arterial blood gases talk

Arterial Arterial BloodBlood Gases Gases

Dr. Michael J. BaffskyDr. Michael J. Baffsky

How to take an ABGHow to take an ABG

Clean procedure (gloves, alco wipe)Clean procedure (gloves, alco wipe) Have gauze for application of pressure post Have gauze for application of pressure post

arterial puncturearterial puncture 4 mins of pressure is ideal4 mins of pressure is ideal Especially in patients who are anticoagulatedEspecially in patients who are anticoagulated

23G needle 23G needle easiereasier, but use what you prefer, but use what you prefer ROTATION and INVERSION of the tube for ROTATION and INVERSION of the tube for

20 seconds to dissolve and mix the heparin20 seconds to dissolve and mix the heparin Ice only necessary if there will be a delay Ice only necessary if there will be a delay

from sampling to testing.from sampling to testing.

Why use ice anyway?Why use ice anyway?

What values do you get?What values do you get? pH - measures acidity/alkalinitypH - measures acidity/alkalinity pCO2 (partial pressure of CO2)pCO2 (partial pressure of CO2)

measures respiratory componentmeasures respiratory component [HCO3-] – (bicarbonate concentration)[HCO3-] – (bicarbonate concentration)

measures metabolic componentmeasures metabolic component derived value (H-H equation)derived value (H-H equation)

pO2 (partial pressure of O2)pO2 (partial pressure of O2) BE (base excess)BE (base excess) O2 saturationO2 saturation Sometimes AG (anion gap)Sometimes AG (anion gap) Sometimes electrolytes and glucoseSometimes electrolytes and glucose

Normal RangesNormal Ranges

• pHpH• 7.35 to 7.457.35 to 7.45

• pCO2pCO2• 36 to 4436 to 44

• pO2pO2• vary with oxygen vary with oxygen

therapytherapy• on room air will be < on room air will be <

100100

• HCO3HCO3• 22 to 2622 to 26

• Anion GapAnion Gap• 8 to 16 mmol/L8 to 16 mmol/L

• Base excess (deficit)Base excess (deficit)• -3 to +3 mmol/L-3 to +3 mmol/L

Questions to ask?Questions to ask?

? Is the patient acidotic or alkalotic?Is the patient acidotic or alkalotic?? Is the primary cause respiratory or Is the primary cause respiratory or

metabolic?metabolic?? Is there compensation?Is there compensation?? Is the patient hypoxic?Is the patient hypoxic?

? (oxygen status, A-a)(oxygen status, A-a)? What is going on?What is going on?

Is the patient acidotic or alkalotic?Is the patient acidotic or alkalotic?

If pH < 7.35, the patientIf pH < 7.35, the patient isis acidoticacidotic If pH > 7.45, the patient isIf pH > 7.45, the patient is alkaloticalkalotic

This is This is IRRELEVANTIRRELEVANT of the cause of the cause

NB: pH drops by 0.017 per NB: pH drops by 0.017 per ˚̊C increaseC increase(not significant in most clinical situations)(not significant in most clinical situations)

High pHHigh pH Low pHLow pH

AlkalosisAlkalosis AcidosisAcidosis

HighHighPaCO2PaCO2

LowLowPaCO2PaCO2

HighHighPaCO2PaCO2

LowLowPaCO2PaCO2

MetaboliMetabolicc

RespiratorRespiratoryy

RespiratorRespiratoryy

MetaboliMetabolicc

Recall, there may be a Recall, there may be a MIXEDMIXED

pattern!! pattern!!

