Author Archives: Pat Neligan

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About Pat Neligan

Pat Neligan lives and works in Galway, Ireland

The Blood Gas Machine – Measuring Oxygen, pH, Carbon Dioxide, Tips and Tricks and Derived Variables

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To round out the year, here are three tutorials on the blood gas machine, blood gas analysis and the blood gas printout.

The first tutorial looks at how oxygen is measured using the Clark Electrode on the blood gas analyser and demonstrates the importance of co-oximetry in modern blood analysis. From that the fractional saturation of hemoglobin with oxygen is derived.

The second tutorial explains the Glass Electrode that measures pH and PCO2. Subsequently I cover problems you might encounter with blood gas sampling. If you don’t want to watch the technical stuff, I strongly recommend you scroll to the middle of the tutorial (12 minutes in) as it covers information that all healthcare practitioners must know.

The final tutorial looks at all of that other data that appears on blood gas printouts that you may never have understood – and it can be really confusing – DERIVED or calculated variables (bicarbonate, temperature correction, TCO2, O2 content, Base Excess, Standard Bicarbonate, Anion Gap etc.). I cover both the Radiometer ABL machines and the GEM 5000. I guarantee you’ll learn something.

TUTORIALS ON CAPNOMETRY AND CAPNOGRAPHY

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Here are three tutorials on inspiratory and expiratory CO2 gas analysis. Tutorial 1 looks at Capnometry and the process behind measuring CO2 in exhaled gas. I cover mainstream CO2 analysis and explain why the end tidal CO2 (EtCO2) may be high or low. Tutorial 2 addresses the Capnograph, the trace and anomalies of the Capnograph at the time of intubation. I also explain Sidestream and Microstream CO2 and gas analysis. The final tutorial will be very helpful to anesthesiologists, particularly those taking exams: I go through a series of abnormal Capnographs, explaining why they are abnormal. I guarantee that you will learn something.

Tutorials on Pulse Oximetry

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There are two tutorials on pulse oximetry. The first looks at the SpO2 and how it is measured. The second looks at the pleth waveform and problems that we commonly encounter with pulse oximetry in general. I guarantee you’ll learn something.

Acid Base Calculations for the ASA

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Here are the calculations that I used in my presentation at ASA 2025.

pH versus  PaCO2 

An increase of 10 mmHg in PaCO2 results in a pH drop of about 0.08

Respiratory Acidosis PaCO2  vs HCO3–

In Acute Respiratory Acidosis (e.g. patient hypoventilating in the OR) the Bicarbonate Increases by 1mmol/L for every 10mmHg Increase in PaCO2

In Chronic Hypercarbia (COPD) the Bicarbonate ↑ by 4mmol/L for every 10mmHg Increase in PaCO2 and Cl– falls by an equivalent amount

Modern Anion Gap = [Na+ + K+] – [Cl– + HCO3– + La–+ βOH–] = Albumin– + PO42- + UMA– mEq/L

[Albumin−]= [Albumin g/L] × (0.123 × pH−0.631)

Albumin Charge is simplified to 2.5 (albumin in g/dl) = Alb in mEq/L

Change in Albumin is (44 – Albumin) / 4

To determine the effectiveness of respiratory compensation use the Winters formula:

(Bicarbonate Version) Expected PaCO2 in Acute Metabolic Acidosis is
1.5 x [HCO3–] + 8 (in mmHg)

(Base Deficit Version) Expected PaCO2 in Acute Metabolic Acidosis is
Normal PaCO2 – BD (in mmHg)

The Base Excess Gap (Fencl Story with my modification)

Identify the BE on the ABG

Calculate the SID for Na+-Cl–+H2O by [Na+ – Cl– – 35] BDNaCl

Calculate the SID for La– and β-OH– (1mmol = 1mEq) BDLβOH

Calculate the Impact of Albumin (44 – Alb– g/L)/4 BEALB

Add these together BENaCl BDLβOH BEALB

Subtract from BE on the Blood Gas

The result is UMA in mEq/L

Finally, the Strong Ion Gap (arguably the gold standard)

The calculation for the strong ion gap (SIG) is:

Strong Ion Gap (SIG) = SIDa-SIDe

SIDa (apparent SID) = ([Na+] + [K+] + [Mg2+] + [Ca2+]) – ([Cl–] + [Lactate–] + [βOH–])

