Dialysis, diuretics and nursing monitoring in renal disease

Roughly two thirds of the sodium filtered at the glomerulus is reclaimed in the proximal convoluted tubule, but no commonly used diuretic is powerful enough to block that segment safely, and a drug that tried would disturb acid–base balance as well. The clinically useful agents are therefore aimed further downstream, at the segments that handle a smaller but decisive share of sodium. The thick ascending limb of the loop of Henle reabsorbs about a quarter of the filtered sodium and is the site where loop diuretics act by blocking the sodium–potassium–2 chloride cotransporter, the carrier protein that pulls these three ions out of the tubular fluid together. Blocking it does two things at once: it washes out the medullary concentration gradient that the countercurrent multiplier builds, so the collecting duct can no longer concentrate urine, and it delivers more sodium and fluid to the distal nephron. That is why loop diuretics such as furosemide, bumetanide and torsemide are the most powerful class and the only one that works well when the glomerular filtration rate is very low, and it is also why they cause a greater loss of potassium, magnesium and calcium in the urine.

The early distal convoluted tubule reabsorbs around five to eight per cent of the filtered sodium through a sodium–chloride cotransporter, and thiazide diuretics block exactly that carrier. Their effect is smaller but longer-lasting, they act on a segment that is less dependent on how much sodium arrives, they reduce calcium excretion rather than increasing it — which is why they are preferred in patients who form calcium stones — and they are the standard first choice for uncomplicated hypertension. They also tend to raise blood glucose, uric acid and cholesterol, which is why monitoring must include diabetes, gout and blood lipids. The late distal tubule and collecting duct handle only one to three per cent of the filtered sodium, but this is the segment controlled by aldosterone, and it is where potassium is secreted in exchange for sodium. Spironolactone and eplerenone block the aldosterone receptor, and amiloride and triamterene block the sodium channel itself; both leave potassium in the body, which is their clinical value and their danger.

The nursing consequences follow directly from the pharmacology. A loop diuretic given in the morning removes the fluid while the patient is up and near a toilet, and its potassium loss is why the level is checked after any dose change; a thiazide prescribed for hypertension may take several weeks to produce its full effect and should not be judged on a single reading; a potassium-sparing agent started in a patient already taking an ACE inhibitor, an angiotensin receptor blocker or a potassium supplement can cause dangerous hyperkalaemia within days, so the potassium is checked early rather than after a routine interval. Two practical comparisons make the classes memorable. First, the more sodium a segment reabsorbs, the more powerful the drug that blocks it, so the ranking loop, thiazide, potassium-sparing is a fact of anatomy rather than of marketing. Second, the site also decides which mineral is lost, so the same class that wastes potassium also wastes magnesium and calcium, while its weaker cousin conserves calcium and can be chosen deliberately for that reason.

Dialysis rests on the same physical rules that govern the tubule, but applied across an artificial barrier instead of a living one. Diffusion, osmosis and ultrafiltration are the three principles on which haemodialysis is based. Waste products leave the blood by diffusion because they move from the higher concentration in the plasma to the lower concentration in the dialysate, a solution made up of all the important electrolytes in their ideal extracellular concentrations. Osmosis describes the movement of water across the membrane when the two sides differ in solute concentration, and ultrafiltration is the removal of water that is driven by the pressure difference across the membrane rather than by concentration alone. The dialysate is therefore not a single fluid but a prescription: by adjusting the dialysate bath, the electrolyte level in the patient’s blood can be brought under control, so a patient arriving with a potassium of 6 mmol/L is dialysed against a low-potassium bath to pull potassium out, while a patient who is acidotic is dialysed against a bath containing bicarbonate as a buffer.

The membrane itself sets the boundary. It is semipermeable, so it allows toxins, fluid and electrolytes to pass while impeding the diffusion of large molecules such as red blood cells and proteins, exactly as the glomerular filter holds them back. Modern dialyzers are hollow-fibre devices containing thousands of tiny straw-like tubes through which the blood is carried; the tubes are porous and act as the membrane, and the constant flow of dialysate on the outside maintains the concentration gradient so that exchange continues along the whole length rather than stopping once the fluid equilibrates. Access has to deliver a flow of roughly 300 to 800 millilitres per minute, far more than an ordinary vein can supply, which is why an arteriovenous fistula is created by joining an artery to a vein, usually in the forearm, and why it needs two to three months to mature: the venous segment dilates under the arterial pressure until it can accommodate two large-bore needles, and hand exercises are taught to speed that enlargement. The fistula has the longest useful life of any access, which is why it is preferred for chronic treatment, while a double-lumen non-cuffed large-bore catheter placed in the subclavian, jugular or femoral vein gives immediate access for acute dialysis with the risks of haematoma, pneumothorax, infection, thrombosis and inadequate flow.

