A comprehensive review on ABG Analysis

Booma

Deputy Nursing Superintendent, Maa Kauvery, Trichy, Tamil Nadu

Background

Arterial blood gases are an important investigation to monitor the acid-base balance of the patient. This may help to

  • To make diagnosis
  • Analyze the severity of the condition
  • Help to assess treatment
  • To wean down from the ventilator

Modified Allen’s Test

Before taking an arterial blood sample from the radial artery, a modified Allen’s test should be performed to assess the collateral arterial supply of the hand from the ulnar artery. The idea behind this assessment is to make sure the patient’s hand isn’t exclusively reliant on the radial artery for its blood supply, in which case sampling should be avoided.

To perform a modified Allen’s test:

  • Ask the patient to clench the fist.
  • Apply pressure over the radial and ulnar artery to occlude both vessels.
  • Ask the patient to open the hand, which should now appear blanched. If the hand does not appear it suggests you are not completely occluding the arteries with your fingers.
  • Remove the pressure from the ulnar artery whilst maintaining pressure over the radial artery.
  • If there is adequate blood supply from the ulnar artery, the normal colour should return to the entire hand within 5-15 seconds. If the return of colour takes longer, this suggests poor collateral circulation Do not perform arterial blood gas sampling on a hand that does not appear to have an adequate collateral blood supply.

It should be noted that there is no evidence performing this test reduces the rate of ischaemic complications of arterial sampling.

Normal levels

pH7.35–7.45 (Exact value is 7.4)
pCO235–45 mm of hg (40 is the exact value)
pO280–100 %
HCO322–26 mEq /Lit (24 is the exact Value)
Oxygen saturation95–100 %
BE2 to + 2
Lactate Less than 2 mmol/L
Anion Gap8 to 12 mmol/L (OR 8 to 16 if potassium is included in calculation)

Step by step Analysis

  • Determine oxygenation (PaO2)
  • Determine pH (acidosis or alkalosis)
  • Determine respiratory component (PaCO2)
  • Determine metabolic component (Bicarbonate)
  • Determine compensation
  • Final interpretation
  • Review other parameters

ABG Analysis

Anything which i

pH less than the normal is called Acidosis – 7.35

pH more than the normal is called Alkalosis – 7.45

In Respiratory Components It will be opposite (R O M E)

More than the normal is called Acidosis –  7.35

Less than the normal is called Alkalosis – 7.45

RO – Respiratory it will be opposite, ME –Metabolic it will be equal

Compensation

  • Fully Compensated: If pH is Normal
  • Partially Compensated: All three values are abnormal
  • Un compensated: pH is abnormal & PaCo2 & HCO3 are  normal

Anion Gap calculation

AG = Na – (Cl + HCO3)

Example 1

Na⁺ = 140, Cl⁻= 98, HCO₃⁻= 23

AG= 140 – (98 + 23) AG = 24

The anion gap is 24. Therefore, the individual is positive for having a metabolic acidosis.

Example 2

Na⁺ = 135, Cl⁻= 100, HCO₃⁻= 25

AG = 135 – (100 + 25)

AG = 10

The anion gap is 10. Therefore, the result is normal and the person does not have metabolic acidosis.

Increased Anion GapNormal Anion Gap
Diabetic KetoacidosisDiarrhea
Chronic Kidney diseaseRenal Tubular Acidosis
Lactic AcidosisAddison's Disease
Alcoholic Ketoacidosis
Aspirin Poisoning
Methanol Poisoning
Ethylene Glycol Poisoning
Starvation

Respiratory Acidosis Causes, Signs & Symptoms

CausesSigns & Symptoms
AsthmaAnxiety
COPDBlurred vision
EmphysemaConfusion
PneumoniaDiscolored (blue, purple, gray-green, grey or white) skin tone from lack of oxygen (cyanosis)
Sleep apneaHeadache
Overuse of substances that affect the central nervous system, including alcohol, amphetamines and opioidsShortness of breath (dyspnea)
Wheezing

Respiratory Alkalosis Cause, Signs & Symptoms

CausesSigns & Symptoms
Anxiety or panic.Dizziness
Fever.Bloating
Overbreathing (hyperventilation)Feeling light-headed
Pregnancy (this is normal)Numbness or muscle spasms in the hands and feet
Pain.Discomfort in the chest area
Tremor.Confusion
Trauma.Dry mouth
Severe anemia.Tingling in the arms
Heart palpitations
Feeling short of breath

Metabolic Acidosis Causes, Signs & Symptoms

CausesSigns & Symptoms
Diabetes-related acidosis.Accelerated heartbeat (tachycardia).
Hyperchloremic acidosisConfusion or dizziness.
Lactic acidosisFeeling very tired – fatigue
Renal tubular acidosis.Loss of appetite.
Headache
Rapid breathing or long, deep breathing.
Nausea and vomiting
Feeling weak.
Breath that smells sweet or fruity.

Metabolic Alkalosis Cause, Signs & Symptoms

CausesSigns & Symptoms
Antacids, which often contain the chemical sodium bicarbonateIrritability.
Baking soda.Muscle twitching
Diuretics or water pills.Muscle cramps
Certain laxatives.Muscle spasms
Steroids.Fatigue
Cystic fibrosisConfusion.
Dehydration.Tremor.
Electrolyte imbalancesTingling and numbness.
High levels of the adrenal hormone aldosterone (hyperaldosteronism)Abnormal heart rhythm (arrhythmia)
Low levels of chloride in the bloodSeizures.
Recurrent vomiting, which occurs with cyclic vomiting syndromeComa

HAGMA: High Anion Gap Metabolic Acidosis

  • G: Glycols (ethylene glycol and propylene glycol)
  • O:Oxoproline
  • L:l-lactate
  • D:d-lactate
  • M:methanol
  • A:aspirin
  • R:renal failure
  • K: ketoacidosis

Errors during Sample Collection

Excessive Heparin

Ideally: Pre heparinised ABG Syringe

Syringe flushed with 0.5ml of 1:1000 Heparin & Empty

Do not leave excessive heparin in the syringe

Result

Increased Heparin → Increased dilutional effect → Decreased HCO3 / Decreased PCO2

Air Bubbles

Ideally: Seal syringe immediately after sampling

Result

Contact with air bubbles → Increased pO2 and Decreased pCO2 (e.g., pO2: 150 mmHg & pCO2: 0 mmHg)

Risk of alteration of results

Increase size of syringe/needle

Decrease volume of sample

Ideally: Syringe must have 50 % blood, Use only 3 ml or less syringe

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