A 5'8" (68 inches) female patient requires mechanical ventilation. What is the appropriate tidal volume range using lung-protective settings?

350-530 mL. First calculate IBW: 45.5 + 2.3 x (68 - 60) = 45.5 + 18.4 = 63.9 kg. For lung-protective ventilation (6-8 mL/kg IBW): 63.9 x 6 = 383 mL to 63.9 x 8 = 511 mL. The range of 350-530 mL is the closest answer. For ARDS, you would use 4-6 mL/kg (256-383 mL).

A ventilated patient has the following ABG: pH 7.28, PaCO2 58 mmHg, PaO2 88 mmHg, HCO3 26 mEq/L. Current settings: VT 450 mL, RR 14, FiO2 0.40, PEEP 5. What is the MOST appropriate ventilator adjustment?

Increase respiratory rate to 18-20. The ABG shows respiratory acidosis (pH 7.28, PaCO2 58) with adequate oxygenation (PaO2 88). The problem is ventilation, not oxygenation. To lower PaCO2 and raise pH, increase minute ventilation by increasing the respiratory rate. Increasing FiO2 or PEEP addresses oxygenation, which is already adequate. Decreasing VT would worsen the respiratory acidosis.

Why is tidal volume calculated using IDEAL body weight rather than actual body weight?

Lung size correlates with height (IBW) not body fat; excess VT causes volutrauma. Lung size is determined by height, not by adipose tissue (body fat). An obese patient's lungs are the same size as those of a non-obese person of the same height. Using actual body weight in obese patients would deliver excessive tidal volumes, causing alveolar overdistension (volutrauma) and ventilator-induced lung injury (VILI).

Which of the following ventilator adjustments would IMPROVE oxygenation without significantly affecting ventilation?

Increasing the PEEP from 5 to 10 cmH2O. PEEP improves oxygenation by recruiting collapsed alveoli, increasing FRC, and improving V/Q matching. It has minimal direct effect on CO2 elimination (ventilation). Increasing RR or VT primarily affects ventilation (CO2 removal). Changing the flow pattern affects pressure distribution but not oxygenation directly.

A ventilated ARDS patient has the following: Pplat 32 cmH2O, PEEP 12 cmH2O. What is the driving pressure and is it within the recommended range?

Driving pressure = 20 cmH2O; this is above the recommended limit of <15 cmH2O. Driving pressure = Pplat - PEEP = 32 - 12 = 20 cmH2O. The recommended driving pressure for ARDS is <15 cmH2O, as higher driving pressures are associated with increased mortality. A driving pressure of 20 is above the target. Interventions include reducing tidal volume or adjusting PEEP to lower the driving pressure.

3.2 Initial Ventilator Settings | CRT/RRT Respiratory Therapist Exam | Free Exam Prep

Key Takeaways

Tidal volume: 6-8 mL/kg of ideal body weight (IBW), NOT actual body weight; use lower VT (4-6 mL/kg) for ARDS
IBW formula: Males = 50 + 2.3(height in inches - 60); Females = 45.5 + 2.3(height in inches - 60)
Respiratory rate: Initially 12-20 breaths/min; adjust based on PaCO2 and pH targets
FiO2: Start at 100% in emergencies, titrate down to maintain PaO2 60-100 mmHg or SpO2 ≥92% (goal FiO2 ≤0.60)
PEEP: Start at 5 cmH2O; increase in increments of 2-3 cmH2O for refractory hypoxemia (ARDS protocol uses higher PEEP tables)
I:E ratio: Normal is 1:2 to 1:3; avoid inverse ratio (I:E >1:1) unless specifically indicated
Flow rate: 40-60 LPM for volume ventilation; affects I:E ratio and peak pressure
Sensitivity/trigger: Flow trigger (2-3 LPM) preferred over pressure trigger (-1 to -2 cmH2O) for reduced work of breathing

Initial Ventilator Settings

Setting up a mechanical ventilator correctly is a critical skill tested extensively on the TMC exam. Initial settings are based on the patient's ideal body weight (IBW), the underlying condition, and gas exchange goals. Understanding the rationale for each setting and how to adjust based on ABG results is essential.

