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

  1. 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.
  2. 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)).
    1. Tidal Volume – most effective if within 6–8 mL kg⁻¹.
    2. 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 SettingPEEPDisplayed PIPMode
30535ΔP (25 ΔP)
25525PIP (ΔP=20)
45545PIP (ΔP=40)
30030Indeterminate → 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.