CRT (CREST) common mistakes
CRT · Understand the traps and how to avoid them.
During patient-triggered breath assessment, the candidate counts every pressure dip on the waveform as a patient effort and reports an elevated triggered breath rate, even when dips coincide with circuit leaks or cardiogenic oscillation.
Trigger sensitivity is set very high in the practice scenario, and the candidate has memorized 'dip equals trigger' without learning to confirm each effort against flow and volume deflections. Oscillations from the heart or from a leak produce small pressure changes that look like triggers at a glance.
What to do instead
For each suspected trigger, verify a simultaneous negative flow deflection and a subsequent volume rise before counting it. Check leak and cardiogenic artifact by examining the timing relative to the cardiac cycle and by temporarily reducing sensitivity to see whether the dips persist. Report only confirmed efforts.
The candidate classifies a breath as a mandatory machine-initiated breath solely because the ventilator display labels it 'mandatory', without determining whether the breath was time-triggered, patient-triggered, or pressure/flow-cycled.
Modern ventilators display breath-type labels that reflect mode logic, and candidates trust the label instead of reconstructing the trigger and cycle variables from the waveforms. This is faster under time pressure and feels safe because the machine 'already decided'.
What to do instead
Independently identify the trigger (time vs pressure vs flow), the target (pressure vs volume), and the cycle criterion (time, flow percentage, or volume) for each breath before reading the label. Use the label only as confirmation, and explain out loud why the breath meets each criterion.
When asked to identify the mode, the candidate names the mode from the ventilator brand or from a single setting (for example, seeing a set rate and concluding 'SIMV') rather than from the breath sequence and control variables.
Mode names vary across manufacturers and some modes combine features, so pattern-matching on one setting is tempting. Candidates also conflate mode with the breath sequence (CMV, IMV, CSV) and with the control variable.
What to do instead
Describe the mode in three layers: control variable (pressure or volume), breath sequence (mandatory only, mandatory plus spontaneous, or spontaneous only), and the specific targeting/target scheme. Then map that description to the manufacturer's label. Practice with a table of modes described by these three layers rather than by brand name.
The candidate interprets an elevated peak airway pressure as a problem with the patient's compliance alone, without checking whether the rise is from resistance, from a patient-ventilator asynchrony, or from a circuit/secretions issue.
Peak pressure is the most visible number on the screen, and candidates anchor on it. They skip the plateau pressure and the flow waveform, which are needed to separate resistive from elastic causes.
What to do instead
Perform a structured check: measure plateau pressure (inspiratory hold) and calculate the peak-to-plateau gradient. A high gradient points to resistance (secretions, bronchospasm, tube obstruction); a high plateau points to compliance or chest-wall issues. Inspect flow waveforms for scooped or truncated patterns and correlate with auscultation.
The candidate treats an auto-PEEP finding as a reason to simply increase the set rate or minute ventilation, rather than addressing the cause and measuring the effect of the change.
Auto-PEEP is taught as a gas-exchange problem, so candidates reach for more ventilation. They also fail to perform an expiratory hold correctly, so they never quantify the intrinsic PEEP they are trying to fix.
What to do instead
Confirm auto-PEEP with an expiratory hold in a passive patient, then reduce the cause: lower respiratory rate, shorten inspiratory time, reduce tidal volume, or treat airflow obstruction. Re-measure after each change. Recognize that increasing rate worsens air trapping when expiratory time is the limiting factor.
When a patient's spontaneous effort is present but ineffective, the candidate increases pressure support without assessing whether the effort is being wasted because of trigger delay, flow starvation, or cycling asynchrony.
Pressure support is the familiar lever for 'working too hard', and candidates apply it reflexively. They do not inspect the pressure-time and flow-time waveforms for the specific asynchrony pattern before changing settings.
What to do instead
Classify the asynchrony first: trigger (delayed, ineffective, auto-trigger), flow (starvation or double cycling), or cycle (premature or delayed). Match the fix to the class, for example adjusting trigger sensitivity or rise time for trigger problems, increasing flow or changing to a decelerating flow pattern for starvation, and adjusting cycle-off criteria for cycling problems.
The candidate recommends extubation based only on a spontaneous breathing trial result, without confirming that the patient can protect the airway, cough effectively, and manage secretions.
The SBT is the most rehearsed step in weaning, and passing it feels like the endpoint. Airway protection and secretion clearance are harder to score and are often skipped under exam time pressure.
What to do instead
Before recommending extubation, systematically verify: adequate cough and peak cough flow, manageable secretion burden, intact airway reflexes, hemodynamic stability, and a passed SBT with acceptable respiratory pattern. State each criterion explicitly in the answer rather than implying it.
During a spontaneous breathing trial, the candidate reports failure based on a single abnormal value (for example, one low tidal volume or one high respiratory rate) rather than on sustained, trended criteria.
SBT failure criteria are memorized as a list of thresholds, and candidates apply any one of them to a single measurement. They do not distinguish transient variation from sustained deterioration.
What to do instead
Apply failure criteria only when abnormalities are sustained over the observation period or accompanied by clinical signs such as diaphoresis, accessory muscle use, or agitation. Report the trend and the duration, and distinguish a single outlying value from a pattern.
The candidate describes the goals of mechanical ventilation only as 'improve oxygenation' and does not specify lung-protective targets, plateau pressure limits, or the distinction between oxygenation and ventilation goals.
Oxygenation is the most intuitive goal, and candidates compress several distinct objectives into one phrase. They omit the evidence-based targets that examiners look for.
What to do instead
State goals in separate categories: oxygenation (adequate PaO2 or SpO2 with acceptable FiO2), ventilation (pH and PaCO2 within target, including permissive hypercapnia where appropriate), lung protection (tidal volume around 6 mL/kg predicted body weight, plateau pressure generally at or below 30 cm H2O), patient comfort and synchrony, and hemodynamic stability. Tie each goal to a measurable target.
When interpreting an arterial blood gas, the candidate stops after identifying respiratory acidosis or alkalosis and does not calculate the expected compensation or check for a mixed disorder.
Primary disorder identification is the first step taught, and candidates treat it as the complete answer. Compensation formulas and anion gap calculation require extra steps that are easy to skip under time pressure.
What to do instead
Use a fixed sequence: identify primary disorder from pH and PaCO2, calculate the expected compensation with the appropriate formula, compare measured with expected to detect a mixed disorder, and calculate the anion gap when metabolic acidosis is present. Write out each step so partial credit is visible.