arterial blood gas analysis in clinical practice (2)

ABG IN CLINICAL PRACTICE

Chairperson : Dr Ardaman Singh Speaker : Dr Ashok Kumar

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What is an ABG•Arterial Blood Gas•Drawn from artery- radial, brachial, femoral•It is an invasive procedure.•Caution must be taken with patient on anticoagulants.•Arterial blood gas analysis is an essential part of diagnosing and managing the patient’s oxygenation status, ventilation failure and acid base balance in medical emergency.

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What is an ABG?

• The Components– pH / PaCO2 / PaO2 / HCO3 / O2sat / BE

• Desire Ranges– pH - 7.35 - 7.45– PaCO2 - 32-45 mmHg– PaO2 - 72-104 mmHg– HCO3 - 22-30mEq/L– O2sat - 95-100%– Base Excess - +/-2 mEq/L

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COMPONENTS OF THE ABG pH: Measurement of acidity or alkalinity, based on the hydrogen (H+)

7.35 – 7.45Pao2 The partial pressure oxygen that is dissolved in arterial blood.

72-104 mm Hg. PCO2: The amount of carbon dioxide dissolved in arterial blood.

32– 45 mmHgHCO3

: The calculated value of the amount of bicarbonate in the blood

22 – 30mEq/L B.E: The base excess indicates the amount of excess or insufficient level of bicarbonate. -2 to +2mEq/L

SaO2:The arterial oxygen saturation. >95%

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Why Order an ABG?

• Aids in establishing a diagnosis • Helps guide treatment plan• Aids in ventilator management• Improvement in acid/base management

allows for optimal function of medications• Acid/base status may alter electrolyte

levels which are critical to patient status.

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Logistics• When to order an Arterial line --

– Need for continuous BP monitoring– Need for multiple ABGs

• Where to place -- – Radial– Femoral – Brachial– Dorsalis Pedis– Axillary

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The Terms• ACIDS

– Acidemia– Acidosis

• Respiratory CO2

• Metabolic HCO3

• BASES– Alkalemia– Alkalosis

• Respiratory CO2

• Metabolic HCO3

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STEPWISE APPROACH TO ABG

• Step 1: Acidemic or Alkalemic? • Step 2: Is the primary disturbance

respiratory or metabolic? • Step 3. Asses to Pa O2. A value below 80mm

Hg indicates Hypoxemia. For a respiratory disturbance, determine whether it is acute or chronic.

• Step 4. For a metabolic acidosis, determine whether an anion gap is present.

• Step 5. Assess the normal compensation by the respiratory system for a metabolic disturbance

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STEPS TO AN ABG INTERPRETATION

•STEP:1•Assess the pH

–acidotic/alkalotic•If above 7.45 – alkalotic•If below 7.35 – acidotic

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• STEP: 2• Assess the paCO2 level.• pH decreases below 7.35, the paCO2

should rise.• If pH rises above 7.45 paCO2 should fall.• If pH and paCO2 moves in opposite

direction – PRIMARY respiratory problem.

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• STEP:2• Assess HCO3 value• If pH increases the HCO3 should also

increase• If pH decreases HCO3 should also

decrease• They are moving in the same direction• PRIMARY problem is metabolic

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Respiratory acidosis

pH PaCo2 HC03normal

Respiratory Alkalosis

normal

Metabolic Acidosis

normal

Metabolic Alkalosis

normal12

• STEP: 3

Assess pao2 < 80 mm Hg - HypoxemiaFor a resp. disturbance : acute or chronic

If the change in paco2 is associated with

the change in pH, the disorder is acute. In chronic process the compensatory

process brings the pH to within the clinically acceptable range ( 7.30 – 7.50)

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Anion GAPSTEP:4• It represent maeasured anions in plasma AG = (Na+ ╋ K+) – (cl- ╋ Hco3-)

(Normally 10-12mmol/L)• * A change in the pH of 0.08 for each 10

mm Hg pCo2 indicates an ACUTE condition.* A change in the pH of 0.03 for each 10 mm Hg pCo2 indicates a CHRONIC condition.

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BASE EXCESS

• Is a calculated value which estimates the metabolic component of an acid- base abnormality.

• It is an estimate of the amount of strong acid or base needed to correct the met. component of an acid- base disorder

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FormulaBE=( Patient’s Anion Gap -10 )

In a 50kg person with metabolic acidosis Hco3 needed for correction is:

Hco3 = 0.3 X body weight X BE = 0.3 X 50 X10 = 150 mmol/L

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EXPECTED CHANGES IN ACID-BASE DISORDERS Primary Disorder Expected Changes

Metabolic acidosis PCO2 = 1.5 × HCO3 + (8 ± 2) Metabolic alkalosis PCO2 = 0.7 × HCO3 + (21 ± 2) Acute respiratory acidosis delta pH = 0.008 × (PCO2 - 40) Chronic respiratory acidosis delta pH = 0.003 × (PCO2 - 40) Acute respiratory alkalosis delta pH = 0.008 × (40 - PCO2) Chronic respiratory alkalosis delta pH = 0.003 × (40 - PCO2)

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STEP: 5 Determine if there is a compensatory mechanism working to try to correct the pH.

ie: if have primary respiratory acidosis will have increased PaCO2 and decreased pH. Compensation occurs when the kidneys retain HCO3.

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Assess the PaCO2• In an uncompensated state – when the pH

and paCO2 moves in the same direction: the primary problem is metabolic.

• The decreasing paco2 indicates that the lungs acting as a buffer response (blowing of the excess CO2)

• If evidence of compensation is present but the pH has not been corrected to within the normal range, this would be described as metabolic disorder with the partial respiratory compensation.

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Assess the HCO3

• The pH and the HCO3 moving in the opposite directions, we would conclude that the primary disorder is respiratory and the kidneys acting as a buffer response: are compensating by retaining HCO3 to return the pH to normal range.

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COMPENSATION• Step 5• A patient can be uncompensated or

partially compensated or fully compensated

• pH remains outside the normal range• pH has returned within normal range-

fully compensated though other values may be still abnormal

• Be aware that neither the system has the ability to overcompensate

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COMPENSATEDpH paco2 Hco3

Resp.Acidosis Normalbut<7.40

Resp.Alkalosis Normalbut>7.40

Met. Acidosis Normalbut<7.40

Met. Alkalosis Normalbut>7.40

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~ PaCO2 – pH Relationship80 7.20

60 7.30

40 7.40

30 7.50

20 7.6023

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PRECAUTIONS Excessive Heparin Decreases bicarbonate

and PaCO2

Large Air bubbles not expelled from sample PaO2 rises, PaCO2 may fall slightly.

Fever or Hypothermia, Hyperventilation or breath holding (Due to anxiety) may lead to erroneous lab results

Care must be taken to prevent bleeding

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ACID/BASE BALANCE The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the no. of (H+) in the blood. The normal pH range is 7.35-7.45.Changes in body system functions that occur in an acidic state decreases the force of cardiac contractions, decreases the vascular response to catecholamines, and a diminished response to the effects and actions of certain medications.An alkalotic state interferes with tissue oxygenation and normal neurological and muscular functioning.Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, may lead to death.

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H2O + CO2 H2CO3 HCO3

+ H+

Acid/Base Relationship

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There are two buffers that work in pairs

H2CO3 NaHCO3Carbonic acid base bicarbonate These buffers are linked to the respiratory and renal compensatory system

BUFFERS

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THE RESPIRATORY BUFFER RESPONSE

• The blood pH will change acc.to the level of H2CO3 present.

• This triggers the lungs to either increase or decrease the rate and depth of ventilation

• Activation of the lungs to compensate for an imbalance starts to occur within 1-3 minutes

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THE RENAL BUFFER RESPONSE

• The kidneys excrete or retain bicarbonate(HCO3-).

• If blood pH decreases, the kidneys will compensate by retaining HCO3

• Renal system may take from hours to days to correct the imbalance.

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RESPIRATORY ACIDOSIS

• Defined as a pH less than 7.35 with a pco2 greater than 45 mmHg.

• Acidosis –accumulation of co2, combines with water in the body to produce carbonic acid, thus lowering the pH of the blood.

• Any condition that results in hypoventilation can cause respiratory acidosis.

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RESPIRATORY ACIDOSISPH, CO2, Ventilation• Causes

– CNS depression(drug,stroke,inf.)– Parenchymal disease(Emphysema,ARDS)– Airway disease(obstruction,asthma)– Musculoskeletal disorders(polio,MG)– Compensation for metabolic alkalosis

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RESPIRATORY ACIDOSIS• Acute vs Chronic

– Acute - little kidney involvement. Buffering via cellular buffering for example• HCO3↑ 1mmol/Lfor 10mmHg in CO2

– Chronic –(>24hr) Renal compensation via synthesis and retention of HCO3 (Cl to balance charges hypochloremia)• HCO3 ↑4mmol/L for 10mmHg in CO2

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SIGNS & SYMPTOMS OF RESPIRATORY ACIDOSIS

• Respiratory : Dyspnoea, respiratory distress and/or shallow respiration.

• CNS : Headache, restlessness and confusion. If co2 level extremely high drowsiness and unresponsiveness may be noted.

• CVS: Tacycardia and dysrhythmias

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MANAGEMENT• Restoration of alveolar ventilation. • Causes should be treated rapidly

include pneumothorax, pain and CNS depression r/t medication.

• Mechanical ventilation.• Rapid correction of hypercapnia

should be avoided.

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RESPIRATORY ALKALOSIS

pH, CO2, Ventilation CO2 , HCO3 (Cl to balance charges

hyperchloremia)• Causes

– CNS Stimulation(Pain,fever,anxiety,tumour)– Hypoxia(high altitude,pneumonia,anemia)– Drugs and harmones(salicylate,pregnancy)– Chest receptors stimulation(Hemothorex,CCF)– Sepsis,Hepatic failure

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RESPIRATORY ALKALOSIS

• Acute vs. Chronic– Acute - HCO3 by 2 mEq/L for every 10mmHg

in PCO2

– Chronic - Ratio increases to 4 mEq/L of HCO3 for every 10mmHg in PCO2

– Decreased bicarb reabsorption and decreased ammonium excretion to normalize pH

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SIGNS & SYMPTOMS• CNS: Light Headedness, numbness,

tingling, confusion, inability to concentrate and blurred vision.