What dialysis does not do matters just as much for the nurse. It prevents death but does not cure renal disease, and it replaces only the filtration and fluid-removal work of the kidney, not its endocrine or metabolic activity. Because the kidney is also the source of erythropoietin, of renin and of activated vitamin D, anaemia, hypertension and renal bone disease continue and may worsen, which is why calcium metabolism deteriorates, why vascular calcification is reported with time, and why hypertriglyceridaemia is accentuated. Cardiovascular disease remains the leading cause of death in patients receiving dialysis, and dialysis itself adds hazards: blood is lost in the circuit, so anaemia is compounded, and peritoneal dialysis adds its own burden, since peritonitis — infection of the membrane lining the abdominal wall — abdominal muscle weakening, hernia, high blood sugar from the dextrose in the dialysate and weight gain are all recognised risks. Two quantitative facts anchor the nursing mind. First, the weight difference before and after a session is the amount of water ultrafiltration removed, so a target weight is a prescription and not a statistic. Second, if the blood lines separate or a needle becomes dislodged, the patient can exsanguinate within minutes through tubing that carries hundreds of millilitres per minute, which is why the circuit is never left unobserved.

The same machine serves two very different patients, and the difference changes what the nurse expects. Acute dialysis is used for a patient who is acutely ill and needs short-term treatment lasting days to weeks, for example after a severe kidney injury, and the goal is to support the patient until the kidney recovers. Chronic dialysis is used for advanced chronic kidney disease and end-stage renal disease, and it is long-term or permanent renal replacement therapy: haemodialysis prevents death indefinitely but does not alter the natural course of the underlying disease and does not completely replace kidney function, so the complications of chronic kidney disease continue and demand more aggressive treatment as time goes on.

Two consequences follow for nursing. First, the patient on maintenance dialysis is not a patient whose kidney problem is solved; he is a patient whose other problems — anaemia, bone disease, malnutrition, cardiovascular disease and sleep disturbance — are still progressing. Second, because dialysis only substitutes for filtration, the dose of every medicine must be re-examined for renal toxicity and for potassium and magnesium content, and a symptom the patient reports must never be attributed automatically to renal failure or to the treatment.

The specific complications fall into groups. Vascular and access problems include clotting of the circuit, bleeding or exsanguination if a needle is dislodged or the lines separate, and access infection. Cardiac and respiratory problems include hypotension during treatment with nausea, vomiting, diaphoresis, tachycardia and dizziness as its signs; pericarditis from retained toxins, detected by substernal chest pain, low-grade fever and a pericardial friction rub, which can progress to a pericardial effusion and cardiac tamponade; chest pain in patients with anaemia or coronary disease; and episodes of shortness of breath as fluid accumulates between sessions. Neurological problems include dialysis disequilibrium, caused by cerebral fluid shifts and more likely in acute renal failure or when the blood urea nitrogen exceeds about 150 mg/dL, a value whose units are easily missed; converted to the units used in most laboratories, that value is approximately 54 mmol/L of urea, and the syndrome presents with headache, nausea and vomiting, restlessness, a decreased level of consciousness and seizures. Gastrointestinal and nutritional problems include gastric ulcers, vomiting during treatment, the metallic taste and nausea of uraemia, malnutrition with hypoalbuminemia as its marker, and the accentuation of hypertriglyceridaemia. Musculoskeletal and skin problems include bone pain and fractures from renal osteodystrophy, itching from phosphorus deposited in the skin, and muscle cramping that comes late in the session as fluid and electrolytes leave the extracellular space quickly. Finally, up to 85 per cent of people undergoing haemodialysis experience major sleep problems, and early-morning or late-afternoon treatment times are reported as a risk factor.

The nurse’s part is not to memorise that list but to know which item each observation belongs to. A patient who becomes restless and confused near the end of a first session is not simply tired: that is the disequilibrium pattern, and it is reported before it progresses to a seizure. A patient with a temperature and a painful, red access site has an access infection, not a cold. A patient who is hypotensive, sweating and cramping in the last hour needs the ultrafiltration rate reviewed, not more antihypertensive medicine. Because patients on dialysis commonly have low white cell counts, low red cell counts and impaired platelet function, infection and bleeding after even minor trauma are more likely than in other patients, so preventing infection and protecting the access are continuing priorities rather than tasks of the treatment hour alone.