Acidosis, Hyperkalaemia and a Painkiller in Chronic Kidney Disease
A 52-year-old man from Cuttack is admitted with weakness and palpitations. He has taken ibuprofen daily for six months for knee pain and eats a high-protein diet. He is not diabetic and has had no diarrhoea. Ultrasound shows small, echogenic kidneys bilaterally. Investigations: plasma creatinine 4.8 mg/dL (reference 0.6–1.2), urea 96 mg/dL, Na⁺ 132 mEq/L, K⁺ 6.3 mEq/L, pH 7.26, HCO₃⁻ 12 mEq/L, pCO₂ 28 mmHg, Hb 8.2 g/dL, 24-hour urine output 400 mL. Blood pressure is 158/96 mmHg. The ECG shows tall, peaked T waves and a widened QRS complex. A repeat arterial blood gas after six hours is unchanged. The resident writes up calcium gluconate, insulin with dextrose, and advice to stop ibuprofen, and asks the students to justify each step.
1. Characterise the acid–base disorder and decide whether the respiratory compensation is appropriate or whether a second disorder is present.
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The pH is 7.26 with HCO₃⁻ of 12 mEq/L, so the primary problem is a metabolic acidosis; the low pCO₂ of 28 mmHg represents respiratory compensation. To judge the compensation, compare the measured pCO₂ with the value expected for that bicarbonate: expected pCO₂ ≈ 1.5 × HCO₃⁻ + 8 ± 2 = 1.5 × 12 + 8 = 26 ± 2 mmHg. The measured 28 mmHg lies inside this range, so the hyperventilation is proportionate and no second respiratory disorder needs to be invoked. The mechanism of the acidosis is loss of functioning nephrons: filtered load falls, proximal bicarbonate reabsorption and distal hydrogen ion secretion are reduced, and ammoniagenesis is suppressed further by the hyperkalaemia, so fixed acids accumulate. The reasoning matters clinically: had the pCO₂ been about 20 mmHg, the patient would be hyperventilating beyond what the acidosis explains and a superimposed respiratory alkalosis should be sought; had it been near 40 mmHg, respiratory failure would be present. The unchanged repeat gas after six hours indicates a stable chronic picture rather than rapidly worsening acute acidosis.
2. Explain why potassium is 6.3 mEq/L although sodium is only mildly low and the patient reports a normal diet. Why is calcium gluconate given first when it does not lower serum potassium?
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Three mechanisms add up. First, the filtered load of potassium falls because GFR is low, and distal potassium secretion depends on sodium delivery and aldosterone, both reduced in advanced renal failure. Second, acidosis shifts potassium out of cells as hydrogen ions enter them. Third, ibuprofen inhibits prostaglandin synthesis and removes the prostaglandin-mediated dilation of the afferent arteriole that protects GFR in a kidney already short of nephrons, so GFR falls further and potassium rises further. The ECG changes - tall peaked T waves and a widened QRS - mean the hyperkalaemia is already threatening the myocardium. Calcium gluconate acts within minutes but does not lower serum potassium at all: it raises the threshold potential, restoring the gap between resting and threshold potentials so that the heart is protected while potassium is shifted into cells with insulin–dextrose and later removed by dialysis. The judgement is that calcium is given first when ECG changes are present, but it is membrane protection, never treatment of the potassium load itself.
3. Account for the haemoglobin of 8.2 g/dL and the bone disease that will develop, and explain why stopping ibuprofen is part of the management of the kidney rather than an unrelated step.
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The kidneys make erythropoietin from peritubular interstitial fibroblasts; with loss of functioning cortex, erythropoietin falls and the marrow cannot mount a normal response, giving a normocytic normochromic anaemia consistent with 8.2 g/dL. Uraemic toxins also shorten red cell survival and impair iron utilisation. Separately, the proximal tubule contains 1α-hydroxylase, which converts 25-hydroxyvitamin D to calcitriol; failing kidneys make too little calcitriol, so intestinal calcium absorption falls and phosphate rises, secondary hyperparathyroidism follows, and renal bone disease develops. Both defects are endocrine functions of the kidney, not excretory ones, which is why anaemia and bone disease appear alongside retained urea and potassium. Stopping ibuprofen is part of renal management because prostaglandins maintain afferent arteriolar dilation in a kidney with reduced effective perfusion; continuing the drug invites a further acute fall in GFR and a sharper rise in potassium. The judgement is that the drug history is part of the diagnosis of this admission, not an unrelated detail.