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.
@ccmtutorials
Abu Dhabi 11/4/13
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.
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.
I decided to do a tutorial on end tidal CO2 as there has been a lot of discussion about it’s merits and limitations in our practice. It is fairly long and can be broken into sections at 20 minutes and 37 minutes if you have a short attention span (I will split it up into smaller segments at some stage in the future).
The content is absolutely essential for doctors and nurses working in anesthesiology and intensive care. In my opinion measuring expiratory CO2 from the ventilator circuit is the most useful clinical measurement tool that we have. It gives us information about cellular metabolic activity, blood flow, venous return, lung unit perfusion, gas exchange and alveolar ventilation. The tutorial commences with a discussion of CO2 as a gas and discusses Henry’s and Daltons’ laws. I then discuss the various different CO2 moieties, particularly bicarbonate. Subsequently I go on to discuss the impact of alveolar ventilation on PaCO2. After 20 minutes I move on to discuss capnometry – the measurement of the presence and quantity of CO2 emerging from the lung at end expiration. I discuss why the etCO2 may rise of fall. I then look at a specific clinical scenario where the etCO2 falls precipitously. After 37 minutes I discuss capnography – initially the normal capnograph and then a series of different capnography traces that you should be able to recognize. As a final thought I mention that CO2 is not the only waste produce or metabolic intermediary that we measure, routinely, in clinical practice.
Clinicians who work in anesthesiology, intensive care or emergency medicine who are involved in the management of respiratory failure must understand the problem of failure to ventilate: “can’t breathe, won’t breathe.” This long tutorial covers a lot of ground and could be viewed in split sessions.
My principle goal is to give you the tools to work the problem of respiratory failure. Along the way I introduce the alveolar gas equation, ventilation perfusion matching and lung volumes; particularly functional residual capacity. In the second half (from 28:20 onwards), I discuss anatomical and physiological dead space, calculate out the dead space to tidal volume ratio and show how you can be inadvertently increasing physiologic dead space by applying PEEP or neglecting auto-PEEP.
Even if you think you know a lot about this subject, I guarantee that you will learn something.
As always, I welcome feedback.
This is the second tutorial on Volume Controlled Ventilation. I discuss the evolution of ventilators from pure controlled mechanical ventilation, to intermittent mandatory ventilation – with spontaneous breathing to synchronized IMV with Pressure Support. This mode remains robustly popular around the world and critical care practitioners and anesthesiologists should be familiar with the mode, along with its advantages and disadvantages. I guarantee you will learn something. @ccmtutorials
I am now moving on to the “meat” of the mechanical ventilation course, starting with volume controlled ventilation. The first of these tutorials is on volume assist control. Even if you think you know a lot about this mode – stick with me, there is a lot of information packed in and I guarantee that you will learn something. Comments always welcome.
This is the last tutorial in the introductory module – “setting up a mechanical ventilator.” In this tutorial I will discuss how the ventilator cycles from inspiration to expiration. In controlled modes this is usually time cycling. However, traditionally volume cycling of volume control was used. On occasion the ventilator pressure cycles – and you must be aware of this as it may cause problems. Finally I will introduce the concept of flow cycling: it is imperative that you understand this process if you use pressure support ventilation. I guarantee you will learn something in this tutorial.
Next week we will be moving on to Volume Controlled Ventilation – specifically Volume Assist Control.
How does the ventilator know that it needs to deliver a breath? The term to describe this is “Triggering.” In this tutorial I will cover time triggering, pressure triggering and the relentlessly confusing concept of flow triggering. I guarantee that you will learn something in this 12.5 minute tutorial.
Most bedside practitioners pay little attention to ventilator waveforms – usually just the tidal volume and, occasionally, the pressure waveform. However, mechanical ventilation is all about flow – if there is no flow there is no breath. In this tutorial I will look at flow patterns in patients attached to a ventilator. Patients who breathe spontaneously, without assistance, draw flow from the ventilator, the positive flow in inspiration is hemispheric in appearance, exhalation is a v shape – reflecting elastic recoil. Volume controlled ventilation may be delivered by either constant or decelerating flow, with or without an inspiratory hold (also known as a pause). The flow pattern in pressure control is always decelerating – as airway pressure rises, flow falls. Tidal volumes are variable in pressure control, as the negative pressure deflection during inspiration increases the inspiratory ramp and and hence the tidal volume.
I guarantee you will learn something from this tutorial and will never look at a ventilator the same way again.
ORtoICU 