Hello, it's my pleasure to be with you for this year's version of the Multiprofessional Critical Care Board Review Course. For my first presentation, we will be discussing mechanical ventilation and weaning. To start, we'll discuss some of the necessary basics on this topic. Then we'll move on to the more interesting aspects of respiratory mechanics and conclude in the last few minutes of this presentation with some ideas and thoughts about weaning and extubation. First we'll start with some basic principles. Number one, mechanical ventilation causes lung damage. The lungs intrinsically are not designed to receive positive pressure ventilation. The human body is designed for negative pressure breathing. Number two, complications result from mechanical ventilation even under the best of circumstances, things sometimes go astray. And lastly, seemingly obviously, the longer a patient is ventilated, the greater the risks. That's our goal for any patient should be to optimize and minimize the length of mechanical ventilation. What are the underlying goals of mechanical ventilation? First is to achieve adequate, not maximal, but adequate gas exchange. The gas exchange is necessary to allow for optimal functioning of the body's organs and tissues. Second, to minimize toxicity, to minimize barotrauma, volutrauma, and electrotrauma and biotrauma. Want to minimize oxygen toxicity as well as to eliminate or lessen any adverse effects of mechanical ventilation on the cardiorespiratory system. Thirdly, we need to optimize the patient worth worker breathing while maximizing patient comfort. And lastly, we want to optimize the patient ventilator interactions. The goals of a ventilator are to gently ventilate the lungs while not causing harm, while minimizing the risk to the patient. We want to try to avoid lung collapse. On the other side, avoid excessive tidal volume or delta P and thus avoid hyperinflation as we optimally set our ventilator to manage individual patients. We also need to maximize patient comfort. We want to optimize patient ventilator synchrony. We need to set the ventilator such that it will inhale when the patient wants a breath and will allow a patient to exhale when she or he wants to. When optimizing mechanical ventilation, there are those entities which are intrinsic in the current situation. Basic physiology, the patient's pathophysiology, the patient ventilator interface, the size of the endotracheal tube, whether it's an endotracheal tube or a tracheostomy tube, and the intrinsic ventilator settings or properties being used. On the other hand, there are those entities that are within the control of the clinician, how one uses the ventilator modes available, how to set the ventilator or to choose the settings for that patient. And then, just as importantly, if not more importantly, how to use graphics and the interfaces of the mechanical ventilator to detect problems and thus direct a clinician response. There are many different ventilator modalities on the market today. Shown on this slide are various current modes that can be used on patients across the country and beyond. Why so many options? The truth is that it's largely determined by individual ventilator companies to try to promote their products as better than others to lead to better outcomes. But the truth is, no study has ever shown that any ventilator modality is better than any others in improving overall patient outcomes. The key is how you use these modalities to optimize lung protection, to promote patient ventilator synchrony, to optimize a patient's work of breathing while minimizing pharmacologic oxidation, to facilitate spontaneous breathing with the goal of trying to minimize the length of invasive mechanical ventilation. When choosing an optimal mode for your individual patient, the approaches can vary based on the available technology in the given ICU. The key is not to know how to use all modes available on the market today, but you do need to know how to use the available modes that you have in your ICUs to be able to use the modalities in an optimal fashion to promote the best possible outcome. As I mentioned, but just to stress, there's no definitive outcome data to prove any mode is superior to any other. The key is to match the patient's pathophysiology to the options currently available. What determines a ventilator breath? Or stated differently, how does a ventilator control the flow of air in and out of the lungs and thus sculpt the desired breath to what the clinician feels is most optimal to manage his or her patient? What is the type of breath? What's the mode type, SIMV, assist control, or spontaneous breathing? What is the quantity of gas flow where it limits the size of the breath? Third, what signals flow to start or what triggers the breath to initiate? What ends inspiratory flow or what cycles flow from inspiration to exhalation? And lastly, what is the type of inspiratory flow or what is the specific flow pattern of the breath being delivered? On this slide, we see a visualization of those parameters I just mentioned. First, we see the trigger. What triggers the start of inspiration? Next, in red, what is the inspiratory flow pattern? Third, the limit. What is the size of the breath? And as inspiratory flow decays, what cycles the breath from inspiration to exhalation? First, let's start with mode types. There are three different ventilator modalities available. First, control modes, second, support