Episode 3 - Basic Mechanical Ventilation I: Fundamentals

From the classroom to the emergency room, OR and beyond. Trauma ICU.

Welcome back to Rounds. I'm your host, Dr. Dennis Kim. Today we're discussing fundamentals of mechanical ventilation. This is gonna be the first of a series of episodes. in which we'll explore common issues, scenarios, as well as topics and mechanical ventilatory support, ranging from patient ventilator desynchrony to proning and high flow oscillatory ventilation.

You know, of the myriad of devices and technology that we use in the ICU, none of them, save perhaps CRRT, generates more confusion, anxiety, or insecurity as does the ventilator. Which I find very interesting, especially given that the need for invasive ventilation is probably the most common indication for ICU admission worldwide. So, what is it exactly that makes ventilators so intimidating? Well, I think there's a few things. Number one, depending on where you practice.

You may not actually have an opportunity to tinker or play with the vent as they may be quote unquote owned by the RTs or RCPs. Shout out RTs. Also, classification and the nomenclature or terminology and acronyms used to describe different modes of ventilation varies tremendously, both across manufacturers. and amongst providers. And I think this generates a lot of confusion as well as inconsistencies in terms of communicating findings or discussing goals of ventilator management.

So having listened to numerous podcasts, watching, attending, and listening to others give talks and presentations on mechanical ventilation. What I've noticed is that the vast majority of discussions around Vents really centers on the differences between modes. as well as how to initiate someone on the ventilator and a lot of talk about the ARDS net protocol for ARDS. And this is all fine and reasonable. And I think that these are all important topics.

But for today's rounds, I'd like to discuss and review the fundamental anatomic and physiologic concepts, as well as the relationships to one another, so that we can get a better understanding of the foundations of mechanical ventilation. So there are four key objectives for today's episode and by the end of rounds you should be able to number one, discuss the indications and goals or objectives for invasive positive pressure mechanical ventilation.

Number two, I'm really hoping that you're gonna walk away from this episode with a deeper understanding of the core physiologic concepts comprising the respiratory equation of motion. also known as the force balance equation, which, as you'll see, really forms the foundation for understanding mechanical ventilatory support. And key concepts that we're going to discuss here include various pressures, as well as flow, volume, resistance, and compliance.

Number three, you should be able to describe the four phases of a breath while on the ventilator. And this is really important insofar as it'll help us to troubleshoot the vent when there's issues related to patient ventilator dynchrony. or if we notice that there are issues with our ventilator graphics or waveforms or physiology of the patient. And finally number four, I'm hoping that you'll have a deeper understanding of what we mean by a mode of ventilation.

And this is something that is debatable and is variably defined across the literature with really no one accepted approach. But I'll share with you my sort of three concept way. of determining the mode of ventilation. So regarding the indications for invasive mechanical ventilation, I think these can be broken down into a few different headings. In the first place, and this may be overtly obvious, if a patient is apnic,

or has absence of breathing, I think it's pretty self-explanatory that this patient would require intubation and ventilatory support. The one thing I would say in this situation is that we do want to take into consideration advanced directives. where available. And so barring any preference not to be maximally and invasively supported, any apnec patient who's not breathing would get snorkeled.

Number two, or in the second place, we have patients who have either impending or established acute respiratory failure. And so whether it's newly diagnosed acute hypoxemic respiratory failure. or acute hypercapnic respiratory failure in the setting of a COPD exacerbation that we've attempted non invasive ventilation on, yet patients remain refractory. as manifested by increased work of breathing

or an ineffective breathing pattern. These are certainly scenarios where patients should be intubated and placed on the ventilator. Now whether or not we want to give them full versus partial ventilatory support. is something that we'll discuss a little later. Outside of acute hypoxemic and hypercapnic respiratory failure.

Other situations where we might want to protect the airway or so-called prophylactic mechanical ventilation. A great example of this would be the patient who's going to the OR and is gonna get a general anesthetic. In the trauma bay, we see a lot of patients who come in with an STBI or severe traumatic brain injury. They're not able to protect their airway because their GCS is less than eight. And so those patients also will buy themselves an E T tube and ventilator.

The fourth and other reason or indication for intubating and placing a patient on the ventilator, at least in the world of trauma, are patients with severe TBI where they have a refractory

intracranial pressure or intracranial hypertension. In these situations we can actually use the ventilator as a therapy to decrease ICP. So by hyperventilating patients, we can reflexively result in an increase in the vasoconstriction in the cerebral arteriolar circulation, thereby decreasing flow into the cranium with the understanding that we may also risk worsening tissue hypoxia with this strategy.

