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Bilevel Pressure Control, BiLevel, BiVent, BiPAP, DuoPAP – a modern mode of ventilation

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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

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.

Volume Guaranteed Pressure Control (Pressure Regulated Volume Control)

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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

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 Assist Control (Part 1)

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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

Pressure Controlled Ventilation – Fundamentals Part 2: Mean Airway Pressure

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In the previous tutorial I introduced some of the fundamental elements of pressure control ventilation – time cycling, decelerating flow, pressure ramps etc. This time I discuss, in detail, the concept of mean airway pressure (Pmaw) and describe why increasing Pmaw is an effective way of treating patients with extensive lung disease. In volume controlled ventilation this can be achieved by titrating PEEP upwards and increasing respiratory rate. Care must be taken to keep the plateau pressure below 30cmH2O in the majority of patients. In pressure control Pmaw is generally increased by increasing inspiratory time – extreme care must be taken, though, to avoid escalating Auto-PEEP as this corrodes tidal volume and actually reduces Pmaw.

If auto-PEEP is unavoidable, as it is with inverse ratio ventilation, then extrinsic PEEP should be reduced to ensure that tidal ventilation is maintained. Pmaw can be achieved in volume control by adding an inspiratory pause, and in pressure control by increasing respiratory rate – but these are less effective approaches – in volume control because of necessary flow limitation and in pressure control because of fixed inspiratory times, and Auto-PEEP.
I guarantee you’ll learn something.

@ccmtutorials

Positive End Expiratory Pressure – Phasic Shunting, Auto-PEEP & ARDS

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In the previous tutorials I explained how hypoxemia results from low lung volumes, resulting in low functional residual capacity, airway closure and atelectasis. We looked at the mechanisms by which CPAP reduces the work of breathing in obstructed airways and how, following lung recruitment, PEEP maintains FRC.

In this tutorial I elaborate on these themes. I look at the problem of phasic V/Q mismatch (shunt) during expiration and how it may cause dis-correlation between pulse oximeters and blood gasses. PEEP prevents this at the expense of increasing dead space and negatively impacting ventilation. Optimal PEEP should restore lung compliance – compliance is low with low and high lung volumes. Compliance may also appear poor in pressure control when there is clinically significant auto-PEEP: the ventilator cannot distinguish auto-PEEP from driving pressure and lower than expected tidal volumes may result.

I explain the concept of the “Waterfall” effect to overcome Auto-PEEP. Finally, in our first visit to ARDS, I introduce the problem of deciding on optimal PEEP in that setting. I guarantee that you will learn something. @ccmtutorials http://www.ccmtutorials.org

@ccmtutorials

Identifying and Quantifying Hypoxemia

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The next part of the course is all about hypoxic respiratory failure. To treat hypoxemia you must understand it. The purpose of this sequence of tutorials is to lead up to discussions on CPAP and PEEP and provide a platform for understanding Pressure Controlled Modes of Ventilation. The first tutorial looks at oxyhemoglobin saturation, why the oxyhemoglobin dissociation curve is essential knowledge for the practicing clinician, how pulse oximeters work and how to quantify hypoxemia (A-aO2 gradient and PaO2/FiO2 ratio).

Pressure Support Ventilation – Part 1

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If you go into most ICUs today, the most commonly used mode of ventilation is Pressure Support. There are many reasons for this: it is widely believed that supporting spontaneous breathing results in less muscular – and in particular diaphragmatic – atrophy; patients require minimum sedation and can be gradually weaned and, because it is a pressure targeted mode, there is biologically variable ventilation. Although not every ICU uses Pressure Support as part of its invasive ventilation strategy, virtually all units use it for non invasive ventilation. If you work in ICU you MUST understand Pressure Support. In my view it is the MOST important mode of ventilation. It is also the easiest mode to get started with and one of the most difficult to master.

These are four tutorials on Pressure Support Ventilation – starting with Triggering, then Breath Initiation, then Setting the Level and, finally, Expiration. The first tutorial introduces the concept of Assisted Spontaneous Breathing and Pressure Support and revisits Triggering – Flow and Pressure Triggering. Although I covered this in the introductory tutorials, I go into much greater detail here. In particular I cover Undertriggering and Overtriggering. I guarantee you will learn something.