Overview of Mechanical Ventilation – Key Exam Points

50 Q&A-style flashcards covering indications, physiologic/clinical goals, initial settings, modes, alarms, and troubleshooting of mechanical ventilation.

1) Acute ventilatory failure 2) Impending ventilatory failure 3) Severe hypoxemia 4) Prophylactic ventilatory support

What are the four primary indications for initiating mechanical ventilation?

Which arterial blood-gas values define acute ventilatory failure in most adults?

A sudden PaCO2 rise to >50 mm Hg accompanied by a respiratory acidosis with pH <7.30

In a COPD patient, what specific change in PaCO2 indicates acute ventilatory failure?

An acute increase above the patient’s usual baseline PaCO2 accompanied by decompensated respiratory acidosis

Give two clinical signs (other than blood gases) that can suggest acute ventilatory failure.

Any of: apnea, bradypnea, acute lung injury (ALI), acute respiratory distress syndrome (ARDS)

How is impending ventilatory failure generally characterized?

Marginally normal blood gases maintained only at the cost of a markedly increased work of breathing, which leads to fatigue and eventual failure

A spontaneous tidal volume below what value (mL / kg) signals impending ventilatory failure?

A spontaneous breathing frequency above what level (breaths / min) suggests impending ventilatory failure?

30 breaths / min

What vital capacity (mL / kg) indicates impending ventilatory failure?

What maximum inspiratory pressure (MIP) value indicates impending ventilatory failure?

MIP less negative than –20 cm H2O (i.e., ≤|20| cm H2O)

A gradual but persistent PaCO2 increase above what level (mm Hg) indicates impending ventilatory failure?

50 mm Hg

List three vital-sign changes that can warn of impending ventilatory failure.

Any three of: tachycardia, arrhythmias, hypertension, tachypnea, use of accessory muscles, diaphoresis, cyanosis

State the formula for estimating normal supine PaO2 in adults.

Supine PaO2 = 109 – (0.43 × age) ± 8 mm Hg

Define severe hypoxemia in terms of PaO2 regardless of FiO2.

PaO2 <40 mm Hg at any FiO2

Define severe hypoxemia in terms of PaO2 when FiO2 >50%.

PaO2

At 100% FiO2, every 50-mm Hg rise in P(A-a)O2 approximates what shunt percentage?

Roughly a 2 % right-to-left shunt

Name the three clinical/radiographic features common to both ALI and ARDS.

Acute onset, bilateral infiltrates on frontal chest radiograph, and pulmonary capillary wedge pressure ≤18 mm Hg

What PaO2 / FiO2 (P/F) ratio defines Acute Lung Injury (ALI)?

P/F ratio ≤300 mm Hg

What PaO2 / FiO2 (P/F) ratio defines Acute Respiratory Distress Syndrome (ARDS)?

P/F ratio ≤200 mm Hg

Write the equation for the P/F ratio.

P/F = PaO2 (mm Hg) ÷ FiO2 (expressed as a decimal)

Give two physiologic goals of mechanical ventilation related to gas exchange.

1) Support or manipulate gas exchange 2) Maintain alveolar ventilation (PaCO2 and pH)

Give two physiologic goals of mechanical ventilation related to lung mechanics.

1) Maintain arterial oxygenation (PaO2, SaO2, SpO2, CaO2, DO2) 2) Increase end-expiratory lung volume / functional residual capacity (FRC)

Give two physiologic goals of mechanical ventilation related to patient comfort or safety.

1) Reduce or manipulate work of breathing 2) Ensure patient-ventilator synchrony or minimize cardiovascular impairment

State two clinical goals of mechanical ventilation concerned with reversing acute problems.

1) Reverse hypoxemia 2) Reverse acute respiratory acidosis

State two clinical goals of mechanical ventilation aimed at reducing oxygen demand or supporting circulation.

1) Decrease systemic or myocardial oxygen consumption 2) Maintain or improve cardiac output

State two clinical goals of mechanical ventilation related to neurologic or structural stability.

1) Reduce intracranial pressure 2) Stabilize the chest or prevent/reverse atelectasis

What three key decisions must be made when initiating mechanical ventilation?

a) Choose the ventilation mode b) Select an appropriate device c) Establish initial ventilator settings

How is full ventilatory support (FVS) defined?

A mode in which the ventilator assumes essentially 100 % of the patient’s work of breathing

How is partial ventilatory support (PVS) defined?

A mode in which the ventilator supplies only a portion of the work of breathing, the patient does the rest

What is the basic difference between a single control mode (SCM) and a dual control mode (DCM)?

SCM uses one control variable (pressure OR volume); DCM uses two control variables (pressure AND volume) governed by separate feedback loops

Name two common volume-targeted ventilation modes.

Volume Assist/Control (VA/C) and synchronized intermittent mandatory ventilation (SIMV-Vt)

Name two common pressure-targeted ventilation modes.

Pressure Assist/Control (PA/C) and Pressure Support Ventilation (PSV)

What initial tidal-volume setting (mL / kg PBW) is recommended for patients with normal lungs?

6–8 mL / kg predicted body weight

What tidal-volume range (mL / kg PBW) is recommended for ALI/ARDS or acute asthma?

4–8 mL / kg predicted body weight

To avoid overdistention, plateau pressure (Pplat) should be kept below what value?

List two reasons why the delivered tidal volume may be lower than the set tidal volume.

1) Gas leakage in the ventilator circuit or around the endotracheal-tube cuff 2) Circuit compressible volume loss

Provide the formula for calculating corrected tidal volume (Vt).

Corrected Vt = Expired Vt – Circuit Compressible Volume

What initial set respiratory frequency is typically chosen to maintain eucapneic ventilation?

10–12 breaths / min

Why are set respiratory frequencies of 20 breaths / min or higher generally avoided?

They promote short expiratory times, leading to air trapping and auto-PEEP

What usual I:E ratio range is employed, and why might it be lengthened?

1:2 to 1:4; it may be lengthened to give patients with obstructive disease more time to exhale and avoid air trapping

What default PEEP level (cm H2O) is commonly applied at initiation?

5 cm H2O

What PEEP level range (cm H2O) is often required for moderate–severe ARDS?

15–20 cm H2O

What initial FiO2 is recommended for patients with severe hypoxemia or major cardiopulmonary compromise?

Start at 1.0 (100 % FiO2)

After stabilization, to what FiO2 level should clinicians attempt to reduce oxygen to avoid toxicity?

For flow triggering, what sensitivity range (L / min) is usually set to avoid auto-triggering yet minimize effort?

1–2 L / min (may need 3–4 L / min with disposable circuits)

During weaning, at what pressure-support (PS) level can extubation be considered if tolerated for 2 h?

When PS is reduced to 5–8 cm H2O without signs of respiratory distress

How should the low exhaled-volume alarm be set relative to measured exhaled tidal volume?

≈100 mL below the patient’s expired mechanical tidal volume

How should the low inspiratory-pressure alarm be set relative to observed peak inspiratory pressure (PIP)?

10–15 cm H2O below the observed PIP

How should the high inspiratory-pressure alarm be set relative to observed PIP?

10–15 cm H2O above the observed PIP

List three common causes of a high inspiratory-pressure alarm.

Water in tubing, ETT kinking/biting, secretions or coughing (also bronchospasm, mucus plug, decreased compliance, tension pneumothorax)

What does the mnemonic D.O.P.E. stand for in ventilator troubleshooting?

Displacement of tube/cuff, Obstruction of tube/circuit, Pneumothorax, Equipment failure