ACID – BASE DISORDERS [Case-based Discussion]

PREAMBLE pH Buffers of Body Significance of [H+] ion conc. pH of Body Fluids [plasma, gastric juice, pancreatic juice]

NaHCO3 = HCO3 = Salt = 20 H2CO3 PCO2 Weak Acid 1 = Metabolic = Dealt by Kidney = X =  Resp Dealt by Lungs  X Buffer =

Cases PCO2 [mmHg] [HCO3] mmol/L pH H+ nmol/L A [ ? ] 25 15 7.4 40 B [Normal] 38 C [ ? ] 45 35 7.5 30

Parameters A B PCO2 [mmHg 25 40 [HCO3] mmol/L 15 pH 7.4 H+ nmol/L 38

METABOLIC ACIDOSIS Renal Failure Diabetic Acidosis NH4Cl Ingestion K+ injections

METABOLIC ALKALOSIS NaHCO3 Ingestion Prolonged Vomiting Too much intake of Antacids K+ Deficiency

RESPIRATORY ACIDOSIS Asphyxia: Drowning Strangulation Emphysema

RESPIRATORY ALKALOSIS Voluntary Hyperventilation Ascent to High Altitude Unacclimatised

Patient A is a 57-year-old man who has been depressed for some time Patient A is a 57-year-old man who has been depressed for some time. In a suicide attempt early that morning he had swallowed the entire contents of a bottle of aspirin [salicylate]. His respiration is appreciably increased and an arterial blood sample gives the following results: Q’s: 1. Which is the type of Acid base upset? 2. Is there any nature of any compensatory response? 3. Which is the likely cause underlying the acid base upset? pH = 7.30 PCO2 = 4kPa [30mmHg] standard [HCO3- ] = 16mmolL-1

Answer Patient A There is a metabolic acidosis (reduced pH and low standard [HCO3- ]) There has been respiratory compensation [increased respiration, reduced PCO2 ]. The cause is gastrointestinal absorption of salicylic acid.

Patient B is a 76-year-old woman who is suffering from an acute exacerbation of chronic obstructive airways disease precipitated by a lower respiratory tract infection. She appears centrally cyanosed [Section 4.7]. Arterial blood analysis gives the following results: Q’s: 1. Which is the type of Acid base upset? 2. Is there any nature of any compensatory response? 3. Which is the likely cause underlying the acid base upset? pH = 7.30 PCO2 = 11kPa [83mmHg] standard [HCO3- ] = 34mmolL-1

Answer Patient B There is a respiratory acidosis [reduced pH and raised PCO2 ]. There is metabolic compensation (raised standard [HCO3- ]). The cause is reduced ventilation caused by chronic obstructive airways disease.

Patient C is a 40-year-old man who is known to be an insulin-dependent diabetic. He was unconscious at admission and is found to be extremely dehydrated. Arterial blood analysis is as follows: Q’s: 1. Which is the type of Acid base upset? 2. Is there any nature of any compensatory response? 3. Which is the likely cause underlying the acid base upset? pH = 7.10 PCO2 = 3.3kPa [25mmHg] standard [HCO3- ] = 10mmolL-1

Answer Patient C There is a metabolic acidosis [reduced pH and reduced standard [HCO3- ]. There is respiratory compensation [reduced PCO2 ]. The cause is likely to be diabetic ketoacidosis caused by insulin deficiency.

Patient D is a 16-year-old girl who is complaining of breathlessness of sudden onset. She appears agitated and is breathing very rapidly but there is no other obvious abnormality on examination. Arterial blood analysis gives the following results: Q’s: 1. Which is the type of Acid base upset? 2. Is there any nature of any compensatory response? 3. Which is the likely cause underlying the acid base upset? pH = 7.55 PCO2 = 3.3kPa [25mmHg] standard [HCO3- ] = 24mmolL-1

Answer Patient D There is a respiratory alkalosis [elevated pH and reduced PCO2 ]. There is no metabolic compensation (normal standard [HCO3- ]). The cause is likely to be hysterical overbreathing.

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