Critical care - ABG interpretation & management Flashcards by Aldrey Neo

What are the components of ABG analysis?

pH
PaO2
PaCO2
HCO3
Base excess (BE)
SaO2

*Note: Difference between PaO2 and SaO2:
- PaO₂ measures oxygen in plasma (dissolved oxygen), while SaO₂ measures oxygen bound to hemoglobin (Reflects oxygen-carrying capacity but not the amount of oxygen dissolved in plasma)

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Normal range of pH in ABG

7.35 to 7.45

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Normal range of PaO2 in ABG

80 to 100 mmHg

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Normal range of PaCO2 in ABG

35 to 45 mmHg

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Normal range of HCO3 in ABG

22 to 26 mEq/L

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Normal range of base excess (BE) in ABG

-3 to +3 mEq/L

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Normal range of SaO2 in ABG

95-100%

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How do we determine respiratory acidosis from pH, pCO2 and HCO3 levels?

Respi acidosis:
pH: LOW (<7.35)
pCO2: HIGH (>45 mmHg)
HCO3: Neutral

*note: pCO2 affected in Respi alkalosis/acidosis

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How do we determine respiratory alkalosis from pH, pCO2 and HCO3 levels?

Respi alkalosis:
pH: HIGH (> 7.45)
pCO2: LOW (< 35 mmHg)
HCO3: Neutral

*note: pCO2 affected in Respi alkalosis/acidosis

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How do we determine metabolic acidosis from pH, pCO2 and HCO3 levels?

Metabolic acidosis:
pH: LOW (< 7.35)
pCO2: Neutral
HCO3: LOW (< 22)

*note: HCO3 affected in Metabolic acidosis/alkalosis

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How do we determine metabolic alkalosis from pH, pCO2 and HCO3 levels?

Metabolic alkalosis:
pH: HIGH (> 7.45)
pCO2: Neutral
HCO3: HIGH (> 26)

*note: HCO3 affected in Metabolic acidosis/alkalosis

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How to tell the difference between partially and fully compensated states?

Look at pH.

Has pH returned to normal?

If yes, it is fully compensated.

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How will the body compensate metabolically if pCO2 is high? Explain the renal control of pH.

The body compensates metabolically via the renal system by:

(Kidneys) Reabsorbing bicarbonate (HCO₃⁻) into the bloodstream
Excreting hydrogen ions (H⁺) in the urine, often as ammonium (NH₄⁺) or dihydrogen phosphate (H₂PO₄⁻).

This increased HCO₃⁻ raises the blood pH, counteracting the acidosis caused by high pCO₂.

*note: High pCO₂ triggers renal compensation.

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What is the anion gap concept? What is it used for?

Used to identify the cause of metabolic acidosis (primarily due to ELECTROLYTE IMBALANCE)

Helps to distinguish between anion-gap and non-anion-gap metabolic acidosis

It represents the disparity between major measured plasma cations (Na+ and K+) and anions (Cl- and HCO3-)

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What is the normal anion gap range?

8-16 mmol/L

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In what cases will you see a raised anion gap? (>16 mmol/L)

Overdoses of paracetamol, salicylates, methanol or ethylene glycol

In what cases will you see a normal anion gap?

If a metabolic acidosis is due to diarrhoea or urinary loss of bicarbonate.

What is base excess?

It represents the amount of acid needed to bring the pH of the blood to 7.40, assuming a normal pCO₂ of 40 mmHg.

How to interpret base excess in ABG?

Base excess guides us whether pt has metabolic acid/alkalosis

Normal range: -3 to + 3 mEq/L (METABOLIC ACIDOSIS TO METABOLIC ALKALOSIS)

e.g. if BE: -8, paO2, pCO2 normal: have metabolic acidosis

Causes of respiratory acidosis

Occurs due to hypoventilation, resulting in accumulation of CO2, which combines with H2O to form carbonic acid

Brainstem trauma
CNS depressant drugs (e.g. morphine)
Impaired respiratory muscle function
Lung disorders (e.g. pneumonia, emphysema)

Treatment of respiratory acidosis

Correct underlying disorders
Hold/discontinue any respiratory depressant drugs, reverse effects of respiratory depressants, improve ventilation/respiration (may require MECH VENTILATION)

Reverse opiate overdose: NALOXONE (0.4-2mg SC/IV/IM, see response within 10 mins)
Reverse benzodiazepine overdose: FLUMAZENIL (0.2 mg IV)

ABG every 2-5 hrs initially
- every 12-24hrs as pH improves
- basic metabolic panel

Causes of metabolic acidosis

large amt of metabolic acids produced (e.g. lactic acids, ketoacids, salicylic acid)
impaired ability to excrete H+ by kidneys
may require HCO3- replacement (bicarb injection), must correct sodium and water deficits too

Treatment of metabolic acidosis

Replacement of sodium bicarbonate useful (for pts w bicarb loss due to diarrhoea or renal proximal tubular acidosis)

