ADD THESE TO YOUR MECHANICAL VENTILATION STUDY GUIDE 15. INSPIRATORY FLOW RATE ⭐ What is inspiratory flow? Flow = how FAST the ventilator delivers the breath. Usually expressed in L/min. For volume control: Flow (L/min) = VT (L) ÷ Ti (sec) × 60 Example: VT = 600 mL = 0.6 L Ti = 1 second 0.6 ÷ 1 × 60 = 36 L/min A constant inspiratory flow around 60 L/min is commonly used when assessing mechanics in volume assist-control, although actual appropriate flow depends on the patient’s needs. HIGH-YIELD FLOW RULE

COPD / Obstructive Disease ⭐⭐⭐

Think:

COPD needs MORE TIME TO EXHALE.

Therefore:

↑ inspiratory flow → ↓ Ti → ↑ Te

This helps decrease:

• Air trapping

• Breath stacking

• Dynamic hyperinflation

• Auto-PEEP

Memory:
COPD = FAST IN, LONG OUT.

Restrictive disease / ARDS

Usually you don’t need the prolonged expiratory time required by COPD.

The bigger priorities are:

• Low VT/lung-protective ventilation

• Appropriate PEEP

• Keep plateau pressure controlled

• Avoid excessive pressures/volumes

AARC recommends assessing VT at 4–8 mL/kg predicted body weight and monitoring plateau pressure.

16. INