Remember at the end of the day all ventilators are just dumb machines that do one thing in a very precise and customizable fashion:

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Rebecca Bruce
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1 Welcome to the exciting world of mechanical ventilators! This is generally the R&R module with the steepest learning curve, depending on your previous experience and comfort with these machines. Ventilators are wonderful inventions capable of supporting respiration in sick patients *almost* as well as our natural breathing process. Modern units also have features which provide much greater support and safety than manual methods of artificial ventilation (e.g. bag-mask ventilation.) Still, at first glance vents can appear overwhelming, and certain models with their screechy alarms can be enough to put anyone off. Further complicating matters, ventilator manufacturers use non-standardized terminology to describe the usual variables and modes, and they are constantly producing new and improved models and selling proprietary software upgrades for existing devices. The consequences: it is rare for the rural physician to encounter more than one unit in the exact same configuration; and the device in front of you probably has subtle differences compared what the RT or ICU doc whom you are consulting, is used to. These are not insurmountable problems, but they can certainly complicate things for the new or less experienced user. Familiarity and regular review & practice are the antidote. Remember at the end of the day all ventilators are just dumb machines that do one thing in a very precise and customizable fashion: That s it. Ventilators blow gas into a tube. We can control what composition of gas they blow. We can control how often, what pressures & volumes are used to inflate, and over what timing & pattern a breath is delivered. We can control residual pressure between breaths and so on. Don t let yourself get overwhelmed or frustrated. Take it one step at a time and learn the fundamental concepts. If you have any questions, the group because chances are someone else is going to be having the same question! If you work at it, it won t take long to master this knowledge, and once you understand how these machines work it will be much easier to make them do what you want. (Not to mention you will be able to provide far superior respiratory support for your patient as compared to human powered bag-mask ventilation!) Enjoy! JW 1 of 20

2 1. Review of Basic Physiology Remember this graph from med school? (Y-axis is lung volume and each letter denotes a different proportion of lung volume during respiration.) Fill in the blanks below. A = TV (Tidal Volume) B = C = D = E = F = G = H = TLC (Total Lung Capacity) How can you estimate TV with respect to ideal body weight? (ml/kg) What is anatomic dead space and how large is this volume (ml/kg)? Define the concept of Minute Ventilation (MV), and what is its formula? 2 of 20

3 Define the following terms and provide normal values: (Which of these can you measure clinically and which can you estimate based on their relationships with each other / rough calculations?) FiO2? FiCO2? PAO2? PaO2? PaCO2? PACO2? ETCO2 ETO2 Variable Description Useful formula? Measurable in ED? FiO2 (%) FiCO2 (%) PAO2 (mmhg) PaO2 (mmhg) PaCO2 (mmhg) PACO2 (mmhg) ETCO2 (mmhg) ETO2 (%) What is the relationship between PaO2 and SpO2? (Can you remember and draw the oxyhemoglobin saturation curve?) How does FiO2 change when you are working in Masset (Sea Level) vs in a King Air with a cabin pressure of 8,000 feet? (Hint: there is an equation.) 3 of 20

4 2. Oxygenation versus Ventilation It s so convenient to imagine oxygenation and the process of CO2 elimination (ventilation) as a 1:1 coupled process. Ah, if only life were so simple In the real world these are of course two very disparate processes which merely happen to share the lungs out of convenience(?) Understanding how and when one process can run amok while the other is completely stable is particularly important in order to master mechanical ventilation. Provide conceptual examples of how Oxygenation can be be decoupled from Ventilation. (Bonus points if you can link your example to a resuscitation type situation.) What is the danger of Hyperventilation? Hypoventilation? (Note we are talking about ventilation, not oxygenation) If you had to pick one state for your patient, which would it be? Simplifying it all now, which variable introduced in Section 1, has the most profound effect on OXYGENATION? (What is the first dial to reach for if you wish to adjust the patient s SpO2?) Similarly, which variable introduced in Section 1, has the most profound effect on VENTILATION? This last concept is so important, please forgive a second question on the same theme: What single variable change could you make to have the most potent effect if: 1. a the SpO2 was low? 2. the ETCO2 was high? 4 of 20