modes, and third, mixed modes. In terms of control modes, it was commonly known or referred to as assist control ventilation. In this situation, each breath is a, quote, mechanical breath as defined by a fixed inspiratory time. Thus, each of the breaths has a set limit, whether that be volume or pressure, as we'll come to in a subsequent slide, and a fixed width or inspiratory time. Second, there are support modes or assisted modes. The most common example is pressure support ventilation. In a supported spontaneous mode of ventilation, the key entity or the key parameter is that each breath has a variable inspiratory time as set by the patient. So in pressure support ventilation, as an example, the patient controls all ventilatory parameters except the size of the breath augmentation, the pressure support level, the PEEP, and the FIO2. And lastly, there are mixed modes or SIMV. For example, SIMV, pressure control, pressure support, which is a combination of the above two breath types. Next, we'll move on to discuss limit, the size of the breath, trigger initiation of the breath, and cycle termination of the breath in this next section. The limit is the size of the breath, which determines the quantity of gas delivered. The limits most commonly are pressure or volume, and these are clinician determined. The trigger is a parameter that initiates the start of an inspiratory flow or the start of a breath. It signals the ventilator response to either patient effort by changes in flow or pressure in the ventilatory circuit, or by time if a patient is not exhibiting adequate spontaneous respiratory effort. The trigger is clinician determined and needs to be carefully titrated, as we'll talk about in a later presentation, to optimize patient ventilator synchrony. The cycle determines when inspiration transitions into exhalation. Termination of inspiration or cycle can be either time triggered, i.e., an inspiratory time, or flow triggered, and we'll talk about that on the next slide. If time triggered, it is clinician determined, and if flow triggered, it is generally patient determined. Here we see a graphical display of a flow-cycled breath. The leftmost portion of this curve, we see inspiratory flow reaching its peak inspiratory flow rate at the top of the graph. In flow, or inspiratory flow, the rate of flow decreases at a variable rate until flow drops to a point that exhalation begins, or the expiratory valve opens, allowing the patient to expire. This expiratory flow rate can vary based on ventilators, and in some ventilators, by clinician determination, but most commonly is set at 25%. To give a specific example, if a patient's peak inspiratory flow rate is 10 liters per minute, and the inspiratory cycle, or the cycle from inspiration to exhalation, is 25%, which is the general default of most ventilators, when inspiratory flow, still positive to the patient, decays to 25% of 10, in the example I gave, or 2.5 liters per minute, the expiratory valve will open in anticipation of the patient's effort, or expected, exhalatory effort. Lastly, we'll talk about the inspiratory flow pattern, the pattern of gas delivery from ventilator to patient. While the inspiratory flow pattern may vary on modern-day ventilators due to individual sculpting approaches, there are two classic inspiratory flow patterns that one needs to be aware of, and probably the only two necessary from a board review question standpoint. The first, constant square wave inspiratory flow, on the left here in orange, flow increases to a clinician predetermined level, and stays at that level throughout the inspiratory cycle, or the inspiratory time, and then decays when inspiration has concluded. On the right side, variable decelerating flow, as shown in blue, inspiratory flow peaks early in the inspiratory cycle, and then decays at a variable decelerating rate until exhalation begins. The key relationship to know here is that between inspiratory flow and airway pressure. In square wave constant flow ventilation, since inspiratory flow is constant, the rate of rise of airway pressure is linear, as shown here on this graphic. However, for decelerating flow ventilation, the rapid rise in inspiratory flow early in the breath leads to a rapid rise in airway pressure, again, early in the breath. However, as inspiratory flow decays, the rate of rise of airway pressure slows as well, thus leading to a curvilinear trace. So for constant inspiratory flow, airway pressure increases linearly, and for decelerating inspiratory flow, airway pressure increases curvilinearly. Due to the physics of gas exchange or gas distribution throughout the lungs, there'll be differences between peak inspiratory flow between a constant and a variable decelerating inspiratory flow pattern, as well there'll be differences in mean airway pressure, and we'll go through these in more detail in the next few slides. But first, a question. A two-year-old boy is intubated for viral pneumonia. The inspiratory flow pattern is changed from square wave constant flow to variable decelerating flow. Which of the following changes in peak inspiratory pressure and mean airway pressure would be expected? And as you can see, there are all the different combinations of increase and decrease. I'll give you a moment to think about this, and then we'll work through the available data and the physics or physiology of gas exchange. As shown here in this now classic study from almost 