RT (CRT/RRT) common mistakes
RT (CRT/RRT) · Understand the traps and how to avoid them.
Given an ABG of pH 7.36, PaCO2 62 mmHg, HCO3 34 mEq/L, the candidate answers 'normal' or 'uncompensated respiratory acidosis' instead of fully compensated respiratory acidosis.
They stop interpreting as soon as the pH falls inside the 7.35-7.45 window instead of checking whether the PaCO2 and HCO3 are both abnormal in the direction compensation rules predict. Stems are deliberately written with a pH that sits just inside the reference range.
What to do instead
Run the same grid on every gas, in order: (1) Is the pH acidemic, alkemic, or normal? (2) Which side of 40 is the PaCO2 on? (3) Which side of 24 is the HCO3 on? (4) Ask whether the pH is normal but CO2 and HCO3 are both abnormal in opposite directions, which confirms that the disorder is compensated. Name the disorder by the primary value. For a chronic CO2 retainer, check that the pH is shifted toward normal and that the HCO3 has risen.
On a hypoxemia item, they jump to 'increase the FiO2' or 'it's a shunt' without ever calculating the P(A-a)O2 gradient, or they plug FiO2 in as 21 instead of 0.21 and derive an impossible PAO2.
They memorized 'normal PaO2 is 80-100' but not the alveolar air equation or the age-based expected PaO2. The stem hands them barometric pressure and an FiO2 with decimals specifically so the calculation can be done, and they read past it.
What to do instead
Write the equations before you start answering: PAO2 = (PB - 47) x FiO2 - (PaCO2 / 0.8); expected PaO2 is about 103 - (age/3); P(A-a)O2 = PAO2 - PaO2. A normal gradient with hypoxemia points to hypoventilation; an elevated gradient that corrects on 100% O2 points to V/Q mismatch; one that stays wide on 100% O2 points to shunt. Keep FiO2 as a decimal throughout and convert only at the end.
Cylinder-duration items: they divide the gauge pressure by the flow (e.g., 2000 / 6) or use the full gauge pressure with no safe residual, and omit the cylinder factor entirely.
Duration needs three separate conversions (psi to liters via the tank factor, subtraction of roughly 200 psi residual, division by L/min), and candidates collapse them into one step to save time.
What to do instead
Use a single memorized formula, then run it in order: minutes = [(gauge psi - 200) x cylinder factor] / flow in L/min. Know the factors well enough to spot them (E ≈ 0.28, M ≈ 1.56, G ≈ 2.41, H/K ≈ 3.14). Sanity-check the units: if an E cylinder at 2000 psi is said to last hundreds of minutes at 6 L/min, you skipped the factor.
They order a 40% air-entrainment mask (or a large-volume nebulizer at 8 L/min) for a tachypneic patient without checking total flow against peak inspiratory demand, and they document 'nasal cannula 6 L/min = 44%' as though a low-flow device delivers a fixed FiO2.
The device-performance table gets memorized as device-to-FiO2 pairs. Candidates never learn that an air-entrainment device holds its ratio only while total flow exceeds demand, and that low-flow FiO2 falls as inspiratory flow and rate rise.
What to do instead
For air-entrainment devices, compute total flow = oxygen flow x (1 + air:O2 ratio), with approximate ratios of 25:1 at 24%, 10:1 at 28%, 5:1 at 35%, and 3:1 at 40% (so 8 L/min driving a 40% device yields about 32 L/min). If total flow is below demand, or the patient is tachypneic or talking, switch to high-flow nasal cannula or a higher-flow device. For low-flow devices, treat FiO2 as an estimate and verify with SpO2 or an ABG.
They pick a postural drainage position from memory of the diagrams and place the patient head-down without screening for raised ICP, recent hemoptysis, uncontrolled hypertension, or aspiration risk, or they treat 'lower lobes' as a single position instead of naming the segment.
Positioning items combine anatomy, secretion location from the film or auscultation, and a contraindication list. Candidates memorize the positions but skip the contraindication screen, and the correct answer is often to modify or abandon the classic position.
What to do instead
Say the target segment out loud from the stem's film or breath sounds, then run a 3-second screen: head-down tilt is out with elevated ICP, recent hemoptysis, uncontrolled hypertension, esophageal or upper GI surgery, or unprotected aspiration risk. If the classic position is contraindicated, the right answer is the modified position (flat or head elevated) or another clearance modality plus suction, not the textbook position.