In terms of the overall goals and objectives, I think you really want to think about these in terms of the indications for intubating and ventilating a patient. So first and foremost we want to support or manipulate pulmonary gas exchange, both in the sense of improving patients' alveolar ventilation and arterial oxygenation. Again, when it comes to augmenting or improving a patient's overall ventilation, We do want to bear in mind that in certain circumstances, for example, patients with ARDS.

We don't want to go too crazy in terms of lung volumes as this may ultimately result in what's known as villi or ventilator induced lung injury. Another important goal of mechanical ventilation is to reduce the work of breathing in the normal, sedentary, steady state. Less than four to five percent of our cardiac output is going to our diaphragms and the muscles of respiration.

You can imagine in someone who's in septic shock or really sick, that amount can go up to twenty percent of your total cardiac output. So that's a fifth. of your blood flow and oxygen delivery that's being diverted away from other critical organs like the brain and the heart or the coronaries and kidneys just to support the work of ventilation.

So, you know, when patients come in really sick, what I have noticed at times is that in addition to being aggressively resuscitated with fluids and let's say being initiated on vasopressors to help augment uh systemic flow. I'll look over at the ventilator and the patient's actually chugging along with minute ventilations of 15 liters per minute. They've got minimal sedation and analgesia on board.

And the whole point of this conversation is just to say that if you've got a sick patient Who need to be on the ventilator at least for the first 24-48 hours while you're actively resuscitating and trying to augment oxygen delivery and reverse critical tissue hypoxia.

Just get them on full ventilatory support. As the patient gets better, that's when we can start to wean things off and then think about partial ventilatory support with the goal of course of ultimately liberating our patient from the ventilator. So moving along to our third objective to understand the core physiologic concepts comprised in the respiratory equation of motion.

I want to take a couple of seconds or minutes just to review a couple of key points when it comes to mechanical or positive pressure ventilation. And the one thing to bear in mind is that there are a host of wide-ranging systemic effects of mechanical ventilation. And I think the best example of this is the cardiopulmonary interactions and dependency that we see and in fact can manipulate at the bedside.

For example, why is innovating a patient with cardiac tamponade a horrible idea and almost as surely going to put them into cardiac arrest? Well, in addition to the sympatholysis associated with the administration of RSI meds.

the increase in intrathoracic pressure is gonna result in a decreased gradient for blood flow from the vena cava Back to the right side of the heart where the filling pressures are already elevated due to compression from blood and clotted blood in the rigid confines of the pericardial sac. Increasing the mean airway pressure with positive pressure ventilation, therefore, is gonna overall result in a decreased preload. If you have no preload, you don't have stroke volume.

I mean, you'll still have contractility and after load, but preload is a major determinant of stroke volume. You don't have stroke volume, you have no cardiac output, which equals no pressure. I bring up this now merely because a lot of these first few talks are on critical care and I obviously miss talking about trauma, but more importantly to highlight the relationships between the lungs and other organs

as well as the predictable responses to interventions among patients receiving positive pressure ventilation. And this includes one of my favorite topics, fluid responsiveness. But moving on.

So let's talk respiratory equation of motion. I would say that this is the single most important equation that really sets up the entire conceptual framework for understanding mechanical ventilation. It's sort of the uh DO2. Counterpart for understanding oxygenation throughout the body. You really want to have a good understanding of this because it really sets up your approach.

when patients get into trouble or when you're thinking about making changes on the ventilator. And it's so important in fact that I would argue that unless you have a strong handle on the meaning of and the relationships between the variables in this equation, you simply do not have a solid understanding as to how ventilators work.

One of my early career mentors and probably one of the smartest and kindest individuals I've ever had the honor of working with and learning from Doctor Rick Hodder at the Ottawa Hospital. May he rest in peace. gave a fantastic lecture entitled A Practical Approach to Mechanical Ventilation. And one of the very first things he asked or posed to me as a critical care medicine fellow in Canada, was to in fact write down the equation of motion for the respiratory system

As you might imagine, I had absolutely no idea what he was talking about. And in fact, and I'm a little afraid to admit this, but it would probably be another four to five years. Before I truly embraced and acknowledged the importance of this concept in overall ventilator management. So in its simplest form, the respiratory system can be visualized as a conductive tube connected to a balloon or elastic compartment. And an important point in ventilation is the basic concept of airflow or flow.

For air to flow through a tube or airway, pressure at one end of the tube has to be higher than the pressure at the other end. Air always flows from a high pressure point to a low pressure point, and this is what we call a pressure gradient. Once again, lung volumes change as a result of gas flow caused by changes in pressure. So when you look at the respiratory uh equation of motion On the left side of the equation you have pressure.