CompensationCompensation

There is no ‘overcompensation’There is no ‘overcompensation’

CompensationCompensationApproximate responses to a primary acid-base problemApproximate responses to a primary acid-base problem

Metabolic acidosisMetabolic acidosispCO2 will fall approx 1.2 times the HCO3- fallpCO2 will fall approx 1.2 times the HCO3- fall

Metabolic alkalosisMetabolic alkalosispCO2 will rise approx 0.7 times the HCO3- risepCO2 will rise approx 0.7 times the HCO3- rise

Respiratory acidosisRespiratory acidosisAcute: HCO3- will rise approx 0.1 times the pCO2 riseAcute: HCO3- will rise approx 0.1 times the pCO2 riseChronic: HCO3- will rise approx 0.35 times the pCO2 riseChronic: HCO3- will rise approx 0.35 times the pCO2 rise

Respiratory alkalosisRespiratory alkalosisAcute: HCO3- will fall approx 0.2 times the pCO2 fallAcute: HCO3- will fall approx 0.2 times the pCO2 fallChronic: HCO3- will fall approx 0.5 times the pCO2 fallChronic: HCO3- will fall approx 0.5 times the pCO2 fall

Also, a 10mmHg change in pCO2 from 40mmHg should cause a change in pH Also, a 10mmHg change in pCO2 from 40mmHg should cause a change in pH of 0.08of 0.08

CompensationCompensation(example)(example)

Set of gases:Set of gases: pH 7.25 pCO2 70mmHg [HCO3-] 31 mmol/LpH 7.25 pCO2 70mmHg [HCO3-] 31 mmol/L Assume the patient has known chronic respiratory insufficiencyAssume the patient has known chronic respiratory insufficiency

Expected rise in [HCO3-] is about 0.35 times the pCO2 rise pCO2 rise is 30 (70-40)pCO2 rise is 30 (70-40) Expect the [HCO3-] rise to be about 10.5Expect the [HCO3-] rise to be about 10.5 So, [HCO3-] should be about 36 (upper norm, 26, + 10)So, [HCO3-] should be about 36 (upper norm, 26, + 10)

In this example, it is In this example, it is LOWERLOWER than expected, and while the than expected, and while the primary problem is respiratory acidosis, there must be primary problem is respiratory acidosis, there must be metabolic acidosis occurring as well.metabolic acidosis occurring as well.

QuestionQuestion pH 7.25 pCO2 70mmHg [HCO3-] 31 mmol/LpH 7.25 pCO2 70mmHg [HCO3-] 31 mmol/L

How would you interpret these if the patient was young, How would you interpret these if the patient was young, and did NOT have chronic lung disease?and did NOT have chronic lung disease?

Expected [HCO3-] rise would be 30 times 0.1 = 3Expected [HCO3-] rise would be 30 times 0.1 = 3 So, the [HCO3-] would be 29 (26+3)So, the [HCO3-] would be 29 (26+3)

In this case, you can say the patient probably has a In this case, you can say the patient probably has a respiratory acidosis with metabolic alkalosis, as you would respiratory acidosis with metabolic alkalosis, as you would not expect the [HCO3-] to be that high in compensation not expect the [HCO3-] to be that high in compensation only.only.

Is there another quicker way to do this?

The Base ExcessThe Base Excess

The BE (or base deficit) is defined as the The BE (or base deficit) is defined as the amount of acid (or base) required to be amount of acid (or base) required to be added to whole blood to achieve a pH of added to whole blood to achieve a pH of 7.4 at 377.4 at 37˚C and paCO2 of 40mmHg.˚C and paCO2 of 40mmHg.

- If the base is in excessIf the base is in excess- may be due to decrease in metabolic acidsmay be due to decrease in metabolic acidsmay be due to increase in buffers (e.g. HCO3-)may be due to increase in buffers (e.g. HCO3-)

- If the base is in deficitIf the base is in deficit- may be due to excess metabolic acidsmay be due to excess metabolic acids

