3.2 Initial Ventilator Settings

CRT/RRT Respiratory Therapist Exam Exam

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1 Introduction: NBRC TMC Exam Overview

NBRC TMC Exam Overview Credential Pathway: CRT to RRT Study Strategies for the TMC Exam Content Domains & Weight Distribution

2 Chapter 1: Patient Data Evaluation

1.1 Patient History & Physical Assessment 1.2 Diagnostic Procedures & ABG Interpretation 1.3 Hemodynamic Monitoring & Waveform Analysis 1.4 Laboratory Values & Diagnostic Data

3 Chapter 2: Airway Management & Oxygen Therapy

2.1 Airway Anatomy & Assessment 2.2 Airway Management Devices & Procedures 2.3 Oxygen Therapy Devices & Delivery 2.4 Humidification & Aerosol Therapy

4 Chapter 3: Mechanical Ventilation

3.1 Ventilation Fundamentals 3.2 Initial Ventilator Settings 3.3 Ventilator Monitoring & Waveform Analysis 3.4 Weaning & Liberation from Mechanical Ventilation

5 Chapter 4: Pharmacology & Equipment

4.1 Respiratory Pharmacology 4.2 Equipment & Troubleshooting 4.3 Specialty Procedures

6 Chapter 5: Infection Control, Ethics & Professional Practice

5.1 Infection Control & Prevention 5.2 Emergency Procedures & Resuscitation 5.3 Professional Practice & Ethics

3.2 Initial Ventilator Settings

Key Takeaways Explain This Topic AI

Tidal volume is set from IDEAL body weight (IBW), never actual weight: 6–8 mL/kg routinely, 4–6 mL/kg for ARDS

IBW (kg): males = 50 + 2.3 × (height in inches − 60); females = 45.5 + 2.3 × (height in inches − 60)

Initial respiratory rate is 12–20/min; in ARDS rates of 20–35 compensate for the low tidal volume

Start FiO2 at 1.0 in emergencies, then titrate to PaO2 60–100 mmHg or SpO2 ≥92% (88–95% in ARDS), aiming for FiO2 ≤0.60

Start PEEP at 5 cmH2O; raise in 2–3 cmH2O steps for refractory hypoxemia, using the ARDSNet PEEP/FiO2 table for ARDS

Oxygenation is driven by FiO2 and PEEP; ventilation (PaCO2) is driven by rate and tidal volume (minute ventilation = VT × RR)

Keep plateau pressure ≤30 cmH2O and driving pressure (Pplat − PEEP) <15 cmH2O to limit lung injury

Default I:E ratio is 1:2 to 1:3; flow trigger of 2–3 L/min lowers the work of triggering versus a pressure trigger

Last updated: June 2026

Ideal Body Weight Drives Tidal Volume

The most-tested calculation in this chapter is ideal body weight (IBW) , because tidal volume is always set from IBW, never from actual scale weight. Lung size tracks height , not body fat; an obese patient's lungs are no larger than those of a lean person of the same height, so dosing volume to actual weight overdistends alveoli and causes volutrauma .

Sex IBW formula (kg)
Male 50 + 2.3 × (height in inches − 60)
Female 45.5 + 2.3 × (height in inches − 60)

Worked example — a 5'10" (70 in) man:

IBW = 50 + 2.3 × (70 − 60) = 50 + 23 = 73 kg
6 mL/kg = 438 mL; 8 mL/kg = 584 mL → routine start 440–585 mL
ARDS at 4–6 mL/kg = 292–438 mL
Initial Settings at a Glance

