Tag Archives: psv

Weaning From Mechanical Ventilation (the basics)

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This tutorial is about weaning from mechanical ventilation. This is not an easy topic because every professional in the ICU has their own weaning method and their own opinions regarding how best to wean and liberate patients. The literature is unhelpful. Some patients, for example those who have been intubated for a brief period of time, can be awoken and the tube removed after a couple of spontaneous breaths. Other patients require careful multidisciplinary activity over weeks to months to liberate. This tutorial focuses on the in-between group patient who have been intubated for a week or so, who require both clinical and mechanical assessment of their ability to wean and liberate from the ventilator.

Generally the first intervention in weaning is to change the patient over to a spontaneous breathing mode – pressure support or volume support and ensure that alveolar ventilation is adequate to maintain CO2 clearance.

Then a number of clinical and mechanical assessments can be made: is the patient awake, do they have a cough, are they triggering adequately, what is their rapid shallow breathing index (RSBI)? One can estimate muscle strength by performing an occlusion test – either a partial occlusion (P0.1) or a longer occlusion (NIF). Once the patient is assessed as being a candidate for weaning, then one can perform a spontaneous breathing trial (SBT) that is either supported (PS, VS, ATC) or unsupported (T-piece, C-circuit, Trach mask, Swedish Nose).

If the SBT is successful after 90 minutes – extubate the patient. SBTs may fail due to worsening hypoxemia, hypercarbia or hypocarbia, respiratory distress (increase RSBI or use of accessory muscles) or cardiovascular instability (hypotension, hypertension, tachycardia, bradycardia, arrhythmias) or falling levels of consciousness, agitation or acute delirium.

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

Pressure Support 4 – Expiratory Cycling

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This is likely the most important of the four tutorials on Pressure Support Ventilation. As you may recall, PS is an unusual mode of ventilation because it is flow cycled – that is – the ventilator cycles to expiration as specific, user set, percentage of peak flow. The default expiratory sensitivity is usually around 25%. Expiratory dys-synchrony is frequently missed by bedside clinicians who have not been schooled in waveform analysis. This tutorial covers everything you need to know. @ccmtutorials http://www.ccmtutorials.org

Next time I am going to commence a series of tutorials on hypoxia-hypoxemia. This will start with a discussion about how we measure hypoxemia – in particular oxyhemoglobin saturation (Tutorial 12). I will then go on to discuss atelectasis, shunt, ventilation-perfusion mismatch and introduce oxygen therapy (Tutorial 13).

Inspiratory Rise Time

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When a pressure limited breath is triggered there is a slight delay between that point and the airway pressure target being reached. This is controlled by a setting on the ventilator known as the “inspiratory ramp” or “inspiratory rise time.”

Although I am covering this topic under the banner of “Pressure Support,” all pressure limited modes include this function, although it may be hidden from sight and each ventilator has a different system for adjustment. Most of the time you will get away with not having to adjust the rise time beyond the factory setting. Nevertheless – having an understanding of the inspiratory ramp is useful for fine tuning breaths in patients who have a tendency to be dys-synchronous. I guarantee you will learn something.

Japan April 5th 2023.

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.