Tag Archives: Mechanical Ventilation

Assessing Mechanical Ventilation

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One of the most intimidating things about entering the ICU for the first time is the “life support machine” – the mechanical ventilator. Although I have posted an extensive series of tutorials on Mechanical Ventilation, covering most of the modes, oxygen therapy and applied respiratory physiology, I have attempted, in this tutorial, to distill everything to the “least you have to know” in 40 minutes. Keep in mind that modern machines look more like iPhones, and are far easier to use than the devices I grew up with that looked to me, on day 1, like something in the cartoon below.

I start with a discussion about the difference between normal breathing, CPAP and Positive Pressure Ventilation (PPV). PEEP is, effectively, CPAP during PPV. I then go on to discuss pressure limited modes of ventilation; worldwide this are the most widely used modes in ICU. I limit my discussion to Pressure Assist Control, Volume Guaranteed Pressure Control (VG_PC) and Pressure Support Ventilation (PSV). VG-PC is a popular and flexible option as an ICU’s default mode. However, as it is a pressure controlled mode, there is significant variability in tidal volume and airway pressure from minute to minute.

Several important rules are emphasized: the tidal volume should, in general be lower than 6ml/kg of ideal body weight, the plateau pressure lower than 30cmH2O and the driving pressure lower than 15cmH2O. I introduce the Spontaneous Breathing Index (SBI = RR/TV in L). The magic number is 100. We use the SBI to determine the success of weaning on PSV.

Volume Controlled Ventilation is the predominant mode use in the Operating Rooms (Theatres), and Volume Assist Control is a popular mode in North America. In ICU you must set a peak inspiratory flow and be aware that this may be insufficient during assisted breaths and lead to dys-synchrony. Volume Control is often used in ARDS to “lock in” the Tidal Volume (TV) but the operator must be aware that the TV that matters is not what is dialed up on the ventilator, but what the patient exhales.

I go on to discuss how to assess the patient on invasive mechanical ventilation, by looking at whether they are breathing spontaneously, in which case we determine whether they are suitable for a Pressure Support wean or not, or whether or not there is a problem with oxygenation (increase FiO2, PEEP, Mean Airway Pressure and seriously consider Prone Positioning) or Ventilation (increase Respiratory Rate, Tidal Volume or both, reduce PEEP).

The final part of the tutorial looks at Non Invasive Ventilation (NIV), and I explain how, in general we only use 2 modes on standalone devices – CPAP and Spontaneous Timed (S/T). The latter is similar to PSV with a backup rate, but I point out that instead of PEEP+PS the breath is EPAP + IPAP and IPAP is not built upon IPAP, as is the case with PSV. If one is delivering NIV on an ICU ventilator, then “leak” adjustment or “leak sync” should be used.

@ccmtutorials

50 Tutorials Uploaded! Now – Help the Patient’s Airway Pressures Are High!

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The alarm goes off like an air raid siren – everybody starts to panic – somebody starts to do the saturation countdown. There is nothing quite as distressing for the anesthesiologist or intensivist than for the ventilator to pressure cycle and fail to deliver tidal volumes due to high airway pressure.

Generally high pressures are caused by one of three things – a problem with the equipment (kinked tubing, patient biting the tubing etc.), an airway resistance problem (e.g. bronchospasm) or a pulmonary compliance problem (e.g. consolidation or pulmonary edema) or a combination of these. The first thing that the clinician should do when there pressure alarm goes off – is to silence the alarm and increase the Pmax.

Then go looking for the problem: start at the mouth and work your way back to the machine. If you can’t find a fault, put the patient on a manual breathing circuit and commence ventilation. If the patient is easy to bag, there is a machine problem, if difficult – then there is a problem with pulmonary resistance or compliance. In this first tutorial I look at assessing airway resistance. I do this in two ways. First I discuss peak to plateau pressure gradients and then look at airway resistance: dynamic versus static and how to calculate it. I will finish the discussion in the next tutorial.

Flow Volume Loops in Anesthesia and Critical Care

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There is a feature on the display of you ICU ventilator or anesthetic machine that you likely pay little attention to – the flow volume loop. Indeed, you may ignore the flow-time waveform also. This is a pity – and you are missing out on tons of information about your patient.

This tutorial commences with a description of the flow waveform (no previous knowledge required!) and the different waveforms that you are likely to encounter – sinusoidal, constant flow, decelerating flow and “shaved-off” decelerating flow (associated with pressure support).

I then show you a series of flow volume loops and – yes you can pause the video and see if you can figure out what is going on with the patients.

If you find this and my other videos useful – do me a favor and subscribe: subscriber only content coming soon.

 @ccmtutorials  http://www.ccmtutorials.org

Volume Support – The Forgotten Mode?