The Anion GapThe Anion Gap

[Na+ + K+] – [Cl- + HCO3-][Na+ + K+] – [Cl- + HCO3-] Measuring of a sample includes most Measuring of a sample includes most

cations, but NOT all anions.cations, but NOT all anions. e.g. proteins, sulphates, phosphates, e.g. proteins, sulphates, phosphates,

some acidssome acids Electroneutrality dictates these must Electroneutrality dictates these must

be equal.be equal. Normal gap 8-16 mmol/LNormal gap 8-16 mmol/L

Interpretation of the anion gapInterpretation of the anion gap

The anion gap increases due to an increase in The anion gap increases due to an increase in unmeasured anions due to a metabolic acidosisunmeasured anions due to a metabolic acidosis Ketoacidosis, lactic acidosis, uraemia, Ketoacidosis, lactic acidosis, uraemia,

salicylate/methanol/ethylene glycol poisoningsalicylate/methanol/ethylene glycol poisoning If the AG is NORMAL with a metabolic acidosis If the AG is NORMAL with a metabolic acidosis

and increased Cl-, think of:-and increased Cl-, think of:- Diarrhoea, pancreatic fistula, renal tubular acidosis.Diarrhoea, pancreatic fistula, renal tubular acidosis. Treatment with either HCl, HN4Cl or acetazolamide.Treatment with either HCl, HN4Cl or acetazolamide.

A decreased gap may be due to low protein A decreased gap may be due to low protein statesstates

Causes of acute respiratory Causes of acute respiratory acidosisacidosis

Respiratory pathophysiology Respiratory pathophysiology respiratory arrestrespiratory arrest airway obstructionairway obstruction severe pneumoniasevere pneumonia chest traumachest trauma pneumothoraxpneumothorax

Acute drug intoxicationAcute drug intoxication especially narcotics, sedativesespecially narcotics, sedatives

Residual neuromuscular blockadeResidual neuromuscular blockade Head traumaHead trauma

Causes of chronic respiratory Causes of chronic respiratory acidosisacidosis

Chronic lung disease (esp CAL)Chronic lung disease (esp CAL) Neuromuscular diseaseNeuromuscular disease Extreme obesity (with OSA)Extreme obesity (with OSA) Chest wall deformityChest wall deformity

Causes of Respiratory AlkalosisCauses of Respiratory Alkalosis

PainPain AnxietyAnxiety HyperventilationHyperventilation HypoxemiaHypoxemia Restrictive lung Restrictive lung

diseasedisease Severe congestive Severe congestive

heart failureheart failure Pulmonary emboliPulmonary emboli

DrugsDrugs SepsisSepsis FeverFever ThyrotoxicosisThyrotoxicosis PregnancyPregnancy Overaggressive Overaggressive

mechanical mechanical ventilationventilation

Hepatic failureHepatic failure

Causes of metabolic acidosis with Causes of metabolic acidosis with raised anion gapraised anion gap

KetoacidosisKetoacidosis diabetic, alcoholic, starvationdiabetic, alcoholic, starvation

Lactic acidosisLactic acidosis hypoxia, shock, sepsis, seizureshypoxia, shock, sepsis, seizures

Toxin ingestionToxin ingestion methanol, ethylene glycol, ethanol, isopropyl methanol, ethylene glycol, ethanol, isopropyl

alcohol, paraldehyde, toluenealcohol, paraldehyde, toluene Renal failureRenal failure

uraemiauraemia

Causes of metabolic acidosis with Causes of metabolic acidosis with normal anion gapnormal anion gap

Renal tubular acidosisRenal tubular acidosis Post respiratory alkalosisPost respiratory alkalosis HypoaldosteronismHypoaldosteronism Potassium sparing diureticsPotassium sparing diuretics Pancreatic loss of bicarbonatePancreatic loss of bicarbonate Acid administration (HCl, NH4Cl)Acid administration (HCl, NH4Cl) Cholestyramine Cholestyramine DiarrhoeaDiarrhoea Carbonic anhydrase inhibitorsCarbonic anhydrase inhibitors

