ARDS case study: ABG, ventilator settings and PEEP

Start with the arterial blood gas, because every later question depends on reading it correctly. **pH 7.31, PaCO2 58, HCO3 28, PaO2 54.** The pH is acidotic, the PaCO2 is raised, and a raised carbon dioxide with an acidotic pH is a respiratory acidosis. The bicarbonate at 28 is mildly elevated, which is the kidney beginning to retain base — partial compensation, not full, because the pH has not returned to normal. And the PaO2 of 54 on an FiO2 of 60 per cent is severe hypoxaemia: the ratio of the two is what defines acute respiratory distress syndrome and places it in the severe band. Name the disturbance, name the compensation state, and name the oxygenation failure separately — they are three findings, not one. Answering in that order also stops the classic error of calling the whole picture respiratory failure and moving on, which loses the compensation and the oxygenation marks in one sentence.

The ventilator question asks for **each setting and the rationale for selecting it**, which means going through them individually rather than describing the mode. Assist control at a set rate of 14 guarantees a minimum minute volume while allowing patient-triggered breaths. A tidal volume of 450 against a 75 kg patient is roughly six millilitres per kilogram, which is the lung-protective range for ARDS and is chosen deliberately to limit alveolar overdistension. An FiO2 of 60 per cent is being used to correct hypoxaemia while staying below the range where oxygen toxicity becomes a concern. PEEP at 5 is the starting point holding alveoli open at end expiration. Each number has a reason and the question is asking for the reasons. Working out the millilitres per kilogram yourself is worth doing, because quoting the number back without relating it to his weight does not show you know why 450 rather than 600.

That sets up the PEEP question, which is the best question in the set. Raising PEEP from 5 to 8 recruits collapsed alveoli and increases the surface available for gas exchange, so the expected outcome is a rising PaO2 — often allowing the FiO2 to be reduced afterwards, which is the real objective. The strong answer also names the cost: higher intrathoracic pressure reduces venous return and can drop cardiac output, and it raises barotrauma risk. In this patient that trade-off is not theoretical, because his peripheral pulses are already weak at 1/4 with capillary refill over four seconds. The PEEP increase may worsen a perfusion problem that is already visible in the assessment data. Naming the cost is also what makes the following question about dopamine and milrinone read as part of the same clinical story rather than as a separate topic.

Which explains the next question rather than leaving it isolated. Dopamine and milrinone are both being used for haemodynamic support and they work differently — dopamine acts on adrenergic receptors with dose-dependent effects, milrinone is a phosphodiesterase inhibitor that increases contractility while causing vasodilation, so it is an inodilator rather than a pressor. Explaining why *each* was chosen means saying what each contributes, and connecting them back to the weak pulses, the delayed capillary refill and the pitting oedema. A patient on rising PEEP with poor perfusion is exactly the situation those drugs are addressing. Milrinone in particular is worth flagging as an inodilator, because a reader who assumes both drugs raise blood pressure has missed why two agents were chosen instead of one.

Two things about the remaining questions. The nursing priorities question says **based on the assessment data**, so it wants priorities drawn from this patient's findings rather than a generic ventilated-patient list — his repeated attempts to pull at the endotracheal tube, the secretions returning brown and yellow, the crackles that do not clear with suctioning, the fever of 102, and the perfusion signs each generate a priority you can point at. And note that his recorded respiratory rate is 14, matching the set ventilator rate exactly, which is worth a comment: he is not triggering above the machine, which tells you something about his drive and his sedation. Each of those findings supports a different priority, so listing them first and converting them into priorities second is faster than trying to rank in the abstract.