THOMAS PIRAINO: Hello, my name is Thomas Piraino. And I'm going to speak to you about basic ventilator settings. There are a few settings common between conventional modes of ventilation. These include the fraction of inspired oxygen, which is simply the amount of oxygen being delivered to the patient. Positive and expiratory pressure, which is the pressure maintained in the respiratory system at the end of exhalation. The purpose of setting PEEP is to try and maintain an open lung by preventing atelectasis. Trigger sensitivity is the criteria used by the ventilator to determine if the patient is making an effort. The two options available on conventional modes of ventilation are flow trigger and pressure trigger. Flow trigger is a change in baseline flow measured in liters per minute that is required for the ventilator to determine if a breath has been requested by the patient. Pressure trigger is the change in baseline pressure measured in centimeters of water that is required for the same purpose. These three settings are present on the three most common conventional modes of ventilation. The three most common modes are volume assist control, pressure assist control, and pressure support. In this table is a summary of how these three modes are different. Please note this is a very basic generalization of the volume flow and pressure differences related to each mode. But it is the basic understanding required for this video. These differences will be described later when discussing considerations for each mode. There are minimum settings and targets that can be considered for mechanical ventilation with any mode. FiO2 can only be set as low as the fraction of oxygen in the atmosphere-- 21%. PEEP is generally in at a minimum level of 5 centimeters of water. Tidal volume should be set between 6 and 8 milliliters per kilogram of ideal body weight. Ideal body weight is the patient's predicted weight based on height and birth gender, not their actual body weight. The respiratory rate when using assist control modes I will describe in a moment should be set at least high enough to achieve a minute ventilation that is predicted for the patient, or set even higher if the patient has a known acid/base disturbance at the time of intubation. Predicted minute ventilation can be determined by multiplying ideal body weight of the patient by 100 milliliters per minute. In this example, an ideal body weight of 70 kilograms leads to a predicted minute ventilation required of 7 liters per minute. Adjustments made based on blood gas analysis or pulse oximetry will be discussed later. Volume assist control is the first mode I will discuss. Assist control mode share the common setting of requiring a frequency, which is respirations per minute. It is the minimum breathing frequency. And patients can trigger some of the breaths, all of the breaths, and more breaths than what is set by the ventilator if they would like. If the trigger sensitivity criteria is met, the ventilator will deliver a breath to the patient. If there is no trigger criteria being met, the ventilator will deliver a breath at the set frequency. When patients begin to actively interact with the ventilator, a trial of a spontaneous mode, such as pressure support, should be considered. Tidal volume is the volume delivered to the patient in milliliters. The acceptable range, as already mentioned, is between 6 to 8 milliliters per kilogram. Flow is the speed at which the tidal volume is delivered. And once the target volume is reached, the breath delivery has ended. A normal setting for flow is between 50 and 60 liters per minute. This minimizes any discomfort when patients begin making spontaneous efforts. An inspiratory pause is a setting typically found in the volume assist control modes. And it allows for a short pause to be set at the end of inspiration. And this functions to distinguish between pressure related to the resistive pressure and the elastic pressure. Resistive pressure is due to the endotracheal tube and airways. And elastic pressure is due to the stiffness of the lung and/or chest wall. Let's first review the settings common to all modes. The PEEP is set at five centimeters of water. The FiO2, which is entered as a percent rather than a fraction in this example, is 30%. The trigger sensitivity is flow, and it is set at 2 liters per minute. Now we'll look at the setting specific to volume assist control. In this example, the patient's ideal body weight is 70 kilograms. With the tidal volume set to 490 milliliters, it is equal to approximately 7 mLs per kilogram of ideal body weight. The frequency is set to 14 breaths per minute. And this gives us a minute ventilation, which commonly has the label VE, of approximately 7 liters per minute, which is the predicted value of minute ventilation based on the patient's ideal body weight. The flow is set to 60 liters per minute, which is the maximum recommended setting for flow. A flow of 60 liters per minute is equal to 1 liter per second when you divide it by 60 to convert, which makes it easy to determine the inspiratory time to deliver 490 milliliters, or 0.49 liters. In this example, would be delivered in 0.49 seconds. Now, 0.49 seconds is considered quite a short inspiratory time. Typically, it should be between 0.75 and 1 second. And there are a few ways to lengthen the inspiratory time when using volume assist control. One is to simply lower the flow rate. In this example, as you lower the flow rate, you can see its impact on the inspiratory time. Please remember that the setting should be between 50 and 60 liters per minute. We'll return it back to 60. Another way to increase inspiratory time would be to change the pattern. Some ventilators allow you to choose different flow patterns. If I choose decelerating, this means the peak flow will be 60 liters per minute, but then it will slow down over time, which of course will extend the time required to deliver the 490 milliliters. If I use further deceleration, you can see it extends the time even longer. However, a constant flow allows easy interpretation and calculation of inspiratory resistance. So we will leave it at constant flow. So with the flow set at 60 liters per minute and using constant flow, the method to lengthen inspiratory time that can also serve as a useful monitoring tool, is to add 0.2 to 0.3 seconds of a pause time at the end of