25 years ago, we see in blue traditional volume control ventilation, square wave constant flow, and in purple, both pressure control ventilation and PRVC, both of which are variable inspiratory flow patterns. For the same tautovolume and inspiratory time, we note that the peak pressure is higher with the square wave constant flow pattern than that of variable decelerating flow. This relates to how gas distributes throughout the lung. This finding has been reproduced in multiple other studies and is consistent with what is known on this topic. So if variable decelerating flow ventilation provides the same tautovolume and square wave constant flow, peak pressure will be lower, leading to a decrease in the risk for ventilator-induced lung injury. And as I didn't stress but will mention now from the prior slide, that that rapid rise of increase in airway pressure leads to a wider base, and thus the area under the pressure curve where the mean airway pressure is higher, leading to improved oxygenation. This is the rationale why modes such as PRVC and pressure control ventilation were more commonly used in those patients with acute lung injury or ARDS. The mechanisms responsible for these effects are unknown, but are most likely related to beta gas distribution throughout the lung of the decelerating inspiratory flow. Here we see an actual patient ventilated with square wave constant flow in the middle of the flow scaler, square wave constant, and associated with that is the linear increase in airway pressure. Square wave constant flow is not used very often anymore today, but it has some advantages that it's available in every ventilator for most breast types, and truthfully, it's adequate for most routine ventilation, especially in our smaller patients. The key disadvantages and the reason that it has fallen out of favor is that inspiratory flow is not matched to patient demand, and such a flow pattern is not responsive to changes in lung mechanics. Here's an actual example of variable inspiratory flow pattern here shown in the middle, flow versus time on this scaler, and on the upper panel, the upper scaler, airway pressure over time, we see the curvilinear increase in airway pressure. The key advantages of a variable decelerating flow is that it matches inspiratory flow to the patient's spontaneous demand, and it is responsive to changes in lung mechanics. The disadvantage, which actually has become much less of a disadvantage with the current of ventilators, is that traditionally it's not been available for all breath types and all ventilators. So back to our question in the example shown, moving from square wave constant flow to variable decelerating flow would lead to a decrease in peak inspiratory pressure and an increase in mean airway pressure. And to recap the key definition point that I made earlier, question two, an invasively ventilated patient, what defines a spontaneous breath? And a spontaneous breath is defined by a variable inspiratory time. OK, we'll now move on to the second section of this presentation, focusing on respiratory mechanics. First, we'll start with two questions to get you thinking in this section of this talk. What is elastance defined as? Change in volume over change in pressure, change in pressure over change in volume, change in pressure over change in flow, change of flow over change in pressure, or pressure over time? Before answering that question, let's look at another one. What is the definition of resistance? And you can read the choices. The key here is that there are some basic definitions, basic equations that are necessary to know for the board examination. So these are the basic equations to remember, compliance, change in volume of the change in pressure, elastance is the inverse of compliance, or the change in pressure over the change in volume. And lastly, resistance, change in pressure over change in flow. Simply three equations that must be memorized. Just general advice for the board, something to think about. If you see a long stem, before spending a lot of time reading through all the details of the stem, look at what's often a simple one line question that follows. There may be a long stem, and then the question simply states, what's the elastance of this patient's respiratory system? In which case, you only need to look at the change in pressure and the change in volume. Again, just to stress, elastance is one over compliance. Moving forward from a mechanical ventilation perspective, dynamic compliance is a change in volume or tautovolume over the difference between PIP and PEEP. Static compliance, obtained in a static situation, eliminates the inspiratory flow portion of the breath, and thus is done during inspiratory hold maneuver, such that static compliance is tidal volume, delta V, over the difference between plateau pressure and PEEP. And lastly, again, elastance is one over compliance. In terms of the spontaneously breathing patient, lung compliance, tautovolume over the difference between the alveolar and pleural pressures. Chest wall compliance, or tautovolume over the difference between pleural and atmospheric pressure. And lastly, total respiratory system compliance, tautovolume over the difference between alveolar and atmospheric pressure. Instead of having to memorize all these different equations on the lower half of this slide, you just need to remember that anything compliance is change in volume or tautovolume over the