Essential to monitor plasma electrolytes during course of therapy as [K+] may decline as pH rises

Goal: Increase [HCO3-] to 10 and pH to 7.20, not to increase these values to normal

Causes of respiratory alkalosis

Caused by hyperventilation due to:
- anxiety, fear, pain
- respiratory stimulants (e.g. doxapram)
- increased metab demands (e.g. fever, thyrotoxicosis)
- CNS lesions

Treatment of respiratory alkalosis

Identify and correct underlying disorders Hold/discontinue any suspected drugs Initiate oxygen therapy in pts with severe hypoxemia (pO2<40 mmHg), change ventilator settings as needed Treat theophylline (bronchodilator) overdoses appropriately

Causes of metabolic alkalosis

Due to either loss of H+ or excess HCO3- Causes: - Prolonged vomiting - Gastric suction (leading to loss of gastric acids) - Excessive antacid usage May require Cl- or K+ replacement

Treatment of metabolic alkalosis

Correct underlying disorder, hold/discontinue suspected drugs If Extracellular fluid (ECF) volume contracted: - NS at appropriate rate - Replace potassium as needed If ECF volume overload: - If no renal insufficiency: Acetazolamide 250-375 daily/twice a day - If ARF or ESRD: hemodialysis or PD (reduced bicarb bath) If caused by hyperaldosteronism: spironolactone or amiloride

Steps to ABG analysis

1. Analyse pH: Is it acidosis or alkalosis?? 2. Analyse the PaCO2 3. Analyse the HCO3 4. Match the CO2 or HCO3 with the pH to determine the case of acid-base disturbances (is it respiratory/ metabolic/ both?) 5. Does the CO2 or HCO3 go the opposite direction of pH? ( is there compensation? full/partial?) 6. Analyse the PaO2 and O2 saturation

Interpret this ABG result: pH: 7.28 (ref: 7.35 - 7.45) PaO2: 80 (ref: 80 -100) PaCO2: 60 (ref: 35 - 45) HCO3: 40 (ref: 22 - 26)

Respiratory acidosis with partial compensation 1. pH (7.28): Acidotic (below the normal range of 7.35–7.45). 2. PaCO₂ (60 mmHg): Elevated (normal: 35–45 mmHg). High PaCO₂ confirms that the primary problem is respiratory acidosis. 3. HCO₃⁻ (40 mEq/L): Elevated (normal: 22–26 mEq/L). This increase in bicarbonate suggests metabolic compensation is occurring. The kidneys are retaining bicarbonate to counteract the acidic effect of elevated CO₂. Why partial? The elevated HCO₃⁻ shows that the kidneys are attempting to buffer the excess CO₂ by retaining bicarbonate. However, the pH is still acidotic (7.28) and has not returned to the normal range (7.35–7.45). This incomplete normalization of pH indicates that compensation is partial, not full.

Interpret this ABG result: pH: 7.48 (ref: 7.35 - 7.45) PaO2: 90 (ref: 80 -100) PaCO2: 37 (ref: 35 - 45) HCO3: 29 (ref: 22 - 26)

Metabolic alkalosis with no compensation 1. Assess the pH: alkalotic 2. Determine the Primary Cause: - PaCO₂ within the normal range: rules out a respiratory cause for the alkalosis - HCO₃⁻ is elevated: indicates a metabolic alkalosis 3. Assess for compensation: - Since the PaCO₂ is still within the normal range, there is no respiratory compensation - In a fully compensated metabolic alkalosis, the PaCO₂ would rise slightly above 45 mmHg to retain CO₂ and bring the pH closer to normal 4. Assess oxygenation: - PaO₂ within the normal range, hence no hypoxemia

Interpret this ABG result: pH: 7.30 (ref: 7.35 - 7.45) PaO2: 49 (ref: 80 -100) PaCO2: 52 (ref: 35 - 45) HCO3: 26 (ref: 22 - 26)

Respiratory acidosis with no compensation Type II RF (Hypercapnia) 1. Assess the pH: Acidotic 2. Determine primary cause: - PaCO₂ elevated : respiratory acidosis - HCO₃⁻ is within the normal range, not elevated: there is no metabolic compensation Hence, this is an uncompensated respiratory acidosis.

Interpret this ABG result: pH: 7.45 (ref: 7.35 - 7.45) PaO2: 58 (ref: 80 -100) PaCO2: 37 (ref: 35 - 45) HCO3: 24 (ref: 22 - 26)

Normal pH, Type 1 RF - PaCO₂ normal: no respiratory disturbance causing the alkalosis - HCO₃⁻ is normal: rules out a primary metabolic alkalosis - PaO₂ significantly below the normal range: indicating moderate hypoxemia Type 1 RF is characterized by hypoxemia (low PaO₂) with normal or low PaCO₂.