Initiating & Adjusting Ventilatory Support

Objectives — Determine initial ventilator settings and input those settings correctly; adjust settings based on patient response and ABG results.

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Mode Selection

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Tidal Volume (Vt)

Initial Setting

Start at 6–8 mL/kg Ideal Body Weight (IBW).

Tidal volume is calculated from IBW, not actual body weight. Large patients with obese body habitus can receive dangerous volumes if actual weight is used.

Calculating IBW

Valid for patients ≥ 4 ft tall.

Example — Male, 5'10" (70 inches):

• IBW = 50 + 2.3 × (70 − 60) = 50 + 23 = 73 kg
• Vt range: 6 × 73 = 438 mL → 8 × 73 = 584 mL
• Appropriate Vt: 450–550 mL

Adjusting Tidal Volume

Adjust Vt to maintain:

• Adequate PaCO₂
• Plateau pressure (Pplat) < 30 cmH₂O — prevents alveolar overdistension

ARDS Lung-Protective Strategy

When Pplat rises above 30 cmH₂O, implement lung-protective strategies:

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Dead Space

Not all delivered volume reaches the alveoli. Three types of dead space reduce the alveolar tidal volume.

Mechanical Dead Space

Any volume lost before reaching the patient's airways.

Sources:

• Ventilator circuit
• Artificial airway (ETT)
• Adaptors: HME, in-line suction catheter, ETCO₂ monitor, aerosol delivery device (HHN or MDI spacer)
• Large-bore corrugated tubing placed distal to the circuit wye (intentionally used to treat hyperventilation by increasing CO₂)

Compressible volume — the circuit expands when pressurized and contracts during exhalation, losing volume:

Example: An adult with a peak pressure of 33 cmH₂O loses 33 × 3 = 99 mL in the circuit.

Most modern ventilators compensate for circuit compliance automatically.

Anatomic Dead Space

Volume that reaches the upper airways (trachea, bronchioles) but does not reach the alveolar-capillary level. It participates in no gas exchange.

• 1 mL/lb IBW
• Approximately 30% of inhaled tidal volume

Alveolar Dead Space

Volume that reaches the alveoli but does not participate in gas exchange because those alveoli are not perfused.

• Caused by: pulmonary embolism, emphysema, shock, pulmonary hypertension, obstruction of pulmonary vessels

Physiologic Dead Space (VD/Vt Ratio)

Physiologic dead space = Anatomic dead space + Alveolar dead space

Calculated using the Bohr equation:

VD/Vt = (PaCO₂ − PECO₂) / PaCO₂

• PaCO₂ from ABG
• PECO₂ from end-tidal capnometry or Douglas bag collection

Alveolar Tidal Volume — Worked Example

Patient: 150 lb, Vt = 500 mL, peak pressure 33 cmH₂O, alveolar dead space 50 mL

• Mechanical dead space (circuit): 33 × 3 = 99 mL
• Anatomic dead space: 1 mL/lb × 150 lb = 150 mL
• Alveolar dead space: 50 mL
• Alveolar Vt = 500 − (150 + 99 + 50) = 500 − 299 = 201 mL

Of the 500 mL set Vt, only 201 mL reached the alveolar level.

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Respiratory Rate (RR / Frequency)

• Initial setting: 10–20 breaths/min
• Primary control for PaCO₂
• RR × Vt = Minute Volume

Cycle time = 60 ÷ RR. At RR = 10: mandatory breath every 6 seconds.

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Peak Inspiratory Flow Rate (PIFR)

• Measured in L/min (mL/sec for neonates)
• Set parameter in volume control; variable in pressure control
• Average: 60 L/min (range: 40–80 L/min)
• PIFR × I-Time = Tidal Volume

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Inspiratory Time (I-Time)

• Normal: 0.75–1.5 seconds (adults)
• Used with PIFR to determine Vt: I-Time × PIFR = Vt

Calculating I-Time from cycle time and I:E ratio:

I-Time = Cycle Time ÷ Sum of I:E ratio

Example: RR = 10 (cycle time = 6 sec), I:E = 1:2

• 6 ÷ (1+2) = 2 sec I-Time

Calculating I-Time from Vt and Flow:

I-Time = Vt (L) / [Flow (L/min) / 60]

Example: Vt = 0.5 L, Flow = 60 L/min

• 0.5 ÷ (60/60) = 0.5 ÷ 1 = 0.5 sec I-Time

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I:E Ratio

Manipulating I:E ratio:

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Peak Inspiratory Pressure (PIP) in Pressure-Control

In pressure-controlled modes, PIP is the primary determinant of ventilation and Vt delivery.

• ↑ PIP → ↑ Vt
• ↓ PIP → ↓ Vt

Initial Vt is calculated the same way as volume control, but PIP is adjusted to achieve it.