Specifically, pressure that needs to be generated by the ventilator and in a spontaneously breathing patient, the negative pressure induced by the muscles of respiration On the right side of the equation, we have a few concepts, specifically flow times resistance plus tidal volume times elastance. So essentially the pressure required to deliver a mechanical ventilator breath has two components. One is the pressure needed to cause flow through the airways and overcome the resistance to airflow.

And that resistive pressure is the product of flow times resistance. The second pressure is the pressure required to overcome the elastic recoil of the lungs and chest walls. The required pressure to overcome the elastic recoil is the product of elastance and tidal volume or volume. Given that elastance is the inverse of compliance, this can also be stated as title volume over compliance. So there's a few things we can take away from this equation right off the bat.

In the first place, we can actually calculate the compliance and resistance of the respiratory system if the ventilatory muscles are relaxed, meaning there's no contribution from the musculature. And if flow, volume, and pressure are measured. Additionally, when there's no flow, for example, at the end of inspiration, then ventilator pressure depends on the products of volume and elastance.

Finally, one simply cannot manipulate all of the variables involved in the equation of motion simultaneously. So for example, if the tidal volume and inspiratory flow rate are set, The airway pressure that's generated is gonna depend completely on the elastance and resistance of the patient's respiratory system.

Now there are several other important pressures that we need to consider and these include number one the trans airway pressure. The trans airway pressure really is the pressure gradient between the airway or mouth opening and the alveoli. And this essentially represents the pressure caused by resistance to gas flow in the airways. The second pressure to consider is the transthoracic pressure.

This is really just the alveolar pressure or alveolar distending pressure minus the body surface pressure. And you might ask, what's the body surface pressure? Well that's zero or equal to atmospheric pressure. unless of course the patient's going for a dive in the hyperbaric oxygen chamber or receiving negative pressure ventilation via an iron lung, which is in fact a much more natural way of breathing.

Similar to the next pressure, transpulmonary pressure, transthoracic pressure represents the compliance of the system. The transpulmonary pressure is the difference between alveolar and pleural pressures. Transpulmonary pressure is what maintains alveolar inflation.

during spontaneous normal negative pressure ventilation at end inspiration and is normally a positive value, usually about plus four to five centimeters of water, which obviously is important to prevent the lung from completely collapsing on itself. Finally, the trans respiratory pressure is composed of two components. One is the trans-airway pressure. And as we said a couple minutes ago, that's the pressure required to overcome airflow resistance.

and transthoracic pressure, which is the pressure required to overcome elastins.

In addition to pressure, the other two primary characteristics of the lungs

are resistance and compliance, and both these variables are constants. In fact the product of these is known as the time constant. And this is a concept that we'll go into further in a future episode. Suffice it to say that it's an important variable to consider Especially when we're placing a patient on pressure control ventilation or when we're attempting to optimize inspiratory and expiratory times to ensure both adequate ventilation and avoid air trapping, respectively.

So what is resistance? Resistance or frictional forces is Primarily occurs result of the anatomic structure of conducting airways and the tissue resistance of both the lungs together with adjacent tissues and organs. Now many things can increase airway resistance and in an inhibated patient that includes the ET tube, but patients can also develop mucus plugs and have bronchospasm, just to name a few causes.

Abdominal compartment syndrome's another great example where in the right clinical context the presence of progressive renal and cardiovascular dysfunction combined with elevated peak pressures in a patient receiving volume control ventilation, might indicate to us that we should get a bladder pressure. But I digress and that's a topic for another round. Compliance is the volume change per unit of pressure. It's really just a measure of the stiffness.

of the respiratory system and as we said earlier is the inverse of elastens or one over elastence. And in the absence of an esophageal balloon, it's not really possible to, at least at the bedside, distinguish differences and compliance between the lung and the contribution from the chest wall.

And this is something to bear in mind as the pressures that need to be generated to develop a specific tidal volume depend not only on lung stiffness, but also stiffness of the chest wall and abdomen. Now compliance is measured in a couple of ways and you'll oftentimes hear people talking about either dynamic or static compliance. And dynamic compliance is a bit of a misnomer.

I say this because it's not truly a measure of compliance. Rather, it reflects the pressure generated to overcome the resistance to airflow as well as the elastic recoil of the lungs. Dynamic compliance is equal to the title volume. divided by the peak or inspiratory pressure minus PEEP, or the positive end expiratory pressure.