They answer the airway clearance item by naming one technique and stopping: no bronchodilator first when bronchospasm is present, no oxygen or SpO2 monitoring during the session, no auscultation or sputum assessment afterwards.
They studied bronchial hygiene as a list of devices rather than a process, so they don't notice that the distractors describe only the middle step of the encounter.
What to do instead
Structure every bronchial hygiene answer as a sequence: assess (breath sounds, sputum, SpO2, ability to cooperate) -> pre-treat (bronchodilator if indicated, oxygen to keep SpO2 in target) -> perform (position or technique, coach huff coughing, watch for fatigue and desaturation) -> reassess (auscultate, note sputum volume and character, compare to baseline). If all choices describe only the technique itself, the correct one includes assessment or reassessment.
They place an oropharyngeal airway in a patient who is still coughing and gagging, or choose a nasopharyngeal airway for a patient with a basilar skull fracture, coagulopathy, or active epistaxis, and they size the nasal airway by eye.
'Airway = airway' recall takes over, so the gag-reflex rule and the NPA contraindication list, which are what the stem is actually testing, never get applied.
What to do instead
Use a two-step rule. Intact gag and cough reflexes -> NPA, measured from nostril to earlobe/angle of mandible (then one size down if needed for the nares), lubricated and directed along the floor of the nose. Absent reflexes -> OPA, measured from corner of mouth to angle of mandible. Screen NPA against basilar skull fracture, severe coagulopathy, and nasal obstruction or epistaxis.
They apply suction while inserting the catheter, suction 15-20 seconds per pass, run the vacuum at 160-200 mmHg on an adult, choose a catheter that nearly fills the tube, and suction a hypoxemic patient without pre-oxygenating.
Suctioning is treated as a task ('clear the tube') rather than a procedure with a size formula, pressure limits, and real hypoxemia, atelectasis, and bradycardia risk. Urgency in the stem pushes them toward the biggest catheter and the most vacuum.
What to do instead
Convert each number into a checklist item: catheter French size ≈ 2 x ETT internal diameter in mm (about 14 Fr for a 7.0 mm tube); adult vacuum 80-120 mmHg (neonate about 60-80); insert without suction and apply only on withdrawal; 10-15 seconds maximum per pass and no more than 2-3 passes with recovery between; pre-oxygenate with 100% for 30-60 seconds and monitor SpO2 and ECG, stopping for desaturation or bradycardia.
They read a high peak inspiratory pressure as 'decreased compliance' and answer 'reduce tidal volume,' without checking the plateau, and repeat that answer even when the peak has risen 15 cmH2O while the plateau is unchanged.
Both numbers sit on the same screen, so peak gets treated as a generic 'pressure' rather than a combined resistance-plus-compliance measurement. The stem supplies the pair specifically to force the distinction.
What to do instead
Never interpret peak without plateau. Get the plateau from a 0.5-2 second inspiratory hold. Peak high with a normal plateau and a gradient above 10 cmH2O means increased airway resistance (secretions, bronchospasm, kinked or bitten tube) - treat the airway. Peak high with plateau also high means decreased compliance (pneumothorax, atelectasis, ARDS, abdominal distension, mainstem intubation) - that is when tidal volume and the cause of the stiffness matter.
Faced with a ventilated patient who is hypotensive, tachypneic, and showing rising peak pressures, they answer 'sedate and increase the FiO2' and miss breath stacking/auto-PEEP; when asked to quantify it they compute nothing or subtract in the wrong direction.
Auto-PEEP usually has no digital readout in the values list candidates scan. It has to be measured with an end-expiratory hold or inferred from expiratory flow that never returns to zero, so it is invisible to a candidate who reads numbers only.
What to do instead
When hypotension, rising pressure, and a fixed set rate with a tachypneic patient occur together, suspect air trapping. Measure total PEEP with an end-expiratory hold and subtract the set PEEP (auto-PEEP = total - set), and inspect the flow waveform for expiratory flow that does not reach zero before the next breath. Then act in order: treat the cause (bronchodilator, suction), lengthen expiratory time by lowering the set rate and shortening inspiratory time, adjust trigger sensitivity to stop stacking, and recheck blood pressure and pressures after each change.