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This tutorial is about Volume Guaranteed Pressure Support – known generally as Volume Support (VS). This, I believe, is an underused mode of ventilation in most ICUs – who prefer to use pressure support. Essentially you specify the desired tidal volume and the ventilator alters to pressure support from breath to breath to deliver something akin to that volume. There is little precision, but – as pressure support is biologically variable anyway – the presence of a volume averaged set of tidal breaths is reassuring, particularly if the bedside practitioner is distracted or inexperienced.
In the tutorial I explain how to set up volume support, what it looks like on three different ventilators – Puritan Bennett, Drager Evita and Servo-i and the strengths and limitations.

Breathe Easy – Automatic Tube Compensation

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This tutorial is about Automatic Tube Compensation (ATC). ATC is a setting that has been included in most modern ventilators. Its aim is to reduce the work of breathing associated with the drop in pressure across the endotracheal tube. The ventilator senses pressure, flow and resistance and changes the pressure during the breath to ensure that the patient has the sensation that they are breathing through their own airway. There are two configurations of ATC – one is as an alternative to pressure support in patients who are essentially weaned from mechanical ventilation: essentially a spontaneous breathing trial. The second configuration is as an accessory to all pressure limited modes – such that the pressure waveform is crafted in inspiration and expiration to reduce the workload of breathing during both phases of respiration. @ccmtutorials http://www.ccmtutorials.org

SIMV-Pressure Control

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This weeks tutorial is on SIMV-Pressure Control. Although this is one of the lesser used modes of ventilation, I sometimes see my colleagues using it in the operating room. And for good reason. Anesthesia ventilators are not set up in the same way as ICU vents. In particular – if you choose “PC” Pressure Control – that is what you get – pressure control; NOT pressure assist control. Hence there is no real provision for patient ventilator interaction. If you choose “SIMV” as pressure control, volume control or volume guaranteed pressure control, then the patient can breath and interact with the ventilator and receive pressure supported breaths. Consequently, conventional SIMV modes, these days, are far more likely to be used in the operating room than in the ICU.

The second reason that I wanted to cover SIMV Pressure Control is to set the groundwork for a different mode “BiLevel Pressure Control” that is built on a similar platform, looks a bit like SIMV, and has significant benefits for those of you who might choose SIMV-PC in ICU.

Most modes of ventilation offer two ways of supporting the spontaneous breath – assist control and SIMV. In SIMV-PC the spontaneous breath can be unsupported, pressure supported or partially supported using Automatic Tube Compensation (ATC). This tutorial covers the type of patient to whom you might deliver SIMV-PC; how to set up the mode; what it looks like on a ventilator screen and the strengths and weaknesses of the mode. @ccmtutorials http://www.ccmtutorials.org

Pressure Controlled Ventilation – The Fundamentals Part 1

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It is time to discuss Pressure Controlled Ventilation. In general if a patient has normal lungs or minimal disease, it really does not matter what mode of ventilation you use, pressure or volume controlled. However, there are some major advantages to using Pressure Control – principally in Acute Hypoxic Respiratory Failure. There are also many disadvantages. This is the first of two tutorials that cover the fundamentals of Pressure Control. I start with a discussion of the terminology that I will be using – the Pressure Limit (PL), the Inspiratory Pressure (IP), the Driving Pressure (DP)/Inspiratory Ramp, the Inspiratory Time (Ti) and the Expiratory Time (Texp). Pressure Controlled Ventilation (PCV) is pressure targeted/limited and volume variable. Breaths are time cycled – in inspiration, expiration or both. The flow pattern is always decelerating.

Following the introduction of a clinical scenario – a patient who is developing ARDS, I describe the process of PCV. I explain that tidal volumes are variable in all settings and all modes of PCV and later describe how changing patient position, chest wall elastance and airway resistance can all impact the tidal volume. I discuss why pressure control is the best option for mechanically ventilating children (particularly where there is no endotracheal tube cuff and a significant air leak) and why you need to pay attention to the rise time and respiratory rate. Finally I discuss the major disadvantages of using PCV. I guarantee you’ll learn something!
@ccmtutorials

Mechanical Ventilation – Setting Up a Ventilator – Flow Patterns

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Most bedside practitioners pay little attention to ventilator waveforms – usually just the tidal volume and, occasionally, the pressure waveform. However, mechanical ventilation is all about flow – if there is no flow there is no breath. In this tutorial I will look at flow patterns in patients attached to a ventilator. Patients who breathe spontaneously, without assistance, draw flow from the ventilator, the positive flow in inspiration is hemispheric in appearance, exhalation is a v shape – reflecting elastic recoil. Volume controlled ventilation may be delivered by either constant or decelerating flow, with or without an inspiratory hold (also known as a pause). The flow pattern in pressure control is always decelerating – as airway pressure rises, flow falls. Tidal volumes are variable in pressure control, as the negative pressure deflection during inspiration increases the inspiratory ramp and and hence the tidal volume.

I guarantee you will learn something from this tutorial and will never look at a ventilator the same way again.