Get Your Pen and Paper Ready – Entrainment Ratios

This tutorial is best experience with a pencil and paper. Before I get into a discussion about high flow oxygen therapy you really need to understand flow. Conventional facemasks, Venturis and nasal cannula deliver modest flows of oxygen to the patient, but to ensure a correct FiO2, oxygen must be blended with air – in the airway or in the device. That air is drawn into the system by entrainment either from the room via the mask or mouth or injected in the case of Venturis into the breathing system. In any case – the inward flow of gas is determined, principally by the patient’s inspiratory effort and the concentration of oxygen during peak inspiratory flow is, hopefully, kept constant. In general, to keep FiO2 constant – a gas flow of at least 30L/min is required. Most devices deliver 40 liters or more, but only at lower FiO2 levels. It is essential to understand that, in this case, the 30 to 40L is NOT high flow – because it is “draw over” flow generated by the patient. High flow, as we will see in the next tutorial is delivered to the patient. For example – when delivering 24% and 28% oxygen to a patient – the total flow may be 44L but the fresh gas flow is only 2 to 4L. The remainder is entrained. This tutorial explains the concept of gas entrainment and how to calculate entrainment ratios and flow rates. If you have never encountered this concept before, I guarantee that you will learn something!

Equations Used In This Tutorial:

The FiO2 vs Flow Equation FiO2 = (Air Flow x 0.21) + (O2 Flow) / Total Flow

The Air:Oxygen Equation: Air/Oxygen = (100% -FiO2)/(FiO2 – 21%)

Oxygen Flow Equation: (Total Flow x (FiO2 -21))/79

High Flow Oxygen Therapy


I have been a “fanboy” for high flow oxygen therapy (HFOT) for a couple of decades, particularly once high flow nasal canula (HFNC) became available. While this was a bit of a cottage industry, coveted by those of us in critical care (and to a lesser extent in anesthesiology), once the COVID 19 pandemic took hold, high flow was everywhere. And everyone, it seemed, had an (ill informed) opinion about this therapy. So, before I introduce this tutorial, about which I procastinated for years, I have to register a disclaimer: the evidence to support a lot of the “beliefs” about high flow oxygen is scant. Most of the claimed “benefits” beyond treating hypoxemia are industry generated hypotheses without rigorous scientific data. Nevertheless, this put me in a difficult predicament when constructing the tutorial – if I limit the discussion to just the facts that I am certain about – it would be very short. Conversely, by describing alternative “benefits” I take the risk of hyping hypotheses (e.g. CO2 clearance) that may be incorrect…..

High Flow Oxygen Therapy (HFOT), particularly when delivered by nasal cannula (HFNC) has revolutionized the management of the patient with hypoxic respiratory failure – in particular in those patients whose lung pathology has plateaued or those resposive to medical treatment (antibiotics, steroids etc). High flow systems have been available for decades – they involve the use of a high pressure oxygen source, and oxygen air blender (air can be entrained into this device), a high flow flowmeter, a humidifier, a heated delivery tube and a delivery device: CPAP mask, T-Piece with PEEP valve, Tracheostomy or specially designed nasal cannula.

In this tutorial I describe the various devices configurations that are available – ranging from very straightforward standalone machines, to full mechanical ventilators. Regardless of device the major goal is to deliver sufficient flow to meet patient demand – resolving the problem of peak flow and separating out the FiO2 from the flow rate. I postulate that, at flows in excess of 30L per minute, and depending on the diameter of the nasal cannula, the patient’s anatomy and whether the mouth is open (and by how much!) – the patient likely receives a couple of cmH2O of pressure support and 3-5cmH2O of PEEP. So it represents mild CPAP (certainly a CPAP device delivering high flow at 5cmH2O will outperform HFNC). There is a dearth of non industry funded data on how HFOT may benefit the patient. Certainly these devices are very effective at targeting SpO2 and reducing the work of breathing. Certainly they increase non hypoxic apneic duration. Conversely – purported impacts on dead space washout, alveolar ventilation and CO2 clearance are currently unproven. I describe how this may work in the tutorial, but point out that this is principally a belief not a fact. HFNO may also improve mucociliary clearance – due to the high flow of humidified gas passing into the airways. However no-one, to my knowledge, has addressed whether constant flow of heated humidified gas for prolonged periods damages the lung mucosa.

In the second part of the tutorial I talk about how HFOT should be used in clinical practice and the scenarios in which it is beneficial (hypoxemia, weaning and liberation) and when it is not (hypercarbic respiratory failure, post op respiratory failure secondary to atelectasis).

I guarantee that you will learn something.


Oxygen Therapy: Variable vs Fixed Performance Devices

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

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

Pressure Support Ventilation – Part 1

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.