5 3. Glossary of Ventilator-Related Terminology Unfortunately ventilators employ a crazy alphabet soup of terms. What s worse is there is no standardization: each manufacturer seems to use a slightly different term when referring to each of the fundamental concepts. As such you must be confident to interpret non-standard terms on the fly. Tip - UNITS and DEFAULT VALUES help in decoding many otherwise confusing ventilator labels! If all else fails, read the manual. (Manuals more often than not can be downloaded free on the web.) Ready? Define each of the following bullet items (with respect to mechanical ventilator settings): (Modes) - CMV - A/C (or AC) - IMV - SIMV - PSV (or PS) - BiPAP - CPAP (Tidal Volume Determination) - VCV - PCV (Pressures) - Plimit (cm H2O) - PIP (cm H2O) - Pplateau (cm H2O) - Pinspiration (cm H2O) - PEEP (cm H2O)(Volumes) (Volumes) - MV (L/min) - TV (ml) (Timing) - Tinspiration (sec) - Texpiration (sec) - I:E ratio [n:n] (Frequency) - frequency ( /min) (Trigger Limit) - trigger limit (L/min) (Phase of Respiratory Cycle) - FI [label] - ET [label] 5 of 20

6 Alphabet Soup! Match each of the following redundant terms to the corresponding concepts already listed above: Ipap (cm H2O) Ppeak (cm H2O) Vpeak (ml) Pressure Control (cm H2O) VTE (ml) High Pressure Limit (cm H2O) Epap (cm H2O) sensitivity (L/min) TE (sec) Inspiration Time (sec) VE (ml) Pmax (cm H2O) VT (ml) Pressure Support (cm H2O) f Psupport (cm H2O) Feeling overwhelmed?! It s natural. Just relax. Remember each ventilator will have its own set of terms. Once you figure out the idiosyncrasies of the machine (e.g. f is the same as Respiratory Rate), it will all come together, provided you understand the fundamental concepts. For this reason: TIP - I highly recommend getting a test lung (e.g. a rubber anesthetic bag will do), sitting down in an empty room with the new-to-you-ventilator, connecting it up and fiddling with the setting to make sure you can adjust all the fundamental things you may need to adjust. 6 of 20

7 4. Ventilator Settings (Note: this section is mostly instructional with fewer questions.) When you first turn a ventilator on, it usually loads with standard variable settings; OR it offers you a choice of several pre-sets (e.g. adult vs peds), then loads standard settings accordingly. Still it is important to review the settings prior to connecting the patient. If you have time, you would do well to review & adjust the settings prior to intubating the patient. Most machines can then be put into standby (and will not alarm but retain your settings) until you are ready to connect. In order of importance: Variable #1 - Respiratory Rate: This is pretty intuitive! What RR do you choose to start? Next: A philosophical decision is required Before proceeding to variable #2 (TV) we must decide how the ventilator will guide its delivery of the breaths. The choices are: (a) push gas until you achieve a certain VOLUME or (b) push gas until a certain airway PRESSURE. These two variables are intrinsically linked. (Think about this relationship until it makes sense to you.) If you choose a volume target (VCV), then the pressures become the slave. You set the vent to deliver say 500mL and it will adjust the Pinsp from breath to breath to achieve that target (unless the pressure demanded triggers an alarm like Plimit.) On the other hand if you choose pressure target (PCV), then the TV (tidal volume) becomes the slave. The vent delivers a constant Pinsp (say 10 cmh2o) and the TV will vary slightly from breath to breath. VCV generally gives you a more consistent MV. (Your RR is fixed, your TV is fixed and the MV becomes a simple and fixed equation! - your PaCO2 management becomes more stable and predictable.) Downside: If the patient condition changes or if the patient fights (bucks the vent) your associated system pressure (PIP) will go up or down. Low airway pressures aren t a concern (provided the patient remains connected to the vent), but high airway pressures put the patient at risk of barotrauma. (Plimit will protect you if set at a reasonable level however you may not deliver the full volume set.) On the other hand PCV will stabilize your PIP and you don t have to worry about highpressure alarms! However your TV may become unreliable (especially if the patient is fighting) and the risk here is hypoventilation or hyperventilation. Similarly, if suddenly there is a drop in resistance (e.g. atmospheric pressure drops as your jet takes off), your set pressure will deliver larger volumes due to decreasing resistance. 7 of 20

