ABG

Question 1

Discuss differential diagnosis for respiratory alkalosis

Answer 1

Stimulated respitaroty drive

• CNS ( CVA, ICH, psychogenic)
• Hypermetabolic (Thyrotoxicosis, pregnancy, sepsis, DT, anxiety, pain, DKA)
• Environmental (hyperthemia)
• Drugs (aspirin, progesterone)
• Liver failure with hyperammonaemia

Hypoxemia induced

• Pneumonia, PE, asthma
• Congenital heart disease
• Chronic altitude compensation
• Early altitude acclimatization

Compensation for metabolic acidosis

Question 2

Discuss differential for respiratory acidosis

Answer 2

Decreased respitatory drive

• CNX (CVA, tumour, infection, haemorrhage)
• Drugs (narcotics and sedatives)

Decreased chest wall movement

• Neurological (NM disorders, GB, Myasthenia, demyelinating disorders, tetanus)
• Tox (muscle relaxants, organophosphates, fentnayl)
• Respiratory (trauma, tension, pleural effuiosn, upper airway obstruction )
• Equipemtn (increased dead space, improper conncetion)

Obstructive pulmonary disease
-COPD, asthma, pneumonia

Question 3

Discuss matabolic alkalosis

Answer 3

Requires an initiating process and a maintenance process

Initiating process

1) gain of alkaline in the ECF
- exogenous source (HCO3 infusion, citrate in transfused blood)
- endogenous source( metabolism of ketoanions to produce hco3)
2) loss of H+ from the ECGF
- kidneys (via diuretics)
- GIT ( via vomiting or NG suction)

Maintenance of alkalosis

• requires a process which greatly impairs the kidneys ability to excrete hco3 ions and return elevated levels to normal
• Four factors that cause maintenance of the alkalosis are
  1) chloride depletion
  2) potassium depletion
  3) reduced GFR
  4) ECF volume depletion

Question 4

Discuss chloride depletion as maintenance of metabolic alkalosis

Answer 4

Commonest cause
Administration of chloride is necessary to correct these disorders
The two commonest causes are
1) gastric loss alkalosis
-most marked with vomiting due to pyloric stenosis or obstruction becuase the vomitus is acidic gastric juice only
-vomiting in other condition is mixed with alkaline duodenal contents and the acid base disturbance is more variable

2) diuretics
- frusemide and thiazides interfere with reabsorption of chloride and sodium in the renal tubules
- urinary loss of chloride exceed those of bicarb
- patients who develop alkalosis on diuretics are also volume depleted and have low dietary chloride intake
- hypokalaemia is also common in these patients

Question 5

Discuss potassium depletion as a maintenance of metabolic alkalosis

Answer 5

• Potassium depletion occurs with mineralcorticoid excess
• Hco3 reabsorption in both the proximal and distal tubules is increased in the presence of potassium depeltion
• Potassium depletion decreases aldosterone release by the adrenal cortex

Causes

1) Primary hyperaldosteronisms
- leads to increased distal tubular Na reabsorption and increased K+ and H+ losses
- increased H+ loss is matched by increased amounts of renal Hco3 leaving in the reanl vein
- the net is metabolick alkalosis with hypochloraemia and hypokalaemia

2) Cushings
3) Severe K depletion

4) Barter’s syndrome
- syndrome of increased Renin and aldosterone levels due to hyperplasia of the juxtagolmerular apparatus
- inherited as an autosomal recessive disorder usually found in children
- the increased aldosterone levels usually result in a metabolic alkalosis

Question 6

Discuss the utility of urinary chloride in the management of metabolic alkalosis

Answer 6

1) urine CL <10
- often asscoaited with voume depletion
- responds well to saline infusion
- causes previous diuretic therapy or vomting

2) urine CL >20mmol
-often assocaited with volume expansion and hypokalaemia
-resistant to therapy with saline infusion
-Causes - excess aldosterone, severe K defieicny, diuretich therapy, Bartters syndrome
D

Question 7

What is the formula for respiratory compensation for metabolic alkalosis

Answer 7

0.7 Hco3 +20 to a max of 55-60