Overview
This lecture explains how to determine initial ventilator settings with limited information, focusing on relationships between tidal volume, inspiratory time (I-time), flow, and respiratory rate for volume control modes.
Determining Mode and Tidal Volume
- Setting a required tidal volume means using volume control mode (AC or SIMV).
- Safe initial tidal volume is 6–8 mL/kg ideal body weight (IBW).
- Example: For a 5'8" male, IBW ≈ 70 kg (154 lbs / 2.2), so tidal volume range = 420–560 mL (0.42–0.56 L).
- Common practice: Select a middle value (e.g., 0.5 L or 500 mL).
Calculating Inspiratory Flow
- I-time (in seconds) = Tidal Volume (L) / Flow (L/s).
- Flow is usually set in L/min but needs converting to L/s (Flow L/min ÷ 60).
- Example: With 0.5 L tidal volume and 0.8 s I-time, Flow = 0.5 / 0.8 = 0.625 L/s; convert to L/min: 0.625 × 60 = 37.5 L/min.
- Preferred flow range is 40–60 L/min; adjust tidal volume or I-time to fit within range.
Calculating Respiratory Rate and Cycle Times
- Total Cycle Time (TCT) = 60 seconds / respiratory rate (breaths/min).
- Example: TCT for 14 breaths/min = 60 / 14 = 4.28 seconds.
- TCT = I-time + E-time (expiratory time).
- E-time = TCT – I-time (e.g., 4.28 – 0.8 = 3.48 s).
Setting I:E Ratio
- I:E ratio = I-time : E-time, normalized (divide both by I-time).
- Example: 0.8 : 3.48 → 1 : 4.35.
- For a set I:E of 1:3 and I-time of 0.8 s, E-time = 0.8 × 3 = 2.4 s; TCT = 0.8 + 2.4 = 3.2 s.
- Respiratory rate = 60 / TCT (e.g., 60 / 3.2 = 18.75 ≈ 19 breaths/min).
Interrelationship of Settings
- Knowing any two of tidal volume, I-time, or flow allows calculation of the third.
- Respiratory rate, I-time, and E-time are interconnected through TCT and I:E ratio.
- Changing one parameter affects others; formulas allow for multiple solution paths.
Key Terms & Definitions
- Tidal Volume (VT) — Air delivered per breath, usually in mL or L.
- Ideal Body Weight (IBW) — Calculated from height, used to set safe tidal volume range.
- I-time (Inspiratory Time) — Duration of breath delivery.
- E-time (Expiratory Time) — Duration of exhalation phase.
- Total Cycle Time (TCT) — Sum of I-time and E-time (full breath cycle).
- Flow — Speed of gas delivery; usually set in L/min.
- I:E Ratio — Ratio comparing length of inspiration to expiration.
- Minute Ventilation — Total air moved per minute (tidal volume × respiratory rate).
- Volume Control Mode — Ventilator delivers set tidal volume each breath.
Action Items / Next Steps
- Practice using the formulas to calculate initial ventilator settings with different scenarios.
- Ensure you can convert between mL and L, and between L/min and L/s.
- Review safe tidal volume ranges and how to calculate IBW for different patients.