8 You risk volutrauma. patient.) (Again, appropriate alarms and limits should protect your TIP - Personally I would recommend for R&R purposes going with a VCV option rather than PCV. Academic discussions aside, it is a bit less finicky and easier to manage if you don t tend to use ventilators on a regular basis! By the way, switching from VCV to PCV is easy! Just be sure to set the current dependent value in the current mode as the target value in the next. For example say you are using PCV at 12 cmh2o and subsequently getting ~ 450mL TV on average (your dependent variable in PCV). You are happy with your SpO2 & ETCO2 in this configuration and so to switch to VCV, set 450mL as your target. You will see that the ventilator titrates itself over a few breaths and will settle with a PIP of roughly 12 cm H2O 1. NB: Most ventilators are smart enough to auto-set the new value based on the current conditions - all you have to do is confirm the new setting. Variable #2 - TV size. So you decided on a VCV mode (whatever your device happens to call it.) volume target do you specify? What Variable #3 - How should the ventilator handle a patient s spontaneous breath effort? Options include: (a) do nothing & ignore it! (CMV or AC) or (b) work around their breath ± assist them. (e.g. Synchronized Intermittent Mechanical Ventilation - SIMV or Pressure Support.) These options may exist in both VCV and PCV forms depending on your machine. Sometimes it is a separate mode e.g. SIMV vs VCV, other times a sync option is available under a PCV menu. (e.g. PCV without sync is a PCV A/C equivalent. PCV with sync is an SIMV equivalent.) Don t get hung up on the specific titles, just be aware of the physiologic differences in what the ventilator is doing. And ideally go learn the specific terms (and options) for the ventilator in your facility before you need it for a patient! TIP - Inconsistency across units is another reason to go play with your vent and understand what features it has and how to access them. Do this before you get a call for a sick patient! Logically one might think it is better to assist the patient - this is not necessarily true! You must look at why you are intubating / ventilating in the first place. e.g. Pooped out asthmatic who was getting narcotized due to CO2 retention? They re too weak and only 1 Typically PCV will achieve similar TVs at slightly lower PIPs than VCV. This is one advantage of PCV over VCV but it comes at the cost of being more finicky. If you are needing high pressures to ventilate your patient consider calling ICU or an RT for advice. 8 of 20

9 interfering with proper gas exchange. e.g. Protection of Head / ICH? You ideally need to target specific physiologic (PaCO2) targets, so giving random extra breaths based on what the?ischemic brainstem thinks it wants is not going to be very precise. At the same time remember you have the patient intubated and on a ventilator! You have the knowledge, the abilities and the drugs Why not just anesthetize and paralyze them? (Just be sure you do it in that order!) TIP - a paralyzed / deeply sedated patient is much easier to manage on a vent than a light & agitated patient. It is also widely accepted that paralyzed & sedated is far safer during transport in a confined ambulance / aircraft where the last thing you want is accidental self-extubation. (Your transport team will thank you.) For this reason, I recommend (all else being equal) to sedate and paralyze your patient. - We ll discuss the specifics more shortly. Variable #3B - If you choose to support respirations, you also get to decide whether you want the vent to simply support their breath efforts only (Pressure Support - PS), or whether you want mandatory (set) underlying breaths if they are apneic for too long. PS is useful when you are protecting the airway for an otherwise healthy person. Say you intubate a drug overdose unable to protect their airway but they are breathing on their own. Assume the SpO2 is acceptable, and ETCO2 is 45 (reasonable, stable and safe.) One option would be put them on PS and let them determine their own rate. You re waiting for their own drugs to wear off and once they start bucking you can weigh the pros & cons of extubation (Admittedly this is a bit of an odd example; generally we let overdoses sleep it off in ¾ prone if necessary rather than tubing. Intubation / extubation is associated with risk of aspiration as well as other complications, but this management could be useful in certain circumstances.) PS requires setting the Pinsp variable. Sometimes in this context it Pinsp is called Pressure Support (cmh2o) - fairly intuitive if not standard. A PS of 10 is a reasonable starting point, but can go as high as you want really. A PS of 5 cm H2O is felt to be equal to the extra pressure necessary to overcome resistance added by the ETT and circuit etc. (It takes more effort to breathe when intubated and through a vent circuit than without the tubing.) Generally if breathing spontaneously on a ventilator a PS of 5cm H2O is always added. Variable #4 - PEEP Where Pinspiration represents the added pressure during inspiration, PEEP is added expiratory pressure. We ll talk more about the importance of PEEP shortly. What is the baseline (physiologic) setting for PEEP generally? 9 of 20