• Dysrhythmias and palpitations• Dry mouth, diaphoresis and tetanic

spasms of the arms and legs.

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MANAGEMENT• Resolve the underlying problem• Monitor for respiratory muscle

fatigue • Hyperventilation syndrome

benefit by rebreath into paper bag• Antidepressents and sedative

avoided41

METABOLIC ACIDOSIS Ph(<7.35), HCO3 (<22mEq/L)• 12-24 hours for complete activation of

respiratory compensation PCO2 by 1.2mmHg for every 1 mEq/L

HCO3• The degree of compensation is assessed

via the Winter’s Formula PCO2 = 1.5(HCO3) +8 2

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METABOLIC ACIDOSIS CAUSES

• High anion Gap metabolic Acidosis

– Lactic Acidosis– Ketoacidosis– Toxins(Ethylene

glycol,Methanol,Salicylate)

– Renal failure(Acute and chr.)

• Non Gap Metabolic Acidosis

– GI bicarbonate loss– Renal

acidosis(k↑,K↓)– Drug induced

hyperkalamia

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• K etoacidosis• U remia• S epsis• S alicylate & other drugs• M ethanol• A lcohol (Ethanol)• L actic acidosis• E thylene glycol

REMEMBERKUSSMALE

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SIGN & SYMPTOMS• CNS: Headache, confusion and

restlessness progressing to lethargy, then stupor or coma.

• CVS: Dysrhythmias• Kussmaul’s respirations• Warm, flushed skin as well as nausea

and vomiting

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MANAGEMENT• Treat the cause• Hypoxia of any tissue bed will produce

metabolic acids as a result of anaerobic metabolism even if the pao2 is normal

• Restore tissue perfusion to the hypoxic tissues

• The use of bicarbonate is indicated for non metabolizable anion with renal failure or if pH<7.10

• 50-100meq of NaHCO3 over 30-45 min• Goal increase HC03 to 10meq/L ,pH to 7.20

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METABOLIC ALKALOSIS• Bicarbonate more than 30m Eq /L with a pH

more than 7.45• Excess of base /loss of acid can cause• Ingestion of excess antacids, excess use of

bicarbonate, or use of lactate in dialysis.• Protracted vomiting, gastric suction,villous

adenoma ,hypokalemia,excess use of diuretics, high levels of aldosterone.

• PCO2 by 0.75 for every 1mEq/L in HCO3

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SIGNS/SYMPTOMS

• CNS: Dizziness, lethargy disorientation, siezures & coma.

• M/S: weakness, muscle twitching, muscle cramps and tetany.

• Nausea, vomiting and respiratory depression.

• Cause should treated.• It is difficult to treat.

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MIXED ACID-BASE DISORDERS

• Patients may have two or more acid-base disorders at one time

• Metabolic acidosis and alkalosis both present pH may normal

• Discrepancy in △AG(prevailing-normal) and △HCO3 (Normal- prevailing) indicate this condition

• DKA with renal failure ,alcoholic ketoacidosis due to vomiting develop metabolic alkalosis.

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• EXAMPLE :1• A 45 years old female admitted with the severe

attack of asthma. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows:

• pH : 7.22• paCO2 : 55• HCO3 : 25• Follow the steps• pH is low – acidosis• paCO2 is high – in the opposite direction of the pH. • Hco3 is Normal.• Respiratory Acidosis• Need to improve ventilation by oxygen

therapy, mechanical ventilation, pulmonary toilet or by administering bronchodilators.

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• EXAMPLE 2:• Mr. D is a 55 years old admitted with

recurring bowel obstruction has been experiencing intractable vomiting for the last several hours. his ABG is:

• pH : 7.5• paCO2 :42• HCO3 : 33• Metabolic alkalosis• Management: IV fluids, measures

to reduce the excess base

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EXAMPLE :3• Mrs. H is admitted, he is kidney

dialysis patient who has missed his last 2 appointments at the dialysis centre his ABG results:

• pH : 7.32• paCo2 : 32• HCO3 : 18• Pao2 : 88• Partially compensated metabolic

Acidosis 52

EXAMPLE: 4• Mr. K with COPD.his ABG is:• pH : 7.35• PaCO2 : 48• HCO3 : 28• PaO2 : 90•Fully compensated Respiratory

Acidosis 53

EXAMPLE: 5• Mr. S is a 53 year old man presented

to ED with the following ABG.• pH : 7.51• PaCO2 : 50• HCO3 : 40• Pao2 : 40 (21%O2)• He has metabolic alkalosis• compensatory respiratory

acidosis.54

TAKE HOME MESSAGE: Valuable information can be gained from an

ABG as to the patients physiologic condition

Remember that ABG analysis only part of the patient assessment. Be systematic with your analysis, start with ABC’s as always and look for hypoxia (which you can usually treat quickly), then follow the four steps. A quick assessment of patient oxygenation can be achieved with a pulse oximeter which measures SaO2.

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