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.
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.
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.
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.
Though the use of IRV does not dictate a specific mode of mechanical ventilation, in execution it is often used as a modification of pressure control mode as this is the most straightforward. In such a case, just as the clinician sets the PEEP and Inspiratory Pressure in conventional pressure control ventilation, in PC-IRV the clinician sets the low pressure (P-low) and the high pressure (P-high). The clinician must also set the frequency of pressure changes and the proportion of time spent at each level, analogous to respiratory rate and I:E ratio. Whether the proportions are dictated through setting a ratio (2:1, 4:1, 10:1 and so on), or by directly setting the P-high time and P-low time is ventilator dependent.
Inverse ratio ventilation can be significantly uncomfortable, and patients may need to be heavily sedated or paralyzed to achieve patient-ventilator synchrony. Some IRV modes will allow a patient-driven respiratory cycle to be superimposed on the IRV cycle to increase ventilation and improve management of dysynchrony.
Notable complications of IRV include lung trauma, accumulation of auto-PEEP, hypoventilation, and hemodynamic compromise.
Though IRV requires lower peak pressures to achieve the same MAP when compared to conventional ventilation, the average pressure in the lungs is overall increased. Thus the patient remains at increased risk for barotrauma. Volume trauma may also occur if there is a high gradient between the P-high and P-low.
Auto-PEEP (also referred to as breath stacking or air trapping) occurs when a patient is unable to completely exhale a breath before the next inspiratory phase begins, resulting in elevated airway pressures. IRV may potentiate this process due to the relatively short expiratory phase or P-low time. There are indications that this auto-PEEP effect may benefit oxygenation in IRV; however, the increased pressures may exacerbate lung trauma and hemodynamic stress. Patients with preexisting obstructive disease (COPD/asthma) who rely on prolonged expiratory times are at increased risk.
IRV increases oxygenation by increasing MAP which has the additional consequence of increasing the average intrathoracic pressure. Similar to the hemodynamic effects seen with high PEEP, increased MAP can cause compromise by increasing intrathoracic pressure, thus impeding venous return to the heart and reducing preload. This can be pronounced in patients who are already preload deficient, such as from hypovolemia or vasodilatory shock, and is especially problematic in patients who are in a significant preload dependent state, such as those in obstructive shock. If the patient develops Auto-PEEP, the risk of hemodynamic compromise is increased.
IRV has not been shown to improve objective clinical outcome measures such as mortality, length of mechanical ventilation, or length of ICU stay. It has been shown in certain studies to increase PaO2, though other studies have not supported this. Currently, more data is needed to evaluate the possible benefit of IRV.
When patients are placed on inverse ratio ventilation, the nurse must be aware of the potential complications that can occur. The patient's hemodynamic status must be closely monitored. Barotrauma may occur, and the patient may require an immediate chest tube. There should be clear communication between the pulmonologist, respiratory therapist, intensivist and the nurse when any ventilatory changes are made.
|||Kotani T,Katayama S,Fukuda S,Miyazaki Y,Sato Y, Pressure-controlled inverse ratio ventilation as a rescue therapy for severe acute respiratory distress syndrome. SpringerPlus. 2016 [PubMed PMID: 27375985]|
|||Daoud EG,Farag HL,Chatburn RL, Airway pressure release ventilation: what do we know? Respiratory care. 2012 Feb [PubMed PMID: 21762559]|
|||Rittayamai N,Katsios CM,Beloncle F,Friedrich JO,Mancebo J,Brochard L, Pressure-Controlled vs Volume-Controlled Ventilation in Acute Respiratory Failure: A Physiology-Based Narrative and Systematic Review. Chest. 2015 Aug [PubMed PMID: 25927671]|
|||Hess DR, Approaches to conventional mechanical ventilation of the patient with acute respiratory distress syndrome. Respiratory care. 2011 Oct [PubMed PMID: 22008397]|
|||Ferdowsali K,Modock J, Airway pressure release ventilation: improving oxygenation: indications, rationale, and adverse events associated with airway pressure release ventilation in patients with acute respiratory distress syndrome for advance practice nurses. Dimensions of critical care nursing : DCCN. 2013 Sep-Oct [PubMed PMID: 23933639]|