blood gases 2

What is the anion gap, how is it calculated, and what does a normal value mean?

Na++K+)–(Cl−+HCO3−​)

• Normal range:
  10–18 mmol/L

• The anion gap represents the amount of unmeasured anions in the blood =Helps determine the cause of metabolic acidosis.

how is CO2 present in the body?

bicarbonate

How do PCO₂ and HCO₃⁻ affect hydrogen ion concentration according to the carbonic acid equation?

Meaning:

• [H⁺] ↑ when PCO₂ ↑ (directly proportional)

• [H⁺] ↓ when HCO₃⁻ ↑ (inversely proportional)

Interpretation:

• More CO₂ → more acid → ↓ pH

• More bicarbonate → less acid → ↑ pH

This explains respiratory (PCO₂) vs metabolic (HCO₃⁻) control of blood pH.

What is acidosis?

• High [H⁺]

• Low pH

What is alkalosis?

• Low [H⁺]

• High pH

what are the types of acidosis/alkalosis?

- non-respiratory (metabolic)

- respiratory

what are the 4 components to the process involved in H+ abnormalities? non respitroy acidosis

- generation - too much H+

- buffering - need sufficient renal perfusion

- compensation - carbonic anhydrase system

- correction - overcorrecting problem

What are the 3 casues of non-respirtory acidosis?

• increase h+ production

• decreased h+ execretion

• Decreased Bicarbonate (HCO₃⁻)

What causes non-respiratory acidosis due to increased H⁺?

• Ketoacidosis / lactic acidosis

• Poisons:

  • Ingestion of acids

  • Salicylate poisoning

• Inherited metabolic disorders

What causes non-respiratory acidosis due to decreased H⁺ excretion?

• Renal tubular acidosis

• Renal failure

• Carbonic anhydrase inhibitors:

  • Acetazolamide

What causes non-respiratory acidosis due to decreased HCO₃⁻?

• Diarrhoea

• Loss from GI tract into peritoneum

How does diabetic ketoacidosis cause increased H⁺? (non respitory acidosis)

• Absolute lack of insulin → body metabolises fatty acids

• Produces ketone acids, increasing H⁺

• High blood glucose causes dehydration, worsening acidosis

How does ethylene glycol (antifreeze) cause acidosis?(non respitory acidosis)

• Ethylene glycol is metabolised into toxic organic acids

• These acids increase H⁺ concentration, causing metabolic acidosis

Why does decreased excretion of H⁺ lead to acidosis?(non respitory acidosis)

• Kidneys fail to remove H⁺

• Hydrogen ions accumulate in the blood

• Leads to metabolic (non-respiratory) acidosis

How does reduced GFR contribute to H⁺ accumulation?(non respitory acidosis)

• Low GFR → ↓ filtration of Na⁺

• Less Na⁺ available for Na⁺/H⁺ exchange

• Less H⁺ is secreted → H⁺ builds up

Why does reduced GFR impair phosphate buffering of H⁺?(non respitory acidosis)

• Low GFR → ↓ phosphate availability

• Phosphate normally buffers & carries H⁺ out

• Without phosphate, H⁺ cannot be cleared efficiently

How do gastrointestinal fistulae cause decreased bicarbonate?(non respitory acidosis)

• GI fluids escape through fistulae

• Loss of alkaline pancreatic secretions

• Loss of acidic gastric secretions → imbalance

• Disrupts normal acid–base neutralisation

How does fluid loss (e.g., fistulae or diarrhoea) worsen metabolic acidosis?(non respitory acidosis)

• Fluid loss → ↓ extracellular fluid (ECF)

• ↓ ECF → ↓ GFR

• Lower GFR weakens renal compensation

• Leads to impaired H⁺ excretion and reduced HCO₃⁻ regeneration

What is the step-by-step mechanism of non-respiratory (metabolic) acidosis?

• ↑ [H⁺]

• H⁺ is buffered by HCO₃⁻, so bicarbonate falls

• Forms H₂CO₃, which dissociates → CO₂ + H₂O

• CO₂ is exhaled → breathing rate increases

• Rising H⁺ stimulates respiratory centre → hyperventilation

• Hyperventilation reduces CO₂ → compensatory respiratory alkalosis

• Compensation is rapid but limited (cannot fully correct H⁺ rise)

what is the treatment of non-respiratory acidosis?

treat underlying problem

- maintain renal function

- Bicarbonate used in patients with pH lower than 7, with constant arterial [H+] monitoring

- chronic acidosis can develop if not corrected

What are the Causes of respiratory acidosis?

• Airway obstruction

• Depression of respiratory centre

• Neuromuscular disease

• Pulmonary disease

• Extra-pulmonary thoracic disease

Key characteristic of respiratory acidosis

• Increased PCO₂ (hypercapnia)

• Coexists with other patterns to confirm the acidosis is respiratory

what are the two types of respiratory acidosis?

• acute

• chronic

What happens to pH, H⁺, CO₂, and HCO₃⁻ in acute respiratory acidosis?

• pH: ↓ (decreases)

• [H⁺]: ↑ (increases)

• PCO₂: ↑ (increases)

• [HCO₃⁻]: Slight ↑ (early renal compensation starts)

What happens to pH, H⁺, CO₂, and HCO₃⁻ in chronic respiratory acidosis?

• pH: slightly ↓ / low normal

• H⁺: slightly ↑ / high normal

• CO₂: ↑ (still high)

• HCO₃⁻: ↑ (kidneys fully compensate)

what is the rate of respiratory response relative to metabolic response?

- respiratory response to metabolic change is rapid

- metabolic response to respiratory change is slow

what is the treatment in respiratory acidosis?

- oxygen main issue, patient will die from hyperoxemia well before hypercapnia

- restoring O2 levels essential with mask/ventilation

What happens in non-respiratory (metabolic) alkalosis?

• increase in bicarbonate levels

• Effect: As [HCO₃⁻] rises → [H⁺] decreases → pH ↑ (alkalosis)

What are the alkali-related casues of non-respiratory (metabolic alkalosis)?

• excessive alkali adminstration

• overcorrection of acidosis

• excess ingestion.

what are the loss-related and other casuses of non-respiratory alkalosis( metabolic)

• loss of free h+ : GI loss (vomitting )

• renal loss: diuretics

• potassium depeltion

• overly rapid correction of chronic respiratory acidosis

what happens to pH, [H+], PCO2 and [HCO3-] in non respiratory alkalosis?

[H+] - decrease

- pH - increase

- PCO2 - increase - rapid resp compensation response

- [HCO3-] - major increase

what is the treatment of non respiratory alkalosis?

usually correction of vol depletion with normal saline infusion

- potassium supplements often provided - as retaining Na+ and losing K+

[H+] = K PCO2/[HCO3-]

What happens in respiratory alkalosis?

• ↓ PCO₂ (hypocapnia)

• From the equation: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Effect: As PCO₂ falls → [H⁺] decreases → pH ↑ (alkalosis)

• Related formula: [H⁺] = K × PCO₂ / [HCO₃⁻]

What causes respiratory alkalosis?

• 1)Hypoxia (↑ respiratory drive → ↓ CO₂):

  • High altitude

  • Anaemia

• 2) Increased respiratory drive:

  • Infections or trauma

  • Hyperventilation

what happens to pH, [H+], PCO2 and [HCO3-] in ACUTE respiratory alkalosis?

- pH - increase

- [H+] - decrease

- PCO2 - decrease

- [HCO3-] - slight decrease

what is the treatment of respiratory alkalosis?

- remove underlying cause

- in cases of hyperventilation, rebreathing CO2 best option - brown paper bag approach