10 Variable #5 - FiO2 Modern ventilators will have an FiO2 setting. This allows you to change the ratio of FiO2 from (sometimes) as low as 0.21 to 1.0 (or 21% to 100%) What FiO2 should you set initially? During prolonged ventilation? During extubation? Other Settings: Trigger - aka Sensitivity - set high enough to avoid accidental / cyclic auto triggering, but not too high that the patient is unable to trigger. 2-4 (L/min) is fairly standard defaults. I:E - or Ti depending on the device. Generally, an i.e. Ratio of 1:1.5 or 1:2 is default. Expiration should be given more time than inspiration. (Asthmatics who are actively retaining gas should have much higher expiration times in the order of 1:4 or more; sometimes as high as 1:7 or beyond.) we ll get to that later too. Pmax - This is a safety setting; the vent will not deliver more than this pressure. Very useful for VCV - the vent will not push too hard and pop the lung in order to achieve the 1500mL you accidentally programmed for the TV! (It will alarm instead.) Default Pmax is generally cmh2o. These are conservative limits (which isn t a bad thing!) CPAP - same concept as PEEP. Technically it is continuous throughout the cycle, but in terms of clinical application it achieves the same things as PEEP. 10 of 20

5. Review of Intermediate Physiology What s the difference between Negative Pressure Ventilation (NPV) and Positive Pressure Ventilation (PPV)? What are the physiologic consequences of PPV?

11 5. Review of Intermediate Physiology What s the difference between Negative Pressure Ventilation (NPV) and Positive Pressure Ventilation (PPV)? What are the physiologic consequences of PPV? (Specifically, what normal processes can it interfere with?) TIP - it is better to attempt to correct extreme shock prior to intubation, all else being equal. Fancier ventilators provide graphic waveform displays (in addition to numeric values). While many do not have this level of detail (particularly the cobwebbed ventilators you typically find in rural centres), it is still valuable to understand the relationships between four of the more important tracings and as displayed below: Pressure, Volume, Flow & ETCO2. Interpret the labels / values on the following anaesthetic machine ventilator display. if you can interpret this, you ll be able to interpret anything! 11 of 20

12 R&R Rounds - Session #3 - Overcoming Ventilatorophobia (Hint, Enf. stands for enflurane, an inhalational anaesthetic. - ignore those values as well as the the N2O references!) What is Barotrauma? How can it be avoided on a ventilator (specific numbers)? How can it be avoided in BMV? What is Volutrauma? How can it be avoided on a ventilator (specific numbers)? How can it be avoided in BMV? Why is PEEP useful? What is the downside to adding PEEP? 12 of 20

13 6. Monitoring & Alarms What two patient monitors provide the best feedback as to the physiologic adequacy of the ventilator? Most ventilator / monitor combos offer several fundamental alarms. These alarms can unfortunately be frightening to the unprepared and can even induce panic. A solid understanding of all the concepts in this module, along with a basic approach to solving each potential problem will best protect the patient. Briefly describe the meaning / consequence of each alarm type and propose a simple Differential for trouble shooting: SpO2 (too low) ETCO2 (too low) ETCO2 (too high) MV or TV (too low) MV or TV (too high) PIP (too low) PIP (too high) Oxygen source (low pressure) PEEP (too high) 13 of 20

14 7. More Problems while on the Ventilator TIP - Remember - if all else fails (the alarms are screaming the patient is desaturating, you can t figure out how to make the darn machine do what you want), you can always disconnect the vent and switch to a BVM with 100% O2 ± a PEEP valve to stabilize. A ventilator is just a dumb machine, but a complicated dumb machine; so many things can (and will) go wrong from time to time! It is beyond the scope of this module to discuss how to solve every last problem one might encounter with a patient on a ventilator, but let s review the most common. For each of the following situations, describe reasonable steps to assess / resolve each problem: Patient is bucking on the vent You assess a high PIP alarm and realize the chest is wheezy: Bronchospasm! The chest is getting larger and larger. The High Pressure alarm is going crazy. Air Trapping aka Breath stacking You have been ventilating away with good MV and FiO2. Your ETCO2 & SpO2 are on target, your PIP is nice and low then gradually there is a bit of a desaturation. 99% down to 95% over 10 minutes but it just keeps going down. The patient is relaxed (not bucking) what is going on, air-entry is equal on both sides? ATELECTASIS You have a gradual desaturation but there is decreased air entry on one side. Your trusty POCUS scan shows no significant lung sliding on the right chest. Pneumothorax?! Whoa, not so fast Tex! Perhaps you just have the tube too far down and wedged in the right main bronchus? Accidental extubation. (Focus on how to prevent this from occurring.) Patient Awareness This isn t a ventilator problem per se, but it was certainly a source of anxiety for me when paralyzing a patient prior to my GPA training If a patient is paralyzed (unable to move / communicate), how do you know if they are adequately sedated and not aware and suffering? Like this: Here are two patients, both intubated, on ventilators and paralyzed. Receiving Morph/ Midaz infusions at 7mg/hr (of each). Which one is at risk of awareness and which one is not? Patient #1 - lying peacefully with regular ventilator waveforms. V/S: 90/ (vent) 99% (FiO2 0.4) Patient #2 - lying peacefully with regular ventilator waveforms. 14 of 20