Parameter Routine start ARDS / lung-protective Why
Tidal volume 6–8 mL/kg IBW 4–6 mL/kg IBW Lower VT cut ARDS mortality from ~40% to ~31% in the ARDSNet trial
Respiratory rate 12–20/min 20–35/min Higher rate offsets the smaller VT to keep minute ventilation up
FiO2 1.0 then titrate Per ARDSNet PEEP/FiO2 grid Target SpO2 ≥92% (88–95% ARDS); aim FiO2 ≤0.60
PEEP 5 cmH2O 8–24 cmH2O per table Prevents alveolar derecruitment, improves V/Q match
Flow rate 40–60 L/min same Sets I-time and comfort
I:E ratio 1:2 to 1:3 1:1 to 1:3 Enough expiratory time prevents air trapping
Trigger Flow 2–3 L/min Flow 2–3 L/min Less work to trigger; avoid auto-triggering
Understanding PEEP

Positive End-Expiratory Pressure (PEEP) holds the airway open at end-exhalation, splinting alveoli that would otherwise collapse. Its benefits and hazards both stem from raised intrathoracic pressure.

Benefits: recruits collapsed alveoli → ↑ functional residual capacity (FRC); improves V/Q matching and oxygenation; reduces intrapulmonary shunt.

Hazards: reduced venous return → ↓ cardiac output and hypotension; alveolar overdistension → barotrauma; pneumothorax risk rises sharply above ~15 cmH2O.

Scenario Typical PEEP
Standard initial 5 cmH2O
Mild hypoxemia 5–8 cmH2O
Moderate ARDS 10–14 cmH2O
Severe ARDS 14–24 cmH2O
COPD with auto-PEEP Applied PEEP set to ~50–75% of measured auto-PEEP to ease triggering
Reading the ABG and Turning ONE Knob

The exam pattern is reliable: oxygenation problems → FiO2 or PEEP ; ventilation problems → rate or tidal volume . PaCO2 is inversely proportional to minute ventilation (VE = VT × RR), so raising VE lowers CO2.

Problem ABG signature First adjustment
Hypoxemia PaO2 <60, SpO2 <90% ↑ PEEP if FiO2 already ≥0.60; otherwise ↑ FiO2
Hyperoxia PaO2 >100 ↓ FiO2 toward ≤0.60
Respiratory acidosis PaCO2 >45, pH <7.35 ↑ RR (or VT) to raise minute ventilation
Respiratory alkalosis PaCO2 <35, pH >7.45 ↓ RR (or VT) to lower minute ventilation

Safety guardrails: keep plateau pressure ≤30 cmH2O and driving pressure (Pplat − PEEP) <15 cmH2O ; if either is exceeded, lower VT before chasing the ABG. When a hypoxemic patient is already on FiO2 ≥0.60, the right move is to add PEEP rather than push FiO2 to 1.0, which risks oxygen toxicity with little added benefit.

Putting the Settings Together at the Bedside

A clean way to remember the initial setup is to walk the parameters in the order you would dial them. First fix the mode and the tidal volume from IBW — this is where the height-based calculation pays off, because dosing a 5-foot-2-inch woman the same 600 mL you would give a 6-foot-4-inch man badly overdistends her smaller lungs. Next set a rate that, multiplied by that volume, gives a starting minute ventilation near the patient's pre-intubation demand; remember that minute ventilation is the product VT times RR, so the same minute volume can be reached with many small breaths or fewer large ones.

Then set FiO2 high and wean it down as the pulse oximeter and the first ABG allow, and finally add PEEP to recruit and hold open alveoli so you can lower FiO2 faster.

The exam tests the interaction of these knobs more than any single value. A common scenario gives a patient who is both hypoxemic and hypercapnic and asks for the one best change: separate the two problems, decide which is more dangerous in that vignette, and turn the matching knob. Another favorite trap presents an ARDS patient whose oxygenation is poor and tempts you to raise the tidal volume; the correct instinct in ARDS is the opposite — protect the lung with 4–6 mL/kg, accept a higher rate, and tolerate a modestly elevated CO2 ( permissive hypercapnia ) as long as the pH stays above roughly 7.20.

Whenever a setting change would push plateau pressure over 30 or driving pressure over 15, the lung-protective limit wins over the textbook gas target. Finally, watch the I:E ratio : increasing the rate or the inspiratory time shortens exhalation, and in an obstructed patient that is the fast road to auto-PEEP and hemodynamic compromise.