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.
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.
This tutorial is about Volume Guaranteed Pressure Controlled (VG-PC) Ventilation – otherwise known as PC-VG, PRVC, VC+ etc. It is a modern mode of ventilation that aims to deliver a desired tidal volume (volume control) using the pressure controlled paradigm (unlimited flow). As such it is a mode that is often labelled “dual controlled” although, in some ways, it is neither volume controlled, pressure controlled nor both. Confused? Most are. I have labelled the mode VG-PC – because that is the best approximation – but volume is not necessarily guaranteed and it is certainly not limited. So why bother using this mode. Simply – it works! As a general use “unit default” mode of ventilation VG-PC has few peers: it is nimble enough to be used as the mode of ventilation of choice for patients admitted to ICU following intubation: postoperatively or with respiratory failure. If the lungs deteriorate – then the mode is versatile enough to deal with it. Being time cycled – mean airway pressure can be easily altered. If compliance or resistance of position changes – then the tidal volume “guarantee” changes the inspiratory pressure from breath to breath to ensure that things remain stable. If the patient breaths spontaneously, using the assist control or SIMV paradigm, flow is increased to meet patient demands. As such it is a very forgiving mode of ventilation, ideal for novices, reassuring to the ICU clinicians. This tutorial explains VG-PC, demonstrates how it is set up in three different ventilators – Puritan Bennetts, Dragers Evitas, Servo I and GE (Aisys) anesthetic machines. I explain the operation of this mode and its strengths and weaknesses. I guarantee you’ll learn something. @ccmtutorials http://www.ccmtutorials.com
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
This is the second tutorial on Pressure Assist Control (PAC).
The tutorial focuses on the use of PAC in acute hypoxic respiratory failure (AHRF or ARDS). In this tutorial I cover the use of Pressure Assist Control in Acute Hypoxic Respiratory Failure (AHRF or ARDS). My major objective is to ensure that you understand that cookbook approaches to ARDS using volume control cannot be ported over to pressure control – in particular the ever increasing use of PEEP.
During previous tutorials I explained that increasing mean airway pressure (Pmean) can be achieved more effectively with increased inspiratory time (Ti) than by increasing PEEP. Pmean can also be increased by increasing respiratory rate but you must be really careful with Auto-PEEP as that reduces tidal ventilation, Pmean and increases dead space. This tutorial starts with an explanation of the Pendulluft effect in hypoxemia and with increased airway resistance – basically prolonging inspiration results in better overall gas distribution. How one manages worsening hypoxemia and lung compliance is key to your skill as an operator of a mechanical ventilator.
Early in the course one tends to maintain driving pressure and inspiratory time in PAC while increasing PEEP. However, ultimately one runs into the 30cmH2O barrier. At that point one must adjust. An important adjustment is to stop the patient from breathing – or more likely – gasping. “Gasping” is a term that I will use in these tutorials to describe the patient generating massive transpulmonary pressures (and likely lung stretch) with minimal impact on ventilation. In fact, the increased work of breathing causes a deterioration in oxygenation due to lower mixed venous oxygen tensions consequent of increased oxygen consumption (not covered in this tutorial).
The second adjustment one must make is to increase the inspiratory time and reduce the PEEP – keeping in mind your tidal volume target. At this point respiratory rate must fall and Auto-PEEP controlled. The final part of the tutorial covers the major drawback of PAC ventilation – expiratory dys-synchrony: what happens when the patient wants to exhale during inspiration.
Virtually all “modern” modes of mechanical ventilation are built on a pressure controlled platform – the original of the species is Pressure Assist Control (PAC). This tutorial introduces PAC as it would be used on a patient admitted, for example, to ICU, with relatively normal lungs. The tutorial commences with a clinical scenario followed by a guide to the settings on both Puritan Bennett and Drager ventilators. At this point in the course I am going to start spending more time on Drager devices as these ventilators were built from the ground up to be used as pressure controlled machines. There are nuances to the Drager ventilator that may be slightly counter-intuitive to clinicians who are familiar to other brands: in particular the use of a pressure limit (Pinsp) rather than a driving pressure above PEEP. I explain this with examples. I then explain how pressure control works and remind you of flow and time triggering. All pressure controlled modes are time cycled with decelerating flow patterns. Care must be taken to ensure that inspiratory time is sufficiently long so as to ensure that the airway is adequately pressurized but not to long as will cause Auto-PEEP. If you want to understand mechanical ventilation you absolutely must be able to interpret and craft ventilator waveforms – and this tutorial focuses on identifying abnormal waveforms in pressure control and correcting them. Hence there is a section on “Crafting the Pressure Waveform” and a section on “Crafting the Flow Waveform.” Finally I discuss inspiratory time and tidal volumes
ORtoICU 