Abelow, Understanding Acid-Base, Williams & Wilkins 1998 The acid base “balance”

(Abelow B, 1998 “Understanding Acid-Base”)

Neuromuscular chain defects -may alter alveolar ventilation

(Abelow B, 1998 “Understanding Acid-Base”) Pulmonary diseases -may alter alveolar ventilation

HENDERSON-HASSELBALCH EQUATION pH = pK + log [HCO 3 - ]/0.03PCO 2 pH = log 24/(0.03 x 40) pH = log 24/1.2 pH = log 20 pH = pH = 7.4 (Abelow B, 1998 “Understanding Acid-Base”)

Abelow, Understanding Acid-Base, Williams & Wilkins 1998 Renal bicarbonate reabsorption

(Abelow B, 1998 “Understanding Acid-Base”) Distal nephron luminal H+/K+ exchanger

Abelow, Understanding Acid-Base, Williams & Wilkins 1998

RESPIRATORY ACIDOSIS - Alveolar hypoventilation : acute airway obstruction with underventilation - Late acute asthma, acute COPD : CNS - opiate overdose : CNS - opiate overdose - stroke - neuropathy, myopathy

UNCOMPENSATED RESPIRATORY ACIDOSIS Abelow, Understanding Acid-Base, Williams & Wilkins 1998 COMPENSATED RESPIRATORY ACIDOSIS

RESPIRATORY ALKALOSIS - Alveolar hyperventilation : Early acute asthma with over ventilation : Pulmonary embolus, pneumonia, pulmonary oedema : Anxiety

RESPIRATORY ALKALOSIS Abelow, Understanding Acid-Base, Williams & Wilkins 1998 Uncompensated Compensated

METABOLIC ACIDOSIS METABOLIC ACIDOSIS - xs production of H + ions : Diabetic ketoacidosis : Acute renal failure : Circulatory shock (eg septic, cardiogenic, hypovolemic)

METABOLIC ACIDOSIS WITH RESPIRATORY COMPENSATION Abelow, Understanding Acid-Base, Williams & Wilkins 1998 UNCOMPENSATED METABOLIC ACIDOSIS

METABLOIC ALKALOSIS - xs HCO 3 - ions : Loss of gastric fluid – vomiting : Diuretics-K+ loss: xs renal HCO 3 reabsorption :Post hypercapnic mechanical ventilation

METABOLIC ALKALOSIS Abelow, Understanding Acid-Base, Williams & Wilkins 1998

When you see “respiratory”, think PCO 2 and When you see “metabolic”, think [HCO 3 - ] Abelow, Understanding Acid-Base, Williams & Wilkins 1998

NameChangeDescription Respiratory acidosis  PCO 2 Hypercapnic acidosis Respiratory alkalosis  PCO 2 Hypocapnic alkalosis Metabolic acidosis  [HCO 3 - ] Hypobicarbonatemic acidosis Metabolic alkalosis  [HCO 3 - ] Hyperbicarbonatemic alkalosis Abelow, Understanding Acid-Base, Williams & Wilkins 1998

EVALUATION OF BLOOD GASES Abelow, Understanding Acid-Base, Williams & Wilkins 1998

RESPIRATORY FAILURE Type 1:  PaO 2  PaCO 2 - Alveolar hyperventilation Type 2:  PaO 2  PaCO 2 - Alveolar hypoventilation

ACUTE ASTHMA Early:Alveolar hyperventilation -  respiratory drive  PaO 2  PaCO 2  give high concentration of O 2 (60%) Late:Alveolarhypoventilation -  respiratorydrive  PaO 2  PaCO 2 : still relying on hypercapnic drive  give high concentration of O 2 (60%) - may need mechanical ventilation

ACUTE EXACERBATION OF COAD Chronic alveolar hypoventilation -  respiratory driveChronic alveolar hypoventilation -  respiratory drive - switch from hypercapnic to hypoxic drive  Use low concentration of O 2 (24%) to avoid suppressing hypoxic drive  Can use central respiratory stimulation (doxapram) to permit higher concentration O 2 (28-35%)

ARTERIAL BLOOD GASES IN ACUTE ASTHMA Late Stage = Fatigue = Alveolar hypoventilation Early Stage = Alveolar hyperventilation If high PaCO 2 (> 6KPa) and low PaO 2 (< 8KPa) at presentation, or if rising PaCO 2 and falling PaO 2 despite treatment  mechanical ventilation (ie call anaesthetist) Always use high flow O 2 mask (> 60% inspired concentration) in acute asthma - even if high PaCO 2 - as patient still relying on hypercapnic drive PaO 2 PaCO 2 1KPa = 7.5 mm Hg