Blood gas, electrolyte and acid-base interpretation

Overview & Definition

Picture a three-year-old brought in breathing fast and drowsy with the smell of ketones on the breath. The glucose is high, the gas shows a pH of seven point oh seven and a bicarbonate of six, and the anion gap is twenty-eight. The gas does not make the diagnosis of diabetic ketoacidosis on its own, but it is the single test that tells you how sick the child is, how hard the body is working to compensate, and how fast you must act. Interpretation is a skill the paediatrician uses in the resuscitation bay, the ward and the PICU, and it rests on a method that is the same every time. [6] [7]

A blood gas is a sample of whole blood analysed at the point of care or in the laboratory for pH, the partial pressures of oxygen and carbon dioxide, the bicarbonate concentration, the base excess, and often the electrolytes, glucose, lactate and haemoglobin on the same cartridge. The gas captures two things at once: how well the lungs move oxygen and carbon dioxide, and how well the kidneys and buffers hold the pH. Together they tell you about oxygenation, ventilation and the metabolic state in under a minute. [3] [4]

Acid-base balance is the defence of the hydrogen-ion concentration of the blood within the narrow range that lets enzymes work. The healthy arterial pH sits between 7.35 and 7.45, and the body holds it there by pairing a weak acid, carbonic acid from carbon dioxide, against its buffer bicarbonate. The Henderson-Hasselbalch relationship fixes the pH at the ratio of bicarbonate to dissolved carbon dioxide, so a change in either lung or kidney side of the ratio moves the pH. Disturbances are named for the side that drives them and for the direction of the pH. [2] [11]

Electrolytes travel with the gas because sodium, chloride, potassium and bicarbonate are the ions that determine the anion gap, the osmolality and the rhythm of the heart. A child who is acidotic is almost always also deranged in one or more electrolytes, and reading the two together is what turns a number on a printout into a treatable diagnosis. [1] [10]

References12Show ledgerHide ledger

1. [1]Zieg J, Ghose S, Raina R Electrolyte disorders related emergencies in children BMC Nephrology, 2024.PMID 39215244
2. [2]Hoorn EJ Intravenous fluids: balancing solutions Journal of Nephrology, 2017.PMID 27900717
3. [3]Konuksever D, Yucel SP, Bölük O, et al Compatibility levels between blood gas analysis and central laboratory hemoglobin and electrolyte tests in pediatric patients: A single-center experience Paediatric Anaesthesia, 2023.PMID 36178754
4. [4]Sheikholeslami D, Dyson AE, Villarreal EG, et al Venous blood gases in pediatric patients: a lost art? Minerva Pediatrics, 2022.PMID 34530585
5. [5]Evans DL, Volsko TA, Capellari E, et al AARC Clinical Practice Guidelines: Capillary Blood Gas Sampling for Neonatal and Pediatric Patients Respiratory Care, 2022.PMID 36002161
6. [6]Dhatariya KK, Glaser NS, Codner E, et al Diabetic ketoacidosis Nature Reviews Disease Primers, 2020.PMID 32409703
7. [7]Wolfsdorf JI, Glaser N, Agus M, et al ISPAD Clinical Practice Consensus Guidelines 2018: Diabetic ketoacidosis and the hyperglycemic hyperosmolar state Pediatric Diabetes, 2018.PMID 29900641
8. [8]Saba L, Hanna C, Creo AL Updates in hyponatremia and hypernatremia Current Opinion in Pediatrics, 2024.PMID 38174733
9. [9]Feld LG, Neuspiel DR, Foster BA, et al Clinical Practice Guideline: Maintenance Intravenous Fluids in Children Pediatrics, 2018.PMID 30478247
10. [10]Powers KS Dehydration: Isonatremic, Hyponatremic, and Hypernatremic Recognition and Management Pediatrics in Review, 2015.PMID 26133303
11. [11]Rodríguez-Villar S, Poza-Hernández P, Freigang S, et al Automatic real-time analysis and interpretation of arterial blood gas sample for Point-of-care testing: Clinical validation PLoS One, 2021.PMID 33690724
12. [12]Calimag APP, Chlebek S, Lerma EV, et al Diabetic ketoacidosis Disease-a-Month, 2023.PMID 35577617