Overview of Inverse Ratio Ventilation
Inverse ratio ventilation (IRV) is a different method used for mechanical ventilation or helping a patient to breathe using a machine. This method adjusts the time between when the patient breathes in and breathes out (known as the inspiratory to expiratory or I:E ratio) to help improve the oxygen levels in the body by increasing the average airway pressure. The concepts needed to understand IRV involve basic knowledge about ventilator management.
Understanding the I:E Ratio
The I:E ratio dictates how much time in each breathing cycle is spent on breathing in (inspiration) and breathing out (expiration). This duration depends on the established ratio and the breathing rate of the patient. If you take 60 seconds and divide it by the breathing rate, you get the total time of a breathing cycle. As an example, let’s say a patient breathes 10 times in one minute (a respiratory rate of 10). Each breath will last 6 seconds total. In most cases, an I:E ratio of 1:2 may be used. Using this ratio, each 6 second breath will be split into 2 seconds of breathing in and 4 seconds of breathing out. If we change the ratio to 1:3, these amounts shift to 1.5 seconds of breathing in and 4.5 seconds of breathing out. Therefore, adjusting the I:E ratio can either lengthen or shorten the time spent on each part of the breathing cycle.
Typically, ventilators are set to an I:E ratio of 1:2, 1:3, or 1:4, favoring a longer expiration phase to mimic natural breathing. However, Inverse Ratio Ventilation uses ratios that reverse this, where the breathing in time is longer than breathing out time, like ratios of 2:1, 3:1, or even up to 10:1.
Understanding Mean Airway Pressure (MAP)
Mean Airway Pressure (or MAP) is the average pressure measured at the opening of the airway over a whole breathing cycle. Numerous factors influence this, including the set “PEEP” (Positive end-expiratory pressure, which is the pressure in the lungs at the end of each breath), the pressure during inhalation, and the time taken for each breathing phase. In basic terms, MAP is higher when the pressure is high for longer periods in the cycle. This is the main goal of IRV – to increase the duration of the high-pressure inhalation phase to increase overall MAP, thereby helping to increase oxygen levels in the body without risking lung injury.
For instance, if a patient is on a ventilator with a PEEP of 5, inhalation pressure of 20, and a I:E ratio of 1:2, the base pressure is around 5 for two-thirds of the cycle (when breathing out) but goes up to 20 for one-third of the cycle (when breathing in). We then calculate 5 x 2/3 (time breathing out) plus 20 x 1/3 (time breathing in), which equals a MAP of 10. If this MAP is high, it helps more gas to exchange in the lungs, improving oxygenation.
Why do People Need Inverse Ratio Ventilation
Inverse ratio ventilation (IRV) is a method used to treat severe oxygen deficiency (or hypoxemia) that isn’t responding well to other treatments. This is often the case for patients with Acute Respiratory Distress Syndrome (ARDS), a severe lung condition. Normally, doctors use a strategy with low volume, high PEEP (positive end-expiratory pressure) ventilation to help manage ARDS. PEEP is pressure in the lungs that remains after releasing a breath. By increasing PEEP, doctors can increase the pressure across the lungs to help improve oxygenation – the delivery of oxygen to the tissues.
However, some patients might not tolerate the increased pressure or volume of air required for ventilation due to barotrauma (injury caused by air pressure changes), volume trauma (injury from the volume of air delivered by positive pressure), and alveolar damage (damage to the tiny air sacs in our lungs where oxygen and carbon dioxide are exchanged). For these patients, IRV might be a better strategy to manage their condition. So, IRV is often used when other methods to improve oxygenation in the body have been attempted to their maximum extent.
When a Person Should Avoid Inverse Ratio Ventilation
Some potential complications may arise from Inverse Ratio Ventilation (IRV). In simpler terms, IRV is a method of breathing support given in intensive care where more time is spent in inhaling than exhaling. Certain health conditions can make a person more likely to experience these complications. For example, if somebody already has a problem with blood flow and circulation in their body, which doctors call a ‘hemodynamic compromise’, IRV might be risky. The same goes for people with a type of lung disease that makes it hard for them to breathe out for an extended time. These health issues make those patients more at risk if they undergo this breathing support method.
How is Inverse Ratio Ventilation performed
Inverse Ratio Ventilation (IRV) is a process that is often used in treating patients with breathing problems. It could be used as a modification of a ventilating technique where pressure is controlled, as this method is straightforward. In simple terms, it is a bit like adjusting the volume and equalizer settings for your music. Your doctors would set the lowest pressure (P-low) and highest pressure (P-high) just like adjusting the volume. Then, they also have to set how often these pressure levels will change and how long each level will last — equivalent to adjusting the rhythm of the music.
Adjusting the rhythm means setting the equivalent of a respiratory rate (how fast or slow breathing occurs) and I: E ratio (the balance between the time spent inhaling and exhaling). Depending on the ventilator used, it can be controlled through a ratio or by directly setting the P-high time and P-low time.
Using IRV can be quite uncomfortable, leading to some patients needing heavy sedation or paralyzing medication to help synchronize their breathing with the ventilator. However, some types of IRV allow the patient’s natural breathing cycle to be blended in with the IRV cycle, which can make it more comfortable and successful.
Possible Complications of Inverse Ratio Ventilation
Some possible problems due to Inverse Ratio Ventilation (IRV), a type of breathing support used in hospitals, include injury to the lungs, building up too much pressure inside the lungs (known as auto-PEEP), not enough air movement in and out of the lungs (hypoventilation), and impact on blood flow in the body.
Although IRV uses lower peak pressures to achieve the desired average air pressure inside the lungs, it still increases the overall pressure inside the lungs. This can increase the risk for damage to the lung from either too much pressure (barotrauma) or too much air volume (volume trauma).
Auto-PEEP, also known as breath stacking or air trapping, happens when a patient can’t fully exhale before the next breath in, resulting in higher pressures in the airway. This can be worse with IRV due to the short time between breaths out and breaths in. Although this might help get more oxygen into the body, the extra pressure can also cause more damage to the lungs and put stress on how the blood flows in the body. People with lung conditions like chronic obstructive pulmonary disease (COPD) or asthma, who rely on having enough time to fully breathe out, are more at risk.
IRV works by increasing the average air pressure inside the lungs to improve oxygen levels, but this also increases the pressure inside the chest. Similar to the effects of high pressure in lung ventilation (PEEP), this can cause issues by increasing the pressure inside the chest, making it harder for blood to return to the heart and reducing the amount of blood the heart can pump out with each beat. This can be a bigger problem in people who are already low on blood volume (hypovolemia), or in due to dilated blood vessels (vasodilatory shock), and can be especially problematic in people with obstructive shock (a severe form of heart malfunction). If auto-PEEP develops, the risk of blood flow problems in the body increases.
What Else Should I Know About Inverse Ratio Ventilation?
Inverse Ratio Ventilation (IRV), a type of breathing support for patients on a ventilator, has not been proven to significantly improve major health outcomes. Factors such as patient survival rates, duration of mechanical ventilation, or the time spent in the Intensive Care Unit (ICU) are not necessarily influenced by IRV. Some research suggests that it could increase the level of oxygen in the blood (PaO2), but this finding is not consistent across all studies. As of now, we need more scientific evidence to determine if IRV truly offers any notable benefits.