A patient with septic shock has a HIGH cardiac output but a low blood pressure. Explain the mechanism. ANSWER: systemic vascular RESISTANCE has collapsed. BLOOD PRESSURE = CARDIAC OUTPUT × SYSTEMIC VASCULAR RESISTANCE In sepsis, inflammatory mediators cause widespread VASODILATATION. Resistance falls sharply. The heart compensates by increasing output — hence the high cardiac output — but the fall in resistance outweighs it, so pressure is still low. THIS IS DISTRIBUTIVE SHOCK. Blood is present and the pump works; it is distributed into a container that has become too large. TREATMENT IMPLICATION: fluid helps fill the enlarged container, but a VASOCONSTRICTOR addresses the actual defect. Contrast with cardiogenic shock, where output is LOW and more fluid is harmful.

Why does a fluid bolus improve a hypovolaemic patient but cause pulmonary oedema in a patient with heart failure? ANSWER: because of where each sits on the FRANK-STARLING curve. 1. Stretching cardiac fibres before contraction makes the contraction stronger — so more filling gives more output. 2. The relationship is a CURVE that FLATTENS. 3. A hypovolaemic patient sits on the STEEP part: extra volume produces a real rise in cardiac output. 4. A failing heart sits on a FLAT part: extra volume adds almost no output. 5. But the pressure behind the ventricle still rises, and is transmitted back into the pulmonary circulation. RESULT: the same fluid becomes cardiac output in one patient and PULMONARY OEDEMA in the other.

A dehydrated elderly patient on an ACE inhibitor and a diuretic is given an NSAID for back pain. She develops acute kidney injury. Explain. ANSWER: all three drugs attack a different support of glomerular filtration — the "triple whammy". FILTRATION depends on pressure across the glomerulus, set by the tone of the AFFERENT (in) and EFFERENT (out) arterioles. DIURETIC reduces the circulating volume arriving at the kidney NSAID blocks prostaglandins that DILATE the afferent → less blood enters ACE INHIBITOR blocks angiotensin II that CONSTRICTS the efferent → pressure inside the glomerulus falls Each alone is usually tolerated. Together, every compensation the kidney would use to defend filtration has been removed. CLINICAL RULE: this is why these drugs are held during acute illness or dehydration.

A blood gas shows a LOW pH and a LOW bicarbonate. What is the primary disorder, and what would you expect the CO₂ to show? ANSWER: METABOLIC ACIDOSIS, with expected respiratory compensation. REASONING — the value that moved in the SAME direction as the pH is the culprit: pH low + bicarbonate low → metabolic acidosis (pH low + CO₂ HIGH would have been respiratory acidosis) EXPECTED COMPENSATION: the lungs blow off CO₂ to reduce acid, so CO₂ should be LOW. This happens within minutes. TWO CHECKS: - Compensation NEVER OVERSHOOTS. If the pH were above normal, there would be a second disorder. - If the CO₂ were normal or high, the respiratory compensation is absent or failing — which is a serious sign.

A patient has a low thyroid hormone level. The pituitary hormone driving the thyroid is ALSO low. Where is the lesion? A. The thyroid gland B. The pituitary C. Autoimmune thyroid destruction D. Iodine deficiency ANSWER: B — the pituitary. This is SECONDARY hypothyroidism. REASONING FROM FEEDBACK: In a working axis, the pituitary hormone and the gland hormone move in OPPOSITE directions, because the gland's hormone restrains the pituitary. GLAND failure → gland hormone LOW, pituitary hormone HIGH (the pituitary is shouting at a gland that cannot respond) PITUITARY failure → BOTH LOW — nobody is shouting Here both are low, so the controller is broken. A, C and D are all causes of PRIMARY gland failure, which would raise the pituitary hormone rather than lower it.

A severely anaemic patient has an oxygen saturation of 100%. Is oxygen delivery adequate? ANSWER: not necessarily. SATURATION IS NOT DELIVERY. Oxygen delivery depends on THREE things: CARDIAC OUTPUT × HAEMOGLOBIN CONCENTRATION × SATURATION Saturation describes the PERCENTAGE of available haemoglobin that is carrying oxygen. It says nothing about HOW MUCH haemoglobin exists. So a profoundly anaemic patient can saturate fully — every one of their few haemoglobin molecules is loaded — while total oxygen delivered to the tissues is dangerously low. CLINICAL POINT: pulse oximetry can be falsely reassuring in anaemia and in haemorrhage. Look at the haemoglobin and the perfusion, not the saturation alone.

Why does serum potassium fall during treatment of a severe acidosis? ANSWER: because correcting the pH moves potassium back INTO cells. 1. Most body potassium is INTRACELLULAR. 2. In acidosis, potassium tends to shift OUT of cells into the serum. 3. So the measured serum potassium can appear adequate — or even raised — while TOTAL BODY potassium is depleted. 4. Treating the acidosis reverses the shift; potassium re-enters cells and the serum level FALLS, sometimes sharply. CLINICAL CONSEQUENCE: potassium must be monitored closely and replaced during treatment of a severe acidosis. The management detail is covered in the diabetic emergencies chapter — the point here is the mechanism that makes it necessary.