SIDe (effective SID) = [HCO3–] + [charge on albumin] + [charge on Pi]

The degree of ionization for weak acids is pH dependent, so one must calculate for this:

[charge on albumin] = [albumin] (in g/L) x (0.123 x pH – 0.631)

[charge on Pi] = [Pi] (in mg/dL) /10 x pH – 0.47

The SIG quantifies UMA

Carbon Dioxide in Acid Base – Three Tutorials

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As part of my fundamentals of Anesthesiology and Critical Care Series I have posted 3 tutorials on the Role of CO2 /HCO3 in Acid Base Balance. These are entirely new tutorials (not part of the previous acid base series – that I have not finished yet! There is some overlap and updated facts and figures) and I have put a lot of work into getting the message of why the respiratory system is so important in acid base. Tutorial 1 is the basics of acid base. Tutorial 2 discusses respiratory acidosis, acute and chronic, and respiratory alkalosis. Tutorial 3 discusses respiratory compensation for acute metabolic acidosis.
Although I cover the respiratory component in great depth, I also explain what metabolic acidosis is, what causes it and briefly discuss the anion gap, expected bicarbonate, base deficit and base deficit gap. I guarantee that you will learn something.

Upcoming Activity

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The next 3 tutorials will be on the impact of Carbon Dioxide in Acid Base. This is different from the tutorials in the Acid Base series – and is complementary – but you don’t have to review that series to follow the tutorials. The tutorials follow directly on from those in the Vapors and Gases series.

I will be speaking at the ASA in San Antonio – on Stewart Acid Base on Oct 11th at 4pm. My updated chapter on that topic in the new edition of Miller should be available shortly. Drop by and say hello if you are in Texas.

I will be instructing on the JFICMI Mechanical Ventilation course in Galway on October 24th.

I will also be co-hosting this year’s Hi-Impact Critical Care Conference 6.0 Sepsis in Galway, at the Clayton Hotel, on November 7th.

I am organizing next year’s Western Anaesthesia Symposium at the Lake House Glasson, near Athlone, Ireland, on April 17th and 18th. It will be an awsome – fully interactive highly clinical meeting in a great location.

I am the co-convenor of the College of Anaesthesiologists Annual Congress, and organizer of the ICSI (Intensive Care Society of Ireland) ASM (at the same conference) on May 21st and 22nd 2026. This will be a really useful meeting for Anaesthesiologists and Intensive Care Specialists, and we (the CAI Council) hope you will be able to attend.

I am delighted to receive feedback on the tutorials and videos that I have posted, so if you come across me at any of those conferences – please tap me on the shoulder and we will have a chat over coffee, beer or wine.

Finally, the Eagle eyed of you will note that I am posting videos on @or2icu and @ccmtutorials. The or2icu channel will be used for rough cuts of videos (to get them out earlier – but there may be some mistakes), and random thoughts and (sometimes unedited) short videos that I will not be posting on the main channel. I will use it for audio only podcasts. On occasion I will post clips from other long videos that I think are worth watching. So, it is worth subscribing to both channels.

Fundamentals of Anesthesiology and Critical Care Series

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Here are the first 9 Tutorials in the Series – the majority are useful for Anesthesiologists and Intensive Care practitioners. Every tutorial contains something that you may not have previously known: I guarantee, who ever you are, that you’ll learn something.

Tutorial 1: Saturated Vapor Pressure

Tutorial 2: The Gas Laws

Tutorial 3: Mixtures of Gases

Tutorial 4: The Alveolar Gas Equation

Tutorial 5: Henry’s Law

Tutorial 6: Carbon Dioxide Solubility

Tutorial 7: Oxygen Solubility

Tutorial 8: Oxygen Content of Blood

Tutorial 9: Oxyhemoglobin Dissociation

New Series – Fundamental of Anesthesiology – Gases and Vapors

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This is a new series on the Fundamentals of Anesthesiology – the first course is on Gases and Vapors. It should serve as a good introduction to the topic for early stage residents in Anesthesiology – but is also applicable in critical care and emergency medicine and nursing. For experienced practitioners it will be a straightforward refresher course – but I guarantee you’ll learn something.

First up I discuss the forgotten gas – water vapor – and why it is really important in our practice. The main concept that you must learn is the Saturated Vapor Pressure.