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PEEP — Positive End-Expiratory Pressure

Indications for PEEP

• Improve oxygenation in refractory hypoxemia:
  • Atelectasis
  • V/Q mismatching
  • Pulmonary edema
  • PaO₂ < 50–60 mmHg with FiO₂ > 50%
• Reduce FiO₂ to non-toxic levels (< 60%)

Contraindications for PEEP

• Untreated tension pneumothorax
• Head trauma with elevated ICP
• Hemodynamic instability
• Bronchopleural fistula (BPF)

Hazards of PEEP

• Alveolar over-distension and barotrauma
• Decreased cardiac output (↓ venous return)
• Decreased renal function
• Increased intracranial pressure

Optimal PEEP

The lowest PEEP that achieves adequate oxygenation at a non-toxic FiO₂ (< 60%). Determined by:

• Oxygen delivery (DO₂ = Qt × CaO₂)
• Compliance measurements
• Pressure-volume curves
• Esophageal manometry

Lung Recruitment Maneuver

A sustained increase in airway pressure to open collapsed alveoli, followed by sufficient PEEP to keep them open.

Protocols:

• 30 cmH₂O for 30 seconds
• 40 cmH₂O for 40 seconds
• "Step-up" — pressure increased in 5–10 cmH₂O increments then held
• Programmed "sigh" during ventilation

CPAP recruitment method:

1. Switch to CPAP mode
2. Increase CPAP to 30–40 cmH₂O
3. Hold for 30–40 seconds
4. Monitor closely for hemodynamic compromise and desaturation
5. Return to previous mode — likely with higher PEEP

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FiO₂

• Adjustable from 21% to 100%
• First setting to wean (to avoid oxygen toxicity)
• Used with PEEP to manipulate PaO₂

FiO₂ Adjustment Algorithm (Test World)

Correcting hypoxemia (low PaO₂):

1. Increase FiO₂ up to 60%
2. Once FiO₂ = 60%, add or increase PEEP
3. Continue increasing PEEP until adverse cardiac effects appear (↓CO, hypotension)
4. Return to increasing FiO₂ from 60% toward 100%

Weaning FiO₂ and PEEP:

1. Reduce FiO₂ first to 60%
2. Then decrease PEEP in increments of 2–5 cmH₂O

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Pressure Support (PS)

Goals of Pressure Support

• Reduce WOB
• Overcome RAW induced by the endotracheal tube
• Increase spontaneous Vt
• Improve patient-ventilator synchrony

PS Levels

Automatic Tube Compensation (ATC) — modern ventilators calculate the minimum PS needed to overcome the resistance of the specific ETT size; set automatically.

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Sensitivity

Pressure Trigger

• Measures negative pressure deflection from baseline
• Range: −0.5 to −2.0 cmH₂O (normal: −2.0 cmH₂O)
• Automatically adjusts to baseline pressure

Flow Trigger (More Common)

• Senses a drop in bias flow running through the circuit
• Base flow: 10–20 L/min
• Flow sensitivity: amount of flow the patient inhales to trigger a breath
• Requires less patient effort than pressure triggering

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Alarms

High Pressure Alarm

• Set 10–15 cmH₂O above measured PIP
• When exceeded → inspiration ends, remaining Vt is not delivered
• Causes: airway/circuit obstruction, patient coughing/secretions, ↑ RAW, ↓ compliance, patient-ventilator asynchrony

Low Pressure Alarm

• Set 10–15 cmH₂O below PIP
• Causes: leak (ventilator, humidifier, circuit, patient), disconnection

Volume Alarms

Rate Alarms

Apnea Alarm / Backup Ventilation

Activated in spontaneous modes (CPAP, PSV) or low SIMV rates (≤ 4 bpm).

Additional Alarms

• Source gas (O₂ or air loss)
• Electrical power failure / battery backup
• FiO₂ (blender or analyzer failure)
• Demand valve malfunction
• Ventilator inoperative (Vent Inop)
• Flow transducer failure

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Initial Set-Up

1. Select ventilator — choose what you know best; most ICU vents have equivalent modes
2. Verify an Operational Verification Procedure (OVP) was performed (calibration and self-test)

OVP Checks

• Electronics check
• Audible and visual alarms
• Calibrates pressure transducers and flow transducer
• Measures/calculates tubing compliance
• Pressurizes circuit to check for leaks

Perform OVP: (1) before connecting to a patient, and (2) after any circuit change or disassembly.

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Adjusting Settings Based on ABG Results

Systematic Approach

1. Interpret the ABG: oxygenation problem or ventilation problem?
2. Evaluate settings:
   • Oxygenation issue → adjust FiO₂ and/or PEEP
   • Ventilation issue → confirm Vt is correct for IBW; if yes, adjust RR; if no, correct Vt first

Adjustment Formulas

New RR = Current RR × (Current PaCO₂ / Desired PaCO₂)

New Vt = Current Vt × (Current PaCO₂ / Desired PaCO₂)

"What you have over what you want"

Adjusting Vt While Maintaining Minute Volume

If you need to correct an inappropriately large Vt but the PaCO₂ is currently normal:

1. Calculate current minute volume: RR × Vt = Min Vol
2. Divide minute volume by new Vt to find the new RR that maintains the same minute volume

Example: 5'10" male

• Current: Vt = 750 mL, RR = 12 → Min Vol = 9.0 L
• Target Vt = 500 mL (correct for IBW)
• New RR = 9,000 ÷ 500 = 18 breaths/min