Acid Base Disorders

Arterial blood gas analysis is used to determine the adequacy of oxygenation and ventilation, assess respiratory function and determine the acid–base balance. These data provide information regarding potential primary and compensatory processes that affect the body’s acid–base buffering system.

Interpret the ABG in a step-wise manner:

1. PaO2. Determine the adequacy of oxygenation
   • Normal range: 80–100 mmHg (10.6–13.3 kPa)
2. Determine pH status
   • Normal pH range: 7.35–7.45 (H+ 35–45 nmol/L)
   • pH <7.35: Acidosis is an abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia.
   • pH >7.45: Alkalosis is an abnormal process that decreases the hydrogen ion concentration and results in alkalaemia.
3. PaCO2. Determine the respiratory component
   • Normal range: PaCO2 35–45 mmHg (4.7–6.0 kPa)
   • PaCO2 <35 mmHg (4.7 kPa): 
     • Primary respiratory alkalosis (hyperventilation) if pH >7.45 and HCO3^– normal. 
     • Respiratory compensation for metabolic acidosis if pH <7.35 and HCO3^– (decreased).
   • PaCO2 >45 mmHg (> 6.0 kPa): 
   • Primary respiratory acidosis (hypoventilation) if pH <7.35 and HCO3^– normal.
   • Respiratory compensation for metabolic alkalosis if pH >7.45 and HCO3^– increased.
   • In chronic respiratory acidosis the metabolic compensation may be acute or chronic. In acute compensation for every10 Δ pCO2 = 0.08 Δ pH and in chronic for every10 Δ pCO2 = 0.03 Δ pH
4. HCO3^– Determine the metabolic component
   • Normal HCO3^– range 22–26 mmol/L
   • HCO3^– <22 mmol/L: Primary metabolic acidosis if pH <7.35. Renal compensation for respiratory alkalosis if pH >7.45.
   • HCO3^– >26 mmol/L: Primary metabolic alkalosis if pH >7.45. Renal compensation for respiratory acidosis if pH <7.35.

Rules and Resources

• Acid Base disorders worksheet – Joshua Steinberg MD
• Acid Base Made Easy – ICU OnePager
• ABG walk though
• Anion Gap – Delta ratio – Strong ion difference
• Base Excess vs Standard Base Excess
• Bedside Rules for Assessment of Compensation – Kerry Brandis

Additional definitions

Osmolar Gap

• Use: Screening test for detecting abnormal low MW solutes (e.g. ethanol, methanol & ethylene glycol [Reference])
• An elevated osmolar gap (>10) provides indirect evidence for the presence of an abnormal solute which is present in significant amounts [Reference]
• Osmolar gap = Osmolality – Osmolarity
• Osmolality (measured)
  • Units: mOsm/kg
  • Measured in laboratory and returned as the plasma osmolality
• Osmolarity (calculated)
  • Units: mOsm/l
  • Osmolarity = (1.86 x [Na⁺]) + [glucose] + [urea] + 9  (using values measured in mmol/l)
  • Osmolarity = (1.86 x [Na⁺]) + glucose/18 + BUN/2.8 + 9 (using US units of mg/dl)
• NOTE: even though the units of measured (mOsm/kg) and calculated (mOsm/l) are different [Reference], strictly they cannot be subtracted from one another… However, the value of the difference is clinically useful so the problem is usually overlooked!

1 2 3 4 5 Rule

Simple table to calculate metabolic compensation in respiratory acidosis and alkalosis (aka the 1-2-3-4-5 rule)

• Respiratory Acidosis: Increased PaCO2. For every 10mmHg rise should have INCREASE in HCO3 by 1 (acute) or 4 (chronic) as compensation
• Respiratory Alkalosis: Decreased PaCO2. For every 10mmHg rise should have DECREASE in HCO3 by 2 (acute) or 5 (chronic) as compensation,

References and Links

• Acid-Base: ABG analysis – Anion Gap – SID – NAGMA
• Metabolic acidosis: Overview – evaluation – DDx
• Metabolic alkalosis: Overview – evaluation – DDx
• Respiratory acidosis: Overview
• Respiratory alkalosis: Overview

CCC Differential Diagnosis Series
NEURO	Anosmia, Ataxia, Blepharospasm, Bulbar and Pseudobulbar palsy, Central Pontine Myelinosis, Cerebellar Disease, Chorea, Cranial nerve lesions, Dementia, Dystonia, Exophthalmos, Eye trauma, Facial twitches, Fixed dilated pupil, Horner syndrome, Loss of vision, Meningism, Movement disorders, Optic disc abnormality, Parkinsonism, Peripheral neuropathy, Radiculopathy, Red eye, Retinal Haemorrhage, Seizures, Sudden severe headache, Tremor, Tunnel vision
RESP	Bronchial breath sounds, Bronchiectasis, High airway pressures, Massive haemoptysis, Sore throat, Tracheal displacement
CVS	Atrial Fibrillation, Bradycardia, Cardiac Failure, Chest Pain, Murmurs, Post-resuscitation syndrome, Pulseless Electrical Activity (PEA), Pulsus Paradoxus, Shock, Supraventricular tachycardia (SVT), Tachycardia, VT and VF, SVC Obstruction
GIT	Abdominal distension, Abdominal mass, Abdominal pain, Asterixis, Dysphagia, Hepatomegaly, Hepatosplenomegaly, Large bowel obstruction, Liver palpation abnormalities, Lower GI haemorrhage, Malabsorption, Medical causes of abdominal pain, Rectal mass, Small bowel obstruction, Upper GI Haemorrhage
GUT	Genital ulcers, Groin lump, Scrotal mass, Urine colour, Urine Odour, Urine transparency,
MSK	Arthritis, Shoulder pain, Wasting of the small muscles of the hand
DERM	Palmar erythema, Serious skin signs in sick patients, Thickened Tethered Skin, Leg ulcers, Skin Tumour, Acanthosis Nigricans
ENDO	Diabetes Insipidus, Diffuse Goitre, Gynaecomastia, Hirsutism, Hypoglycaemia, SIADH, Weight Loss
HAEM	Splenomegaly
PAEDS	Floppy infant
MISC	Anaphylaxis, Autoimmune associated diseases, Clubbing, Parotid Swelling, Splinter haemorrhages, Toxic agents and abnormal vitals, Toxicological causes of cardiac arrest
IMAGING	CHEST: Atelectasis, Hilar adenopathy, Hilar enlargement on CXR, Honeycomb lung, Increased interstitial markings, Mediastinal widening on mobile CXR, Pulmonary fibrosis, Pseudoinfiltrates on CXR, Pulmonary opacities on CXR, ABDO: Gas on abdominal X-ray, Kidney mass, BRAIN: Intracranial calcification, Intracranial structures with contrast, Ventriculomegaly, OTHER: Pseudofracture on X-Ray,
LABS	LOW: Anaemia, Hypocalcaemia, hypochloraemia, Hypomagnesaemia,  HIGH: Bilirubin and Jaundice, Hyperammonaemia, Hypercalcaemia, Hyperchloraemia, Hyperkalaemia, Hypermagnesaemia,  ACID BASE: Acid base disorders, Resp. acidosis, Resp. alkalosis, Creatinine, CRP, Dipstick Urinalysis, Laboratory Urinalysis, Liver function tests (LFTs), Pleural fluid analysis, Urea, Urea Creatinine Ratio, Uric acid, Urinalysis, Urine Electrolytes

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