How to Survive a

GAS ATTACK!in 10 minutes…

A Talk by Chris Nickson

No $

Photo by Misserion

Overview

A-a GradientCompensation

Strong IonsAnion Gap

All aboutA-a gradients(not really…)

A-a gradient = PAO2 – PaO2

PAO2 = FiO2 (PB – PH20) – (PaCO2/R)

Quick check:expected PaO2 = %FiO2 x 5

Caveats:high PaCO2

A-a gradient varies with age and FiO2

Rule of thumb:expected A-a gradient =

(age/4) + 4

Be aCompensation

King (or Queen)

Respiratory compensation = rapid

Metabolic compensation = slow

Metabolic compensation

The 1-2-3-4-5 Rule

1

12

12

HCO3

1 42

HCO3

1 42 5

HCO3

1 42 5

HCO3

acute chronic

1 42 5

HCO3

acute chronic

10 mmHg

PaCO2

Respiratory compensation

PaCO2 = 1.5 x HCO3 + 8 (+/-2) if metabolic acidosis

PaCO2 = 0.7 x HCO3 + 20 (+/-5) if metabolic alkalosis

Rule of thumb:If pH = 7.xy then

PaCO2 ≈ xy

Strong ions… WTF?

The real determinants of pH are:

Strong ionsWeak acids [ATOT]

PCO2

Strong Ion difference:

Na – Cl

<38 = NAGMA>38 = metabolic alkalosis

EMCrit by Scott Weingart http://emcrit.org/podcasts/acid-base-i/

Acid-Base Physiology by Kerry Brandishttp://www.anaesthesiamcq.com/AcidB

aseBook/ABindex.php

High Lactate withNormal Anion Gap… WTF?

Anion gap

Na – (HCO3+Cl)

Normal = 8-12 mM

LTKR

Anion gap is only ~50% sensitive

for lactic acidosis

What causes a low anion gap?

Low Anions: Albumin

High Cations

Bromism

Analytical error

Correcting for low albumin is essential

AGadj = AGobs + 0.25 x (42 - Albobs)

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