Is there anything more frustrating in the ICU when you decide to start weaning a patient – they look like they’re assisting the ventilator. You switch them over to a “spontaneous” mode and then……nothing…..no breaths….eventually the backup starts.
This tutorial is about triggering of mechanical ventilation. I will revisit how patients trigger the ventilator, the different systems used and introduce I-Sync – a new method of triggering.
Finally I will discuss the problem of Auto-PEEP and explain why, in the setting of Auto-PEEP, there is no point fiddling with the flow by or negative pressure.
Expiratory dysynchrony is a major unrecognized problem in critical care. Usually it takes one of two forms: a terminal upstroke on the pressure waveform, indicating pressure cycling (breath too long) or a W shaped anomaly in the expiratory flow waveform – indicative of the breath being too short or too long. I call this the “Wibbly Wobbly Waveform”.
This tutorial looks at expiratory dysynchrony – why it happens and how to make adjustments to resolve the problem. I also introduce a relatively new technology: IE Sync.
The patient is turning purple in the bed, alarms are going off, he is desaturating: he is “fighting the ventilator.” Although a widely used description I believe that it is misused to redefine the problem away from an issue of ventilator operator competency and reframe it as a patient problem. It is not. Most of the time that patient have negative interactions with the ventilator it is a problem of triggering, flow or expiratory cycling. The treatment is not deep sedation and controlled ventilation. The treatment requires skill and nuance, and does not always work. This tutorial looks at inspiration and reasons why it may go wrong.
The most frequently seen patient ventilator dysynchrony is scooping of the pressure waveform, usually associated with flow limited volume controlled ventilation. This can be resolved by increasing the peak flow or changing to pressure control.
In general the ambition to establish a patient on spontaneous assisted ventilation is laudable, but oftentimes we have no idea about what is going on underneath the pressure, flow and volume waveforms. In this tutorial I try and correct the narrative about patient-ventilator interaction when using pressure support. I suggest that volume support in some situations may be a superior approach. I point out that the tidal volume in pressure support has little to do with patient effort and more to do with lung compliance.
I finish the tutorial with a discussion about the inspiratory rise time and explain why you must be careful when using older ventilators.
In the previous tutorial we looked at the problem of high airway pressures and addressed inspiratory airway resistance in two ways: peak to plateau pressure gradient and dynamic and static inspiratory resistance.
In this tutorial we will look at three more ways of assessing airflow resistance: the identification and measurement of Auto-PEEP, Flow-Volume Loops and capnography.
Subsequently I discuss high airway pressure due to low total respiratory system compliance. I explain that when “compliance” is low – this may be a problem with the lungs as well as the chest wall – including the abdomen. I finish with the introduction into this course of Abdominal Compartment Syndrome.
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.
This tutorial looks at the pressure waveform in patients undergoing anesthesia or mechanically ventilated in ICU. All modern ventilators will provide a pressure time waveform and display volume pressure (often called “pressure volume” loops).
This tutorial commences with a discussion about pressure-flow loops – to demonstrate the relationship between flow and airway pressure. I then discuss and describe normal airway pressure versus time waveforms.
Subsequently I explore normal and abnormal dynamic volume pressure loops. I briefly discuss static VP-curves and why they are important in ARDS. Finally I demonstrate how you can measure real plateau pressure and static compliance by pushing one button and performing an inspiratory hold.
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.
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For the majority of patients admitted to ICU with hypoxic respiratory failure, a conventional ventilatory strategy using volume, pressure or dual control modes with PEEP is usually very effective. With severe lung injury it may be necessary to administer neuromuscular blockade, turn the patient prone and increase the mean airway pressure using PEEP or inverse ratio ventilation (IRV). If these interventions are unavailable, ineffective or inadequate, rescue therapies may be required.
One easily available rescue therapy is Airway Pressure Release Ventilation (APRV). APRV is an extreme version of IRV that looks analogous to using CPAP at high airway pressure levels (e.g. 28cmH2O). Intermittently that high airway pressure is released to remove CO¬ – the release time (less than 1 second) being too short to cause lung derecruitment. Using modern ventilators it is possible utilize the inspiratory capacity to oxygenate the patient (flipping the respiratory cycle from expiration as the primary time of gas exchange to inspiration) and allow the patient to breath spontaneously.
The spontaneous efforts have been shown to improve both gas exchange and cardiovascular performance – but they are not necessary when using this ventilator strategy. Gasping should be avoided. This tutorial covers the science behind APRV, how to set it up, how to use it as part of a ventilator strategy in ARDS, the strengths and limitations of this approach and how to wean it.
The introduction of the active expiratory valve was a disruptive technology in critical care mechanical ventilation. This valve flutters when the airway pressure rises above the targeted level – to vent off surplus gas, but maintain airway pressure. It led to the development of newer modes of ventilation (and adjustments to older modes) that allowed the patient to breathe spontaneously independent of the ventilator. As such this was a development of intermittent mandatory ventilation (IMV) – without the risk of breath stacking and expiratory dys-synchrony.
The major mode of ventilation that evolved from the active expiratory valve has several different aliases – BiLevel, BIPAP, BIVENT, DuoPAP etc. but they are all, essentially, pressure controlled intermittent mandatory ventilation modes – that allow the patient to breathe supported or unsupported at a high (Phigh) or low (Plow) airway pressure.
I have chosen the term “Bilevel Pressure Control (BL-PC)” to describe this mode. This tutorial introduces BL-PC, from the perspective of IMV, explains the technology and then discusses the setup and use of the mode. It is a mode of ventilation that is used widely as the “default mode” in many ICUs and can be used in any patient at any time. @ccmtutorials http://www.ccmtutorials.org
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.