Causes of Metabolic AlkalosisCauses of Metabolic Alkalosis

DiureticsDiuretics NG suctionNG suction VomitingVomiting Post hypercapnicPost hypercapnic Cushing’s syndromeCushing’s syndrome Excessive alkali intakeExcessive alkali intake

Mixed Acid-Base DisturbancesMixed Acid-Base Disturbances

Metabolic acidosis and respiratory acidosisMetabolic acidosis and respiratory acidosis- e.g. aspirin and sedative overdosee.g. aspirin and sedative overdose

Metabolic acidosis and respiratory alkalosisMetabolic acidosis and respiratory alkalosis- e.g. sepsise.g. sepsis

Metabolic acidosis and alkalosisMetabolic acidosis and alkalosis- e.g. vomiting with DKAe.g. vomiting with DKA

Metabolic alkalosis and respiratory acidosisMetabolic alkalosis and respiratory acidosis- e.g. CAL and vomiting or diuretic usee.g. CAL and vomiting or diuretic use

Metabolic alkalosis and respiratory alkalosisMetabolic alkalosis and respiratory alkalosis- e.g. pregnancy and vomitinge.g. pregnancy and vomiting

Oxygen statusOxygen status

• HypoxaemiaHypoxaemia• decreased oxygen content of blood decreased oxygen content of blood • pO2 less than 60 mmHg and saturation pO2 less than 60 mmHg and saturation

is less than 90%is less than 90%

• HypoxiaHypoxia• Levels of pO2 sufficiently low to have an Levels of pO2 sufficiently low to have an

adverse effect on tissue functionadverse effect on tissue function

Types of HypoxiaTypes of Hypoxia Hypoxic hypoxiaHypoxic hypoxia

due to low blood pO2due to low blood pO2 e.g. due to lung disease processese.g. due to lung disease processes

Anaemic hypoxiaAnaemic hypoxia inadequate O2 delivery to tissuesinadequate O2 delivery to tissues

e.g. in anaemia or CO poisoninge.g. in anaemia or CO poisoning Circulatory hypoxiaCirculatory hypoxia

inadequate blood flow to tissuesinadequate blood flow to tissues e.g. shocke.g. shock

Histotoxic hypoxiaHistotoxic hypoxia inability of tissue to use the oxygeninability of tissue to use the oxygen

classically, in cyanide poisoningclassically, in cyanide poisoning

Causes of HypoxemiaCauses of Hypoxemia

Inadequate inspiratory pO2Inadequate inspiratory pO2 HypoventilationHypoventilation Right to left shuntRight to left shunt V/Q mismatchV/Q mismatch Problems at the gas exchange Problems at the gas exchange

surfacesurface

The A-a gradientThe A-a gradient

• Difference between alveolar and arterial Difference between alveolar and arterial pO2pO2• used as an index for shunting and V/Q used as an index for shunting and V/Q

mismatch (also diffusion problems)mismatch (also diffusion problems)• normally about 5 mmHg (up to 15, in older normally about 5 mmHg (up to 15, in older

patients, esp. lying down)patients, esp. lying down)• pAO2 - paO2pAO2 - paO2• pA02 obtained from the Alveolar Gas Eq.pA02 obtained from the Alveolar Gas Eq.

• pAO2 = pIO2 – (paCO2 / R)pAO2 = pIO2 – (paCO2 / R)

A-a gradientA-a gradient

Normal A-aNormal A-a Hypoxaemia due to:Hypoxaemia due to:

HypoventilationHypoventilation Decreased FiO2Decreased FiO2

Increased A-aIncreased A-a Hypoxaemia due to:Hypoxaemia due to:

VQ mismatch (e.g. PE (increases dead space))VQ mismatch (e.g. PE (increases dead space)) Shunting (increasing FIO2 NOT help!)Shunting (increasing FIO2 NOT help!) Diffusion problemsDiffusion problems

ANY QUESTIONS?ANY QUESTIONS?

THE ENDTHE ENDThank YouThank You

arterial blood gases talk

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