15 V/S: 195/ (vent) 99% (FiO2 0.4) On closer inspection you would likely see tears in the corners of the eyes of Patient #2 (a sign of severe sympathetic response to pain / discomfort.) If you see this what do you do? The flip side of the coin: many doctors worry about hypotension while their patients are ventilated & sedated. Profound hypotension is concerning of course, but you must keep in mind that a sedated / paralyzed patient (at rest & pain free and without awareness or sympathetic drive), is going to naturally be a bit hypotensive in comparison to the rest of the population. TIP - Target your sedation to achieve a MAP of ~ mmhg or so. No lower but no advantage to higher either. Always look for other signs of sympathetic drive: elevated HR, tearing, etc. A Mean Arterial Pressure of 65 or so will ensure sufficient cerebral and coronary flow. It will also be a good and reassuring sign that a paralyzed patient is in fact quite comfortable. HR too is a valuable indicator but don t be fooled by a low HR restricted by beta-blockade! 15 of 20

16 8. Weaning / T-piece A good question to start this section with, is why on earth would you need to wean a patient? If you re already tubing and ventilating, this patient is sick and is probably going to ICU! Nevertheless the concepts here are important and you never know, this information may come in handy one day. In order to wean a patient from a vent you need to ensure that they are able to adequately breathe on their own. Disconnecting the vent and pulling the tube could lead to some serious problems if they aren t ready! How would you go about assessing a patient s ability to support their own ventilation prior to disconnecting from a ventilator? (Mode & settings?) What is a T-piece? How is it useful, how does it work? 16 of 20

17 9. CPAP vs BiPAP Define IPPV vs NIPPV. What is the difference between CPAP machines vs BiPAP machines? What is benefit / limitation of BiPAP? Initial settings for BiPAP? 17 of 20

18 10.Quiz Time! Check out the following 4 columns, each representing a different mode of ventilation (albeit some are very similar.) Can you determine which one (or more) is PCV and which one (or more) is VCV? Can you identify in each column where inspiration starts? Where expiration starts? where expiration ends(subtly different from inspiration starting)? 18 of 20

19 11. Vent adjustments based on pathophysiology: Ventilators can be adjusted to assist in compensating for various pathophysiologic states. Consider each condition and describe whether any VENT changes should be made, and if so what vent settings changes would be beneficial for the condition: (assume all other medical treatments / optimizations have been made.) 1. Bronchospasm 2. Pulmonary Edema 3. Pneumothorax (decompressed with chest tube in place) 4. Obesity 5. Obstructive Sleep Apnea 6. Non-respiratory Sepsis 7. Pneumosepsis 19 of 20

20 12.Pneumothoracies, Decompression & Chest Tubes What are the signs of a Tension Pneumothorax? What is your immediate action? How is ultrasound useful for the detection of a pneumothorax? (i.e. non tension PTX!) What is the procedure for inserting a chest tube? 20 of 20

Basics of Mechanical Ventilation. Dr Shrikanth Srinivasan MD,DNB,FNB,EDIC Consultant, Critical Care Medicine Medanta, The Medicity

Basics of Mechanical Ventilation Dr Shrikanth Srinivasan MD,DNB,FNB,EDIC Consultant, Critical Care Medicine Medanta, The Medicity Overview of topics 1. Goals 2. Settings 3. Modes 4. Advantages and disadvantages