressure. Increasing the time spent at the higher pressure portion of the cycle allows the elevation of MAP without increasing the pressure itself, which is associated with pulmonary injury. A higher MAP results in a higher transpulmonary pressure which improves gas exchange and arterial oxygenation.[1][2][3][4]

The primary indication for inverse ratio ventilation is the management of hypoxemia refractory to other ventilation strategies, particularly in patients with hypoxemia secondary to ARDS. Conventional management of patients in ARDS consists of low volume, high PEEP ventilation. Increasing PEEP is used to increase transpulmonary pressure to improve oxygenation; however, some patients cannot tolerate the escalating PEEP or inspiratory pressures required for ventilation due to barotrauma, volume trauma, and alveolar damage. IRV is one possible alternative strategy in these circumstances. IRV is often used as a rescue strategy when other methods of oxygenation have been maximized.[1]

There are multiple possible or expected complications of IRV, discussed below. Relative contraindications to IRV are those conditions that put the patient at higher risk for developing these complications, such as preexisting hemodynamic compromise or obstructive lung disease requiring a prolonged expiratory phase.