Static compliance, on the other hand, is a much better measure of compliance because we measure compliance in the absence of airflow resistive forces or when airway pressure is static or zero. And so the question becomes, well, how do you do that?

This is accomplished by measuring the plateau pressure, which in addition to the peak pressure and mean airway pressure, comprise the three variables that we can both monitor and manipulate in order to identify and improve patient ventilator interactions and gas exchange. Static compliance is measured by performing an inspiratory hold or pause. So a set tidal volume is delivered to the patient and then held with a zero flow rate.

And graphically, this is displayed on the airway pressure versus time curve or graph as a decrease in the pressure waveform from the peak insertory pressure to a plateau pressure. And the plateau pressure is obviously one of those key measures that we follow in patients with ARDS. And our goal in these patients is to keep the plateau pressure less than 30 centimeters of water in order to avoid ventilator induced lung injury in the form of volue or barrel and or bio trauma.

So what if we deliver an 8cc per kg title volume to a 60 kilogram male with ARDS? And at the end of the delivery of the 480 C C's, we perform an inspiratory pause and the plateau pressure comes back as 35 centimeters of water. What's the first thing we should do?

Uh, if you answer dial down the title volume, then you're absolutely correct. And this is of course in addition to the other targets we want to be meeting, including judicious or liberal use of PEEP, permissive hypercapnia, and my favorite, restricted Fluids.

Now while we continue on the topic of pressures, it's probably makes a lot of sense to discuss two more important pressure than those are the peak airway or inspiratory pressure. And this is the highest pressure measured during inspiration, during volume control ventilation, And this is a mode where volume is our primary control variable. The peak pressure is gonna vary based on the underlying respiratory system compliance and resistance.

The key point here is that the peak inspiratory pressure doesn't always reflect the alveolar pressure or compliance because of the resistive component that contributes to the overall peak inspiratory pressure. Finally, in a pressure support or control mode, the peak inspiratory pressure represents the sum of the set peep and set pressure.

The mean airway pressure is a super important concept and it's really the average pressure over the entire respiratory cycle and includes PEEP or the positive end expiratory pressure. PEEP is obviously a critical variable, and increasing PEEP will increase the overall mean airway pressure by preventing alveolar collapse or atelectasis.

and through the recruitment of FRC. FRC or functional residual capacity, you may recall, is composed of the sum of the residual volume together with the expiratory reserve volume. Now several variables affect the map or mean airway pressure in addition to PEEP, including how long someone spends in inspiration, also known as the I to E ratio, inspiration to expiration ratio. the peak inspiratory pressure, size of tidal volume, as well as the flow rate and pattern.

Oftentimes when patients become hypoxemic on positive pressure ventilation, the knee-jerk reaction I've noticed is to just crank up or dial up the FIO2, which isn't necessarily wrong. But I do think that increasing the mean airway pressure is nice as higher concentrations of oxygen can be biologically toxic.

With that said, one important point to bear in mind is that the mean airway pressure correlates with the hemodynamic effects of positive pressure ventilation. So one can see this in patients who are, for example, placed on a high frequency oscillatory ventilation where in general we target higher maps than patients would be receiving on conventional mechanical ventilation.

Not surprisingly, as has been demonstrated in a few previous RCTs, oscillated patients generally require more vasopressor support, probably due to the hemodynamic effects. of such high mean airway pressures. So just to summarize before we go into the phases of uh mechanical ventilator breath The respiratory equation of motion is a great way of thinking about the variables that are involved in delivering or generating a positive pressure breath on the ventilator in order to ventilate patients.

And again, the pressures that the ventilator must overcome are the sum of both the resistive pressure to airflow and the pressure to overcome the elastic properties of the lung.

So that now brings us to our third objective, which is to describe the four phases of a mechanical ventilator breath. And these four phases are number one, trigger, number two, the inspiratory phase. Number three, cycling, and number four, the expiratory phase. So let's talk about each one of these individually. Regarding triggers, the trigger really describes the changeover from expiration to inspiration.

And the trigger may be either the patient or it may be the ventilator. It really depends if patients are spontaneously breathing or not. If patients aren't spontaneously breathing, then the trigger essentially is going to be based on time. So if we set a rest rate of 12, that's a breath every five seconds. If the patient's paralyzed, not breathing on the ventilator, they get a breath every five seconds.

The two most commonly used triggers, however, are pressure and flow. So with pressure triggering, the ventilator requires the patient to generate a small negative inspiratory pressure in the ventilator circuit. This is typically somewhere in the range of one to three centimeters of water, but we can change this pressure if we need to. Once that negative pressure is sensed by the ventilator, the machine changes over to inspiration from expiration to deliver the next breath.