Everything You Need to Know About End Tidal CO2

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.

Tutorial 6: Synchronized Intermittent Mandatory Ventilation

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

Tutorial 4 – Cycling The Mechanical Ventilator – Available Now

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.

Plan For Upcoming Tutorials (Available Wednesdays – 09.00 GMT)

[These Dates are Subject to Change as the Course Progresses – I will try to keep this list updated].

March 1st 2023
Tutorial 5: Volume Assist Control Ventilation

March 8th 2023
Tutorial 6: Synchronized Intermittent Mandatory Ventilation

March 15th 2023
Tutorial 7: Understanding Ventilation & CO2 Clearance

March 22nd 2023
End Tidal CO2 – Everything You Need to Know

March 29th 2023
Tutorial 8: Pressure Support Part 1 – Triggering the Breath

April 5th 2023
Tutorial 9: Pressure Support Part 2: Controlling the Initial Flow (the rise time)

April 12th 2023
Tutorial 10: Pressure Support Part 3: Setting the Pressure Support Level

April 19th 2023
Tutorial 11: Pressure Support Part 4: Controlling Exhalation (the EXPsens)

April 26th 2023
Tutorial 12: Identifying and Quantifying Hypoxemia

May 3rd 2023
Tutorial 13: Mechanisms of Hypoxemia Part 1

May 10th 2023
Tutorial 14: Mechanisms of Hypoxemia Part 2, Introduction the Oxygen Therapy & Hyperoxia

May 17th 2023
Tutorial 15: Oxygen Therapy

May 24th 2023
Tutorial 16: Why Low Lung Volumes are Bad

May 31st 2023
Tutorial 17: CPAP and PEEP

June 7th 2023
Tutorial 18: Non Invasive Ventilation

June 14th 2023
Tutorial 19: Pressure Controlled Ventilation – Part 1 Assist Control

June 21st 2023
Tutorial 20: Pressure Controlled Ventilation – Part 2 Volume Guaranteed Pressure Control

June 28th 2023
Tutorial 21: Pressure Controlled Ventilation – Part 3 BiLevel Pressure Control and APRV

July 5th 2023
Tutorial 22: How To Read A Blood Gas – Part 1 – Carbon Dioxide

July 12th 2023
Tutorial 23: How To Read A Blood Gas – Part 2 – Metabolic Disorders

July 19th 2023
Tutorial 24: The Patient with High Airway Pressures – Part 1 – Airway Pressure Monitoring and Flow Volume Loops

July 26th 2023
Tutorial 25: The Patient with High Airway Pressures – Part 2 – Treating the Problem

August 2nd 2023
Tutorial 23: The Patient is Fighting the Ventilator – Part 1

August 9th 2023
Tutorial 24: The Patient is Fighting the Ventilator – Part 2

August 16th 2023
Tutorial 25: ARDS – Part 1 – Understanding the Disease

August 23rd 2023
Tutorial 26: ARDS – Part 2 – Treating the Patient with ARDS

August 30th 2023
Tutorial 27: Pulmonary Edema

September 6th 2023
Tutorial 28: Pulmonary Embolism

September 13th 2023
Tutorial 29: Ventilator Weaning and Liberation

September 20th 2023
Tutorial 30: Tracheostomy

Announcing Mechanical Ventilation Course

I have received quite a lot of feedback over the years regarding the original ccmtutorials website and what is apparent to me is that the Mechanical Ventilation Tutorials were unfailingly popular.
Fortunately I have delivered quite a few Mechanical Ventilation lectures and tutorials over the past few years. I have redone, reworked and rethought all of this material through and now I am announcing a new version of the Mechanical Ventilation Tutorials for the 2020s. The tutorials will start to stream this week.

The first part of the course is titled: “Setting Up A Mechanical Ventilator” and this contains four tutorials.
1. Ventilator Control: Volume Control and Pressure Control
2. Flow Patterns
3. Triggering
4. Cycling
Please follow these tutorials sequentially – you might think you know a lot about mechanical ventilation; you do not.
Here is the intro spiel to the first module of the course:
If you have ever sat into an unfamiliar car – a rental car or a new car for example – you need to take some time to figure out the controls. How does it start ? Manual or Automatic? Left or right sided drive? Where is the hand break – lever or button? The lights? The wipers? The de-mister? The radio? How you connect your phone etc. Before you ever put a patient onto a ventilator you need to understand how the machine works and how to set it up. This tutorial will look at the basics of setting up a ventilator – and this is essential material. Do not skip onto the next tutorial until you have learned this material.

(note I haven’t forgotten the fluid course – Hyponatremia and Hypernatremia to feature soon).

Pat Neligan Jan 2023