inspiration. Without this pause time, all the ventilator will display is the peak pressure. However, with the pause time, there will be a moment of zero flow that will deliver a plateau. Let's watch the screen over here as we change. Currently, you can see that there is only a peak pressure being demonstrated. And this includes resistive and elastic pressures. And we can't distinguish between how much of this is resistance and how much is due to the stiffness of the lung. So let's add a pause. Now you will see there is an inspiratory pause in the waveform. And on the side here, you will see now there is a plateau pressure that is being read by the ventilator, and also driving pressure which is just plateau minus PEEP. And this can be used to calculate respiratory system compliance. Also, peak pressure minus plateau can also be used to calculate resistance. Peak minus plateau divided by the flow in liters per second is resistance. Because we're using 60, this is 1 liter per second. So 23 minus 15, which is 8 centimeters of water, divided by 1, is 8. So the resistance of the patient's respiratory system is 8 centimeters of water per liter per second. Now let's discuss pressure assist control. For pressure control settings, the frequency is the same as previously described with volume assist control. Pressure is the pressure delivered to the patient in centimeters of water. On some ventilators, this is the absolute pressure that will be reached with each breath. For the majority of ventilators, it is the change of pressure above PEEP, however. The pressure is typically set at a level that will deliver a tidal volume to the patient based on their ideal body weight, which is their predicted body weight based on height and birth gender, not their actual body weight. Again, the acceptable range for patients is between 6 to 8 milliliters per kilogram of ideal body weight. Inspiratory time is the amount of time the pressure will be applied. Rise time is the time the ventilator will take to reach the pressure that you have set. The default setting is usually acceptable and somewhere around 0.1 seconds. Because the pressure is set as a target, it will not change. And monitoring it is much less important than monitoring tidal volume. As resistance or elastance of the respiratory system changes, the result will be a change in the tidal volume delivered. Therefore, keeping the tidal volume within a reasonable target requires closer monitoring. This also means that minute ventilation will also change when these conditions change. As you can see in the graphics, the pressure waveform is at the bottom. And the pressure is being maintained at a setting of 12 centimeters of water above PEEP. And it's being maintained there for 0.8 seconds. As I mentioned before, if the respiratory system mechanics change, the pressure will not change because it is set. And what will change is the slope of the decelerating flow, here. And changes in the shape will result in differences in volume, and, of course, minute ventilation delivered to the patient. Let's see what happens when we increase resistance. Watch the shape of the flow change on the next breath. See, the deceleration is a little bit more gradual. But what happens is the tidal volume now changes from 505 to 426 in terms of exhaled volume. Now, let's return the resistance back to where it was. Watch, the exhaled volume will come back again. Now we'll change the compliance. And we'll pay attention to the volume changing here, but also the minute ventilation. Let's watch the volume as the compliance changes. Notice the shape change. It becomes much more rapidly decelerating. The tidal volume has dropped. And as you watch the minute ventilation, it will update, breath by breath, and become lower. This of course would cause a change in your arterial pH as CO2 rises. Now let's talk about breath timing. Every time the flow rate, inspiratory pause, shape pattern, inspiratory time, or respiratory frequency is changed, you must evaluate the time left to exhale. And the simplest parameter to determine whether there is sufficient time to exhale is the inspiratory-to-expiratory ratio. In general, if inspiratory time is less than one second or used, the IE ratio is not a concern until the respiratory rate or frequency is greater than 20 breaths per minute. Or if the patient has severe expiratory flow limiting conditions, such as a severe asthma exacerbation or COPD. When the respiratory frequency is greater than 20, we must ensure the resulting IE ratio is at least one to two or higher, as there may not be sufficient time to exhale the delivered tidal volume. An easy visual assessment can determine if the IE ratio is no longer sufficient after increasing respiratory rate. If there is evidence of air trapping indicated by a lack of expiratory flow reaching zero, as shown here. And please keep in mind that even though the settings you have used in assist control modes provide an appropriate IE ratio, if the patient begins to trigger the ventilator above the set rate, the IE ratio will be changed, and the patient should be assessed for liberation and/or weaning if this continues. I've now added spontaneous breathing to the patient on pressure assist control. Now in many ventilators, it is easy to notice a little dip in pressure prior to the onset of the breath. It's difficult to see in this simulator, so I will change the flow sensitivity to pressure sensitivity for trigger. And then you will see a drop of negative two is required to trigger the ventilator. Now you can see that there is an active effort being made by the patient. The ventilator set frequency is 14. And the patient is breathing 18. This patient is now ready to be transitioned to a spontaneous mode of breathing and assessed for liberation from mechanical ventilation. Pressure support is nearly identical to pressure control except for two things-- there is no respiratory frequency set. Instead, there is a backup setting that will apply on assist control mode if the patient stops breathing. And it is important to check the settings for that to make sure they are applicable for the patient. The second difference is the termination criteria for the breath. With pressure assist control, it is based on time and seconds. With pressure support. it is a percent of the inspiratory flow. The appropriateness of the setting depends on the resistive and elastic