With flow triggering, this requires the patient to generate a change that's detected by the ventilator, and ventilators are very smart. They're continuously monitoring both upstream and downstream flow. And when the patient begins to take a breath, the downstream flow decreases relative to the upstream flow, resulting in the start or triggering of a breath.

Now compared to pressure triggering, flow triggering may actually be a little bit more comfortable or at least require less work. Typically the trigger is set at somewhere between 1.5 to 3 liters per minute. And I think that's really the one thing to bear in mind that when it comes to triggers and patience triggering the vent, the flow trigger typically requires a little less work.

Now regarding the inspiratory phase, this is the phase of the ventilatory cycle that begins with the initiation of the breath and ends when the ventilator cycles into expiration. This is also known as the limit variable, which means to restrict the magnitude of a specific variable. Uh a limit variable is the one that can reach and maintain a preset level before inspiration ends, but does not end the inspiration.

I think this specific variable or phase of the breath creates a lot of confusion among clinicians because the limit variable and cycle variable oftentimes get kind of confused in our minds. To cycle means to end inspiration. A cycle variable always ends inspiration, whereas a limit variable doesn't terminate inspiration. It only sets an upper boundary for something like pressure, volume, or flow. The third variable is cycling.

And this is the parameter that ends inspiration and initiates the changeover from inspiration to expiration. Cycling really defines how the ventilator recognizes that the inspiratory phase is over. And again, this is something that may be cycled as a result of the flow rate. decreasing in patients who are on, let's say, pressure support ventilation or in someone who's on pressure control ventilation, maybe the inspiratory time has been reached. and now the ventilator cycles into expiration.

The fourth and final phase is the expiratory phase, and this is what's happening during expiration. There's not really a lot that's going on during this period. I mean, we have some peep applied. And we can certainly determine the length or duration of the expiratory phase by manipulating the I to E ratio. In patients who have significant air trapping, for example, or auto peep.

These are patients where we want to prolong the expiratory time. And sometimes we partner that up with increasing the inspiratory flow rates to decrease the eye time. So try to get breaths in really quickly. and then give them a prolonged expiratory phase so that they can return to baseline and blow off the CO2.

So before we end, I do want to quickly go over how to describe or how we think about modes of ventilation. And what do we mean by a mode? Well, quite simply a mode is a particular pattern of spontaneous and mandatory breaths delivered by the ventilator. And in my mind, I always have three key factors that I wanna think about when I'm trying to determine the mode or describe a mode of ventilation. Those factors are number one, the type of breath.

Number two, the targeted control variable. And number three, timing of breath delivery. So when it comes to the type of breath delivery, there's really three types of breaths. They're either mandatory, and these are breaths for which the ventilator controls the timing or tidal volume and or both. We have spontaneous breaths in which the patient controls the timing and title volume, or assisted breaths.

And these have the characteristics of both mandatory and spontaneous breaths. In an assisted breath, all or part of the breath is generated by the ventilator. In terms of the targeted control variable Again, when we think about the respiratory equation of motion, the key variables there are pressure, volume, and flow that we can independently manipulate when we place patients on the ventilator.

uh by far the two most common target control variables are going to be volume and pressure, so volume control ventilation or pressure control ventilation. In the next episode, we'll do a deeper dive into what differentiates volume from pressure control. And we'll also go into some of the advantages and disadvantages. of these two target control variables. One thing I will say is that volume control ventilation overall is conceptually quite easy to grasp and understand.

Pressure control ventilation, I personally like that mode of ventilation, both from the standpoint of a patient comfort and and synchrony standpoint, but also the conceptual framework that goes into thinking about it, because again, this does bring up the respiratory equation of motion, as well as the concept of time constants. So more on that another time. The third factor to consider when we're discussing a mode of ventilation is the timing of breath delivery.

And in my mind when I think about the timing of breath delivery, there's really three general techniques. If you look at the front of the ventilator screen and look at all the acronyms when you go into the different modes. There's really three common or basic breath delivery techniques, and they are continuous mandatory ventilation or CMV.

synchronized intermittent mandatory ventilation or SIMB and spontaneous modes. So in the next episode as we get into the different types or modes of ventilation, we'll talk a little bit more about this. Well that's gonna end our rounds for today. I wanna thank you for joining me. I'm your host, Dr. Dennis Kim. Please do visit the website at traumaicurounds.ca for more information and materials.

And also please do visit us on iTunes and subscribe. And I'd appreciate you sharing the show with your friends and colleagues. In the meantime, stay safe. Talk.