differences in pressure at the extremes of what is being asked or considered. Thus, again, in the respiratory system compliance, it's atmospheric pressure all the way on the outside to alveolar pressure all the way on the inside. This next point is one that everyone knows, but when it comes to a board review question, if you haven't thought about it ahead of time, often leads or can lead to a wrong or incorrect answer. It's a concept borrowed from electrical engineering. And it describes the phenomena whereby a given percentage of a passively exhaled breath requires a constant time to be exhaled, regardless of the starting volume, given constant lung mechanics that the overall system isn't changing. And the key here is that it's a logarithmic function. Continuing on with time constants, at the start of exhalation, initial flow of gas out of the lungs will depend on the drive pressure, the difference between the alveolar pressure and the mouth pressure, as well as the airways resistance. So for a given gas volume, the alveolar pressure at the start of exhalation is going to be dependent on the lung compliance. This time constant is the product of resistance times compliance, where TC equals R times C. So for any value of resistance and compliance, the time constant equals the time necessary for the lungs to empty by 63%. Again, this is a logarithmic equation. When expiratory time equals the time constant, the patient will passively exhale 63% of the tidal volume. On a board exam, rather than asking for a simple equation when it comes to time constants, it's much more likely that an examiner will ask about a clinical application. And the most common one that one might see is the example of small airways disease in infants, in which there's a long time constant leading to longer airway emptying times. This could be exacerbated in a patient such as this, who is being ventilated with a long eye time, and thus has an excessively short expiratory time. One example would be RSV bronchiolitis. And the other example would be an infant with chronic lung disease or prematurity undergoing exacerbation, whether that be viral induced or some other entity. So the increase in airways resistance leads to a long time constant. Again, time constant being R times C. These patients tend to be tachypneic. So now we have tachypnea, a long time constant. And in this situation, an inability to fully exhale. Thus, the lungs do not empty. And the resulting clinical entity is one of air trapping. Let's move on to another entity. And let's talk a little bit about dead space ventilation. Total minute ventilation is going to equal the sum of dead space ventilation plus alveolar gas exchange ventilation. Now, obviously, minute ventilation is volume times respiratory rate. So if you look at this from the perspective of just an individual breath, tidal volume of a given breath will be the sum of the dead space volume plus the volume of gas that's participating in gas exchange at the alveolar level. I carefully chose those words because dead space or total dead space, which is often referred to as physiologic dead space, is going to be the sum of airway or anatomic dead space and alveolar dead space. So if I take a step back, we have to think of total tidal volume as the sum of the portion of the tidal volume that is participating at gas exchange at the alveolar level plus alveolar dead space, that portion of the tidal volume that reaches the alveoli but is not participating in gas exchange, plus that portion of the tidal volume that is, quote, lost in the airways or the anatomic dead space. So physiologic dead space is airway anatomic dead space, but alveolar dead space. How do we relate dead space to tidal volume or what percent of the total breath is dead space or the VDVT ratio is determined or defined by the difference between an arterial CO2 value and a mixed expired CO2 divided by the arterial CO2. So to determine alveolar dead space, one must have an arterial CO2 gas value. If we look at dead space more globally, let's start with ventilation-perfusion relationships. In the middle here, we have a normal VQ ratio of 0.8 where ventilation and perfusion are reasonably balanced. If we go on the right side of this slide where I just was discussing dead space ventilation, that again is going to be the situation of high ventilation to normal perfusion or normal ventilation to low perfusion, a high VQ ratio. And this is the world of capnography. On the left side of the slide, shunt perfusion is low ventilation to perfusion or the world of oximetry and desaturation. And the last portion of this presentation will conclude with a brief discussion of weaning and extubation. In terms of prolonged ventilation, the side effects or the adverse effects are well-known and well-reported. An increase in length of ventilation will almost always lead to an increase in both ICU and hospital length of stay, be associated with increased cost, increased risk for ventilator-associated pneumonia, potentially increased risk for airway injury, especially of the upper airway, increased risk for ventilator-induced or associated lung injury, and of course, decreased patient and family satisfaction because no one wants themselves or their loved ones on a ventilator any longer than necessary. So why not try to extubate patients sooner, potentially taking a chance that they may or may not be ready? The problem with that is that failed extubation leads to an