Identifying and Quantifying Hypoxemia

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 4 – Expiratory Cycling

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

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).

Pressure Support 3 – Setting the Level

This is the third tutorial on Pressure Support Ventilation. This tutorial discusses the thoracic pump – the inspiratory drive and transpulmonary pressure. It looks at when one can use Pressure Support and how to adjust the settings. I introduce the Rapid Shallow Breathing Index and provide a series of steps in adjusting the PS level. Finally I introduce an alternative version of PS known as Volume Support.
Abu Dhabi 11/4/13

Inspiratory Rise Time

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.