properties of the lung and the inspiratory drive of the patient. Many ventilators have default values of either 25% or 30%. These default settings are important to adjust in patients with abnormal resistance or elastins, such as patients with COPD and asthma. Let's see how this works. The breath profile and pressure support looks identical to pressure control in terms of the pressure is increased and maintained for a certain amount of time. However, in pressure support ventilation, it is not a time based on seconds. It's based on cycle off percent. And it's 25% in this example, which means it's 25% of this peak flow is at which point the ventilator will cycle off and allow exhalation. So let's see what happens when we change this. First, let's change the scale so that it's more clear where it's cycling off, which you can see there. Now I'm going to increase the cycle-off percent to 50%. Now you can see this breath was still at 25% cycle-off of the peak flow, whereas here it's 50% of the peak flow. So halfway between peak and baseline of zero, it's cycled off at 50%. So as you can see, although there's no time being set in seconds, we do have the ability to set a timing in pressure support, which is important because it's possible to set a timing that is too short, which, in fact, if you see here, the expiratory flow, there's a peak and it's exponential. Whereas here, there's a little bit of a dampening in the peak. And that's because the effort is actually still continuing, even though the ventilator cycled off. This is a form of dyssynchrony or asynchrony called premature cycling, or short cycling. So the ventilator cycled off before the patient was done the effort. CPAP is Continuous Positive Airway Pressure. Consider it similar to PEEP, relating to its purpose to maintain end-expiratory lung volume. Many ventilators do not have a specific mode called CPAP. Rather, it is delivered by simply turning the pressure support level and pressure support ventilation down to zero. And what you have left is a constant pressure in the system based on your PEEP setting. Flow is delivered to the patient by the machine in order to maintain the set pressure. It does not directly provide support to reduce inspiratory workload. But if it maintains end-expiratory lung volume, it can help to minimize workload due to atelectasis. Here's an example of CPAP, the pressure's set at 5 centimeters of water. And it's constant, lead delivered. The pink line here represents patient effort, which generates negative pressure in the pleural space. As you can see with each effort, the machine maintains pressure by providing flow, and therefore tidal volume. Let's review some of the special considerations for monitoring, specifically between volume assist control and pressure assist control. In volume assist control, because flow is controlled and tidal volume is the target, the pressure generated depends on the resistance and the elastance of the respiratory system. Peak pressure and plateau pressure is extremely important to monitor. Abnormal resistance and elastance due to ARDS, COPD, asthma, intra-abdominal hypertension, and other conditions will result in high levels of peak and/or plateau pressure. In pressure assist control, because pressure is controlled and time is the target, the tidal volume delivered depends on the resistance and elastance of the respiratory system. Tidal volume and minute ventilation is extremely important to monitor. Abnormal resistance and elastance due to ARDS, COPD, asthma, intra-abdominal hypertension, et cetera, will require higher pressures to deliver an acceptable volume. And acute changes can cause significant changes in your tidal volume and minute ventilation. And therefore can change your arterial pH. When it comes to understanding the ventilators role in acid/base and oxygenation issues, two key concepts need to be understood. Ventilation refers to the alveolar ventilation, generally determined by minute ventilation. And this addresses carbon dioxide in the blood, and therefore can alter pH. Oxygenation refers to oxygen diffusion into the blood. And it is addressed by changes in the fraction of inspired oxygen, and PEEP. Improvements in oxygenation by providing more oxygen is easy to understand, since it increases the partial pressure of oxygen available for diffusion. PEEP is often used to increase the end-expiratory lung volume available for gas exchange. However, considerable care needs to be taken when adjusting PEEP for oxygenation. Due to the potential to over-distend the lungs. And caution should be taken when plateau pressure is greater than 27 centimeters of water. PEEP can also affect perfusion within the lung, which can negatively impact ventilation, resulting in worsening pH. Oxygenation is classified by severity. And when the SpO2, or saturation, is well correlated with arterial blood oxygen levels, it can be used to estimate PA02, which is the partial pressure of arterial oxygen. An easy rule to remember is that a saturation of 90% under normal conditions correlates to an oxygen level in the blood of 60 millimeters of mercury. Normal arterial pH is between 7.35 and 7.45. Levels below this are consistent with acidosis. And levels above this are consistent with alkalosis, generally referring to respiratory or metabolic conditions. The arterial pH has an inverse relationship with arterial carbon dioxide. If the carbon dioxide in the blood increases, pH will decrease. If carbon dioxide is reduced, the pH will increase. These are considered respiratory disturbances. The pH has a direct relationship with arterial bicarbonate. If bicarbonate increases, pH will increase. If bicarbonate decreases, pH will also decrease. These are considered metabolic disturbances. The simplest thing to remember, regardless of whether the issue is carbon dioxide or bicarbonate, respiratory or metabolic, is that there is a direct relationship between the pH in the blood and minute ventilation. If you want to increase the pH of the blood, increase your minute ventilation. If you want to decrease the pH in the blood, decrease the minute ventilation. But please note, controlling for metabolic acidosis can be challenging when the increase in minute ventilation that is required results in a carbon dioxide level in the blood less than 30 millimeters of mercury. Levels this low are not advised and may impact cerebral perfusion.