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

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!)

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 5 Now Available: Volume Control Ventilation – featuring Volume Assist Control

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.

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

Mechanical Ventilation Tutorial 3 TRIGGERING

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

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

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,

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

Fluids In Hospital Medicine (Part 1)

Intravenous fluid, fluid management, the physiology of body fluids – all relentlessly controversial and complicated issues. I decided a couple of years ago to put together a course that covers the whole spectrum of fluids – from basic chemistry to basic and advanced physiology, applied physiology, fluid and electrolyte disorders and therapy and acid base chemistry. I will also cover diseases and disorders associated with fluids – either as therapies for, or iatrogenic causes of, disease. I am posting Part 1 on the fluids course in its entirety. Subsequent parts of the course will be posted ad-hoc depending on when each tutorial is completed (I will set aside pages on this website for the tutorials in order and as playlists on you-tube). I hope you find this useful. All of the tutorials on this set (plus a number that I have not posted yet) were road tested as Galway University Hospital in 2021-2022.

Pat Neligan Dec 22nd 2022

Introduction to the Course

This is a quick introduction to the course, explaining what I am proposing to cover over four parts.

Preliminary Material

This is some really basic chemistry that will allow you to understand the content of subsequent tutorials.

Tutorial 1 Water and Concentrations

This tutorial convers the physical properties of water, what a mole and mmol is and what is g%. I use dextrose as my major example and look at the different ways that glucose concentration is measured in the USA (mg/dl) versus the rest of the world (mmol/L). The end of the tutorial covers the alcohol and calorie content of drinks and drink driving limits.


1 Supplement

I rather like caffeinated drinks and am frequently the subject of sanctimonious comments about my caffeine habit. This tutorial covers caffeine content. Subsequently I look at the issue of 1% versus 2% lidocaine and explain exactly what 1:200,000 epinephrine (adrenaline) is.

Tutorial 2 Salts

This tutorial explains how to calculate out the quantity of electrolytes released from salts as they are dissolved in intravenous fluids. I also take an early look at hypertonic saline solutions.

Tutorial 2 Supplement 1 – More Salt

This tutorial goes through a couple of conundrums where I look at intravenous fluid products and show you how to calculate out the electrolyte contents when you are only given the salts in g/L

Tutorial 2 Supplement 2

This is an early look at calcium supplement products that we typically use in critical care. What exactly is the difference between Calcium Chloride and Calcium Gluconate?

Tutorial 3 Osmosis

Fundamental to understanding how water behaves in body fluids is the concept of osmosis. It is also very important when we visit renal replacement therapies in Part 4 of the course. In this tutorial I use traumatic brain injury and mannitol as my main example.

Tutorial 4 Osmolality and Tonicity

What is the difference between osmolality and osmolarity? What are mOsm? How do you calculate Osmolarity? This tutorial looks at the concept of Osmolality and the Tonicity of intravenous fluids, and why understanding this concept is essential for practitioners of hospital medicine. The clinical scenario is of a patient with hypotonic hyponatremia. I will revisit hypertonic saline solutions and look at the concept of the Osmotic Co-efficient.

Tutorial 5 Electrolyte Distribution

This tutorial looks at the distribution of electrolytes in the body – between the intracellular and extracellular compartments. I look at the needs of a patient who is unable to take oral fluids and electrolytes. I emphasize the importance of maintenance fluids in this situation rather than resuscitation fluids. This tutorial also looks at the interstitial matrix and how it is vulnerable to hydraulic fracturing (“fracking”) caused by intravenous fluids.

This is the end of Module 1.


Tutorial 6 The Adaptive Perioperative Stress Response

Whether we are injured, assaulted or undergo surgery, our bodies respond with an inflammatory response that involves endocrine, metabolic and immune components. The “adaptive” stress response is predictable and its magnitude mirrors the degree of injury. To understand emergency and perioperative medicine and critical illness you must understand the stress response. Having explained the basic physiology, I then go on to discuss fluids and fluid balance and describe the conventional approach (that I do not necessarily subscribe to) to perioperative fluid therapy.

Tutorial 7 Critical Illness and Resuscitation

A patient presents with an “acute abdomen.” His bowel is obstructed and he is losing fluid and becoming both dehydrated and electrolyte depleted. This tutorial looks at the different types of body fluids that may be lost – how they all resemble extracellular fluid and suggests a type of fluid that can be used for resuscitation. I then progress to describing the maladaptive stress response of critical illness, and why it is associated with capillary leak syndrome. There follows a discussion of fluid overload and the need for de-resuscitation. Finally I introduce the topic of chronic critical illness and death.

Tutorial 8 The Macro Circulation

What happens to the body when there is major blood loss? This tutorial looks at the different components of the circulation and how blood flow is redistributed in shocked states. I also look at the assessment of hypovolemic shock, oxygen consumption versus delivery and the mixed venous oxygen saturation. Finally I address resuscitation strategies in acute blood loss.

This ends Part 1 Module 2.


Tutorial 9 Venous Return

Since the 1970s the venous (and lymphatic) side of the circulation and the right side of the heart seem to have been ignored by doctors. At worst is the widely held belief that central venous pressure represents an appropriate measure of blood volume and resuscitation status. This tutorial looks at the concept of cardiac output versus venous return. I discuss the Guyton concept of mean systemic pressure, the stressed and unstressed blood volume and vascular compliance. I then go on to look at venous return during anesthesia, the impact of low and high dose vasopressors and the impact of fluid overload.

Tutorial 10 The Microcirculation & Capillaries

For the past 125 years or so, the vast majority of clinicians have based their understanding about transendothelial fluid flux on the work of Ernest Starling. Problem is that his hypothesis – the Starling Principle – is wrong. The presence of the capillary glycocalyx and enhanced understanding of fluid kinetics has changed our view of fluid therapy, in particular the role of colloids in treating critically ill patients. This tutorial looks at the capillary network, the traditional Starling method, the “Revised” Starling method, the glycocalyx, oncotic pressure gradients, the impact of fluid extravascation and the lymphatic system.

Tutorial 11 Albumin & Colloids

Colloids, whether they are hydroxyethyl starches, dextrans, gelatins or even albumin, were popular resuscitation fluids until the 2010s. Multiple studies failed to demonstrate the effectiveness of these agents. However, the use of hyperoncotic human albumin solution has gained popularity, based on no real evidence, in recent years. Given our knowledge of the microcirculation, is there any compelling reason to be treating a patient with human albumin solution in the 2020s?

Tutorial 12 Fluid Kinetics

In this last tutorial in Part 1 of this course, we are returning to the operating room. What happens to intravenous fluid once it is injected into the veins a) in normal volunteers, b) during anesthesia, c) during the stress response? This tutorial is all about fluid or volume kinetics and is based on the work of Robert Hahn, from Sweden. I discuss fast versus slow boluses, resuscitation with crystalloid in hypovolemic states, the urinary output during surgery and what happens during hypervolemia.

If you have enjoyed this course, please subscribe on youtube and post lots of likes and positive comments. I will do my best to answer queries and comments below, time permitting.