increased risk of mortality. Weaning is the transition from full ventilator support to full spontaneous breathing. The patient gradually assumes the responsibility for effective gas exchange in which positive pressure ventilation is reduced. The reduction in vent support is, of course, at the discretion of the patient care team. An optimal weaning process allows for the recognition of the readiness to wean or extubate as early in the process as clinically indicated and occurs efficiently 24 hours per day. An optimal process needs to address the many complex variables related to readiness for and tolerance of weaning. Protocols may or may not be helpful, but the key is to test the patient periodically through spontaneous breathing trials, extubation readiness testing, to find, to determine that point in time the patient no longer needs mechanical support. Gradual weaning may not even be necessary. The key is to extubate when clinically indicated. So some classic studies have shown that weaning occupies the great proportion or the vast majority of a total length of ventilation. With such a prolonged weaning phase, extubation might be delayed. On the other hand, without weaning, one could worry that premature extubation may occur. And as I've shown previously, both can be harmful. One of the goals or approaches with weaning protocols is to try to operationalize best practice. And the key for a successful weaning protocol is for it to be iterative with frequent reviews of protocol deviations and ongoing improvements based on new literature available. Best practices are meant to bring the masses up to the available evidence-based medicine, the currently available literature, and to reduce the occurrence of bad practices by avoiding unnecessary variability, and to over time allow a process to evolve to encourage good deviations and reduce bad ones. While protocols have often been shown to be beneficial, there is still some uncertainty into the use of protocols and some controversy as to whether or not they are the ideal approach. In this study, we have found that in this study shown on this slide, the RESTORE study, Protocolized Sedation versus Usual Care in Pediatric Patients Mechanically Ventilated for Acute Respiratory Failure, Martha Curley and colleagues showed that protocolized approach to sedation using bedside nurse-controlled algorithms was not more beneficial than routine care. And in this case, protocolized approaches to sedation and mechanical ventilation did not lead to a change in length of ventilation. As we think about weaning, there are obviously various aspects that one needs to consider from intubation to extubation, and then follow up for 24 hours after extubation. There's not much more at this point that I'm gonna go through in detail on this slide, but just again, thinking about when you are weaning a patient where you are in the disease process. Spontaneous breathing trials or testing has become standard of care in most situations. It's a test of whether the underlying disease process necessitating mechanical ventilation has improved sufficiently to allow adequate gas exchange with spontaneous breathing. A spontaneous breathing test should be as objective as possible, but often does have a subjective component when it comes to worker breathing, worker breathing, retractions, and overall respiratory effort. Various recipes have been published, and a spontaneous breathing trial or test is a component of an overall approach to extubation readiness assessments. When doing an extubation readiness test, the worker breathing or the pressure rate product needs to be considered. There's various parameters or various approaches that need to be considered, ranging from press support to CPAP to T-piece trials through to the extubated state. Various recipes, various approaches have been published, and I don't believe that there's likely to be a question on any various approaches because it still remains controversial. The key here is just the concept, overall concept at hand. Overall assessment of extubation failure or extubation readiness testing failure, either way, are often clinical with clinical assessments of worker breathing, as well as various components of respiratory rate, heart rate, tidal volume delivery, saturation. If we tried to be a little bit more specific, there are monitoring laboratory values that can be used, increase in entile CO2 greater than 10, often a pH decline of greater than 0.07, changes in PF ratios or a fall in saturation by 5%. Wouldn't worry too much about all the details here. There are very much institutional differences in approaches, institutional biases, but the key here is to have set criteria for extubation failure. So with that, I'll conclude with this overview, initial presentation on mechanical ventilation and weaning as we review the necessary basics, respiratory mechanics, and I really just touched on weaning and extubation. In follow-up or closing, my email address is listed. I'm always very open to receiving questions and having ongoing discussion. And I also look forward to participating in the live question and answer discussion sessions that are being scheduled for August and September. Hopefully I'll get to see many of you in one of those sessions. And I appreciate your attention to this introductory talk.

mechanical ventilation

weaning

critical care

ventilation modes

respiratory mechanics

extubation readiness testing

weaning protocols