Ideal Body Weight (IBW) Calculation

Tidal volume is ALWAYS based on IBW, NOT actual body weight. This is one of the most commonly tested concepts.

Sex	IBW Formula
Males	50 + 2.3 x (height in inches - 60) kg
Females	45.5 + 2.3 x (height in inches - 60) kg

Example: A male patient who is 5'10" (70 inches):

IBW = 50 + 2.3 x (70 - 60) = 50 + 23 = 73 kg
VT at 6 mL/kg = 438 mL; VT at 8 mL/kg = 584 mL
Initial VT range: 438-584 mL

Initial Settings Summary

Parameter	General Setting	ARDS/Lung Protective	Rationale
Tidal Volume (VT)	6-8 mL/kg IBW	4-6 mL/kg IBW	Prevent volutrauma; lower VT in ARDS reduces mortality
Respiratory Rate (RR)	12-20 breaths/min	20-35 breaths/min	Higher RR compensates for lower VT in ARDS
FiO2	100% initially, then titrate down	Per ARDS Net table	Target PaO2 55-80 or SpO2 88-95% (ARDS)
PEEP	5 cmH2O (physiologic)	8-24 cmH2O (per ARDS table)	Prevents alveolar derecruitment; improves V/Q matching
Flow Rate	40-60 LPM	Same	Affects I:E ratio and patient comfort
I:E Ratio	1:2 to 1:3	1:1 to 1:3	Adequate expiratory time prevents air trapping
Trigger Sensitivity	Flow: 2-3 LPM	Flow: 2-3 LPM	Lower threshold = less work to trigger; avoid auto-triggering
Flow Pattern	Square wave or decelerating	Decelerating preferred	Decelerating reduces peak pressure

Understanding PEEP (Positive End-Expiratory Pressure)

PEEP maintains positive pressure in the airway at the end of expiration, preventing alveolar collapse (atelectasis).

Physiologic Effects of PEEP:

Recruits collapsed alveoli → increases functional residual capacity (FRC)
Improves V/Q matching → improves oxygenation
Reduces intrapulmonary shunt
May decrease preload (can reduce cardiac output if excessive)

PEEP Titration:

Clinical Scenario	PEEP Level
Standard/initial	5 cmH2O
Mild hypoxemia	5-8 cmH2O
Moderate ARDS	10-14 cmH2O
Severe ARDS	14-24 cmH2O
Obstructive disease (COPD)	May need applied PEEP to counterbalance auto-PEEP

Complications of Excessive PEEP:

Decreased venous return → decreased cardiac output → hypotension
Overdistension of alveoli → volutrauma/barotrauma
Increased dead space ventilation
Pneumothorax (especially at PEEP >15 cmH2O)

ABG-Based Ventilator Adjustments

The two primary goals are correcting oxygenation and ventilation:

To Improve Oxygenation (PaO2 / SpO2):

Increase FiO2
Increase PEEP
FiO2 and PEEP affect oxygenation; rate and VT primarily do not

To Improve Ventilation (PaCO2):

Increase respiratory rate (most common adjustment)
Increase tidal volume (use caution — avoid exceeding 8 mL/kg IBW)
PaCO2 is inversely proportional to minute ventilation (VE = VT x RR)

Problem	ABG Finding	Ventilator Adjustment
Hypoxemia	PaO2 <60, SpO2 <90%	Increase FiO2 or PEEP
Hyperoxia	PaO2 >100	Decrease FiO2 (maintain FiO2 ≤0.60 when possible)
Respiratory acidosis	PaCO2 >45, pH <7.35	Increase RR or VT (increase minute ventilation)
Respiratory alkalosis	PaCO2 <35, pH >7.45	Decrease RR or VT (decrease minute ventilation)

CRT/RRT Respiratory Therapist Exam

1Introduction: NBRC TMC Exam Overview

2Chapter 1: Patient Data Evaluation

3Chapter 2: Airway Management & Oxygen Therapy

4Chapter 3: Mechanical Ventilation

5Chapter 4: Pharmacology & Equipment

6Chapter 5: Infection Control, Ethics & Professional Practice