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

Posted on June 21, 2023 by Pat Neligan

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

Oxygen Therapy: Variable vs Fixed Performance Devices

Posted on May 17, 2023 by Pat Neligan

Oxygen is probably the most used and misused drug in a hospital. The purpose of oxygen therapy is to restore the PaO2 or SpO2 to a safe level for that patient. One of the major issues with targeted oxygen therapy is the problem of peak inspiratory flow.

During peak inspiration the FiO2 must be constant. That means that flow delivery must meet flow demand. Oxygen therapy can be delivered with variable or fixed performance devices. Variable performance devices include nasal cannula and simple (“Hudson”) facemasks. In both cases oxygen and air are blended in or near the airway. Nasal cannula are remarkably efficient and can deliver low inspired oxygen concentrations. Due to issues with dead space and rebreathing, simple facemasks are unreliable below 35% (5L). Both devices struggle where there is rapid breathing, particularly with large tidal volumes.

Venturi devices, which are really jets use a narrow injection port to entrain and blend oxygen and air proximal to the facemask. They are more precise but less efficient (in terms of total flow) than variable performance devices. Performance is remarkably robust between 24% and 40% inspired oxygen. They perform less well with rapid deep breathing particularly at high FiO2 levels. Non rebreather facemasks use a reservoir to store fresh gas during expiration and facilitate the delivery of FiO2 of approximately 80% with 10 to 15 liters of flow. As such they are highly efficient, although unreliable and non titratable. These devices can be used with modest oxygen flows for transporting hypoxic patients, but are short term remedies. @ccmtutorials http://www.ccmtutorials.org

Tutorial 14 Mechanisms of Hypoxemia Part 2

Posted on May 10, 2023 by Pat Neligan

This tutorial explains ventilation perfusion mismatch. It will provide you with a platform for understanding oxygen therapy – which I introduce towards the end. I also deal with the concept of oxygen induced hypercarbia. I guarantee you will learn something.

Contents of This Tutorials:

Ventilation-Perfusion Relationships

Gravity and Blood and Gas Distribution Through the Lungs

Gas and Blood Distribution Through Diseased Lungs

Simplistic Ventilation-Perfusion From Dead Space to Shunt

Stale Gas Within Alveoli

Ventilation Perfusion Relationships – Slimy, Soggy and Stick Alveolar Units

Supplemental Oxygen Therapy For Bronchopneumonia

“Targeted Oxygen Therapy”

When Does Oxygen Therapy Fail? [Shunt]

COPD Flair

Why Does Hyperoxia Cause Hypercarbia (VQ mismatch theory)

The Haldane Effect

Mechanisms of Hypoxemia – Part 1

Posted on May 3, 2023 by Pat Neligan

If you treat patient with hypoxic respiratory failure you really need to understand what is going on in their lungs. These two tutorials look at diseases of the lung parenchyma and how blood flow and gas flow interact. The first tutorial focuses on alveolar oxygen content and how it is impacted by disease. I explain the concept of airway closure (which will will revisit in detail several times during this series), stale alveolar gas, the various causes of atelectasis and the six S approach to figuring out what is going on in the airways (Slimy, Soggy, Sticky, Stiff, Squished, Shunty).

Identifying and Quantifying Hypoxemia

Posted on April 26, 2023 by Pat Neligan

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

Posted on March 29, 2023 by Pat Neligan

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.

Tutorial 7: Understanding Ventilatory Failure, Alveolar Gas, Lung Volumes and Dead Space.

Posted on March 15, 2023 by Pat Neligan

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.

Don’t Be Scared of Respiratory Physiology – it makes sense (well, most of it anyway!)

Mechanical Ventilation Tutorial 3 TRIGGERING

Posted on February 15, 2023 by Pat Neligan

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.

Mechanical Ventilation – Setting Up a Ventilator – Flow Patterns

Posted on February 8, 2023 by Pat Neligan

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.

Mechanical Ventilation – Control

Posted on February 1, 2023 by Pat Neligan

As promised – here is the first tutorial from Module 1 (“Setting Up a Mechanical Ventilator”) of the course on Mechanical Ventilation. I discuss the difference between Volume Control and Pressure Control and Dual Control – including the advantages and disadvantages associated with each mode.

Module 1 Tutorial 1 of the Mechanical Ventilation Course

New Tutorial Every Wednesday,

ORtoICU

Tutorials in Anesthesiology & Intensive Care

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