3.1 Ventilation Fundamentals
Key Takeaways
- Mechanical ventilation is indicated for acute hypercapnic failure (PaCO2 >50 mmHg with pH <7.25), refractory hypoxemia (PaO2 <60 mmHg on FiO2 ≥0.60), apnea, or inability to protect the airway (GCS ≤8)
- Volume-controlled ventilation (VCV) guarantees a set tidal volume; airway pressure rises when compliance falls
- Pressure-controlled ventilation (PCV) guarantees a set inspiratory pressure; delivered tidal volume falls when compliance falls
- Assist/Control (A/C) gives a full breath for every patient and time trigger, so a high drive can cause respiratory alkalosis and breath stacking
- SIMV synchronizes a set number of mandatory breaths; spontaneous breaths in between get only pressure support, raising work of breathing if PS is low
- Pressure Support Ventilation (PSV) is patient-triggered with no mandatory rate; the patient sets rate, VT, and inspiratory time
- CPAP delivers one continuous pressure during spontaneous breathing and gives no inspiratory assist, so it does not lower PaCO2
- BiPAP/bilevel delivers two pressures (IPAP and EPAP); pressure support = IPAP − EPAP, which is why BiPAP helps clear CO2 while CPAP does not
Why This Chapter Matters
The Therapist Multiple-Choice (TMC) Examination is the National Board for Respiratory Care (NBRC) entry test for both the Certified Respiratory Therapist (CRT) and Registered Respiratory Therapist (RRT) tracks. It has 160 items (140 scored, 20 unscored pretest) in a 3-hour window; a score of 86 earns the CRT and 92 earns RRT eligibility for the Clinical Simulation Examination. Adult critical care — dominated by mechanical ventilation — is one of the largest content domains, so mastering modes, settings, and waveform troubleshooting moves your score more than any other single chapter.
Indications for Mechanical Ventilation
Ventilation is started when a patient cannot maintain gas exchange or protect the airway despite non-invasive measures. Memorize the numeric triggers — the exam loves a specific arterial blood gas (ABG) value.
| Indication | Objective criterion |
|---|---|
| Acute hypercapnic (ventilatory) failure | PaCO2 >50 mmHg with pH <7.25 |
| Refractory hypoxemic failure | PaO2 <60 mmHg on FiO2 ≥0.60, or P/F ratio <200 |
| Apnea / impending arrest | Absent or agonal breathing |
| Airway protection | Glasgow Coma Scale (GCS) ≤8, absent gag, aspiration risk |
| Excessive work of breathing | RR >35, accessory-muscle use, paradoxical (abdominal) breathing |
| Neuromuscular failure | Vital capacity <10–15 mL/kg, NIF less negative than −20 cmH2O (Guillain-Barré, myasthenic crisis, high cervical injury) |
A classic trap: a normal or low PaCO2 in a tiring asthmatic is ominous, not reassuring — a "normalizing" CO2 during a severe attack signals fatigue and impending failure.
Volume-Controlled vs. Pressure-Controlled
The single most tested fundamental: in volume control you set the volume and pressure floats; in pressure control you set the pressure and volume floats. Whatever you set is guaranteed; whatever floats becomes your monitoring alarm.
| Feature | Volume-Controlled (VCV) | Pressure-Controlled (PCV) |
|---|---|---|
| Set (guaranteed) | Tidal volume (VT) | Inspiratory pressure |
| Floats | Airway pressure | Tidal volume |
| Flow pattern | Square or decelerating (selectable) | Decelerating (always) |
| If compliance falls (stiffer lungs) | Pressure rises → barotrauma risk | VT falls → hypoventilation risk |
| If resistance rises (bronchospasm) | Peak pressure rises | VT falls |
| Watch this alarm | High peak/plateau pressure | Low tidal/minute volume |
Worked example: a VCV patient set to VT 500 mL develops a mucus plug. Volume stays 500 mL, but peak pressure jumps from 28 to 45 cmH2O, tripping the high-pressure alarm. The same plug in PCV at 18 cmH2O would instead drop VT from 500 to 300 mL.
The Five Core Modes
- Assist/Control (A/C) — Every breath, whether patient- or time-triggered, is a full mechanical breath (set VT or set pressure). Best for the apneic, sedated, or paralyzed patient needing full support. Risk: a high respiratory drive triggers many full breaths → respiratory alkalosis and breath stacking / auto-PEEP.
- SIMV (Synchronized Intermittent Mandatory Ventilation) — Delivers a set number of mandatory breaths synchronized to patient effort; breaths in between are spontaneous and get only the pressure support you add. Useful as a transitional mode; the downside is high work of breathing if PS is set too low.
- PSV (Pressure Support Ventilation) — Purely patient-triggered, no set rate (a backup apnea rate is a safety net). The patient owns rate, VT, and inspiratory time. Ideal for weaning and spontaneous breathing trials, but requires a reliable drive.
- CPAP — One continuous pressure during fully spontaneous breathing; no inspiratory boost, so it improves oxygenation (recruits alveoli) but does not lower PaCO2. Used for obstructive sleep apnea and mild cardiogenic edema.
- BiPAP / bilevel — Two pressures, IPAP (inspiration) and EPAP (expiration); the difference is the pressure support that unloads the muscles and clears CO2. First-line for COPD exacerbation with hypercapnic acidosis and for cardiogenic pulmonary edema.
Rule of thumb for the exam: if the gas shows CO2 retention with acidosis, the patient needs ventilatory help → A/C or BiPAP, not CPAP. If the problem is pure oxygenation with normal CO2, CPAP/PEEP/FiO2 is the lever.
Matching the Mode to the Stage of Illness
Think of the modes as a continuum from full machine control toward full patient effort, and let the clinical stage pick the point on that line. A freshly intubated, deeply sedated patient in acute respiratory distress syndrome (ARDS) belongs on A/C, where every breath is identical and predictable, the minute ventilation is locked in, and the therapist can hold tidal volume and rate exactly where the lung-protective protocol demands.
As sedation lightens and the patient begins to interact with the machine, partial-support modes such as SIMV with added pressure support or straight PSV let the patient shoulder a growing share of the work while the machine fills the gaps. At the spontaneous end of the continuum, CPAP and a low PSV are the bridge to extubation.
A frequent distractor on the exam is to offer a mode that is technically a ventilation mode but is wrong for the stated drive. PSV with no backup rate is dangerous in a patient with an unreliable drive — an apneic episode produces no breath until the safety rate fires. CPAP is offered as a tempting answer for the hypercapnic COPD patient precisely because it raises airway pressure, but because it adds no inspiratory boost it cannot move the trapped CO2; BiPAP is the right answer there.
Conversely, choosing full A/C for an awake, comfortable patient with a strong drive risks over-ventilation, respiratory alkalosis, and dynamic hyperinflation as the patient triggers many full breaths. The exam reliably rewards the candidate who reads the scenario's respiratory drive and gas-exchange deficit together and then selects the least amount of support that safely meets the need.
A patient on volume-controlled A/C develops a sudden drop in lung compliance from worsening pneumonia. With the tidal volume unchanged, what will the respiratory therapist observe?
Which mode delivers a full mechanical breath for every patient-triggered AND every time-triggered breath?
A cooperative COPD patient has pH 7.28 and PaCO2 62 mmHg and is tiring. What is the most appropriate initial ventilatory intervention?
In pressure-controlled ventilation, what happens to the delivered tidal volume when lung compliance suddenly decreases?