Mechanical Ventilation—Assist-Control: Volume & Pressure Control Comprehensive Notes
Mode Classification & General Principles
- Full-support Assist-Control (AC) only delivers mechanical breaths.
- Two possible triggers patient-triggered OR time-triggered, yet both produce a ventilator (mechanical) breath.
- Absolutely no spontaneous breaths permitted in this mode.
- Rationale: allows the patient to rest while primary disease (cardiac arrest, MI, stroke, drug OD, COPD exacerbation, metabolic acidosis, etc.) is treated.
- Breath sequence summary
- Trigger: patient effort (negative-pressure or flow) OR elapsed time.
- Limit/Target: flow (in VC) or pressure (in PC).
- Cycle: volume (VC); time (PC).
- “Full support” implies minimal to zero patient work; weaning begins only once the primary cause of respiratory failure is controlled.
Assist-Control Volume Control (AC-VC)
Classification
- Patient or time triggered.
- Flow-limited (clinician sets inspiratory flow 40–60 L min⁻¹).
- Volume-cycled.
Initial Tidal-Volume (VT) Target
- General range 6 – 8\;\text{mL kg}^{-1} Ideal Body Weight (IBW).
- Normal or obstructive lungs (asthma, COPD) start toward 8\;\text{mL kg}^{-1}.
- Decreased compliance (ARDS, chest trauma, inhalation injury) \le 6\;\text{mL kg}^{-1} (may fall to 4\;\text{mL kg}^{-1} in lung-protective strategy).
IBW Equations (shortened version allowed)
- Male: IBW{M}=50+2.3(Height{in}-60)
- Female: IBW{F}=45.5+2.3(Height{in}-60)
- Some clinicians drop the 0.3 for speed ("2 × (# inches > 5 ft)").
Example Calculations
- Pt 5’7’’ (64 kg IBW)
- 8\times64≈512\;\text{mL} (upper end)
- 6\times64≈384\;\text{mL} (lower end)
- Acceptable exam answers ≈ 350 – 550 mL; pick higher end for obstructive, lower for restrictive.
- Pt 6’2’’ (78 kg IBW)
- VT{hi}=624\;\text{mL},\;VT{lo}=468\;\text{mL}
- NBRC answer keys often offered in 50-mL steps (e.g., 450, 500, 550, 600).
Consequences of Improper VT
- VT ABOVE max range (over-distension)
- Barotrauma → pneumothorax, pneumomediastinum.
- ↑ Peak P & ↑ mean airway pressure ((\bar P_{aw})) → ↓ venous return → ↓ SV & CO → hypotension.
- VT BELOW min range
- Hypoventilation → ↑ PaCO₂ (hypercapnia) → respiratory acidosis.
NBRC Test-Taking Tips
- Board questions supply entire ventilator package (mode, RR, VT/pressure, PEEP, FIO₂) – evaluate all variables, not VT in isolation.
- Choose the answer that best balances VT range and appropriate FIO₂/PEEP for the given clinical scenario.
Peak Inspiratory Pressure (PIP) Dynamics in VC
- PIP is variable; depends on airway resistance (Raw) + compliance (Cₗ).
- ↑ Raw or ↓ Compliance → ↑ PIP (bad → barotrauma risk).
- ↓ Raw (bronchodilator, suction) or ↑ Compliance (PEEP recruitment) → ↓ PIP.
- Clinically track PIP trends:
- Normal ≈ 20 cmH₂O.
- “Worry zone” 30–39 cmH₂O.
- Critical > 40 cmH₂O → evaluate (bronchospasm? secretions? atelectasis?).
- Document PIP pre-/post-therapy to prove benefit (analogous to breath-sound documentation off-vent).
Manipulating PaCO₂ in VC
- Only two dials affect minute ventilation ((\dot V_E=RR\times VT)).
- Tidal Volume – most effective if within 6–8 mL kg⁻¹.
- Rate – used once VT ceiling reached.
- Effect chain:
- ↑ VT or ↑ RR → ↑ (\dot V_E) → ↓ PaCO₂ → ↑ pH.
- ↓ VT or ↓ RR → opposite.
Desired PaCO₂ (VT-change) Formula
VT{new}=VT{current}\times\frac{PaCO2{current}}{PaCO2{desired}}
- Standard desire: PaCO2_{desired}=40\;\text{mmHg}.
- Worked Example (corrected):
- 70-kg pt, VT 400 mL, PaCO₂ 65 mmHg, goal 40 mmHg.
- VT_{new}=400\times\frac{65}{40}=650\;\text{mL} → exceeds 8 mL kg⁻¹ (560 mL). Cannot raise VT safely; must ↑ RR instead.
Winter’s Formula (less commonly tested)
PaCO2{desired}=1.5\,[HCO3^-]+8
- Some Redlands clinicians use this to compute desired minute ventilation:
\dot V{E,new}=\dot V{E,current}\times\frac{PaCO2{current}}{PaCO2{desired}} - Remember: no direct (\dot V_E) knob – you’ll adjust VT and/or RR to hit that target.
Assist-Control Pressure Control (AC-PC)
Classification
- Patient or time triggered.
- Pressure-limited.
- Time-cycled.
Initial Settings & Lung-Protective Strategy
- Set ΔP (driving pressure) ≈ 20–25 cmH₂O (or plateau – 5 cmH₂O from prior VC trial).
- Adjust ΔP or PEEP to obtain exhaled VT ≈ 4–6\;\text{mL kg}^{-1} (ARDS net protocol).
Variable in PC = Tidal Volume
- Same Raw & compliance relationships but reversed focus:
- ↑ Raw or ↓ Compliance → ↓ VT.
- ↓ Raw or ↑ Compliance → ↑ VT.
- Document VT pre-/post-therapy (e.g., bronchodilator: VT ↑ validates reduced Raw).
Permissive Hypercapnia
- To avoid excessive pressures we allow ("permissive"):
- PaCO₂ up to ≈ 55 mmHg.
- pH down to ≈ 7.30.
- Overall pressure goals
- PIP < 40 cmH₂O.
- Plateau < 30 cmH₂O.
Two Flavors of PC
1. ΔP (Driving Pressure) Style
- Ventilator setting label shows "Pressure Control = 25" while PEEP = 5.
- Total PIP = ΔP + PEEP = 30 cmH₂O.
- Change either ΔP or PEEP → PIP shifts correspondingly.
2. PIP (Absolute) Style
- Setting reads "Pressure Control = 25" AND monitored PIP = 25; PEEP = 5.
- Implied ΔP = 20 cmH₂O (25 – 5).
How to Tell Which Style at Bedside
- If PC setting and displayed PIP match → PIP-based PC.
- If they differ by exactly PEEP → ΔP-based PC.
- If no PEEP is set, add 3–5 cmH₂O PEEP and observe:
- PIP rises → ΔP mode.
- PIP unchanged → PIP mode.
Quick Practice Table
PC Setting | PEEP | Displayed PIP | Mode |
---|---|---|---|
30 | 5 | 35 | ΔP (25 ΔP) |
25 | 5 | 25 | PIP (ΔP=20) |
45 | 5 | 45 | PIP (ΔP=40) |
30 | 0 | 30 | Indeterminate → add PEEP test. |
Manipulating PaCO₂ in PC
- Principal dial = ΔP (raises or lowers VT).
- If ΔP maxed out (risking barotrauma) use RR changes – but remain aware of I:E and auto-PEEP.
Documentation & Clinical Integration
- VC patient: chart PIP trends; PC patient: chart exhaled VT trends.
- Always correlate changes with interventions (suction, bronchodilator, recruitment, PEEP change) so physicians can judge therapy efficacy.
Ethical / Practical Implications
- Lung-protective ventilation prioritises long-term morbidity (VILI) over immediate blood-gas "perfection" (permits hypercapnia).
- Board/exam questions often force you to choose the safest answer (pressure & volume limits) over the superficially “normalising” answer.
- Real-world practice still guided by physician orders; exams test your ability to predict what the test-writer thinks is correct.
Formulas & Quick References
- \text{Minute Ventilation}=VT\times RR
- Desired VT formula (see above).
- Winter’s formula (PaCO₂ target) (see above).
- IBW (male/female) (see above).
Study Strategy Suggestions
- Break material into compartments: study VC for an hour, rest, then PC.
- Construct comparison tables (Trigger / Limit / Cycle / Variable / Typical Targets / How to manipulate PaCO₂).
- Practice setting ventilators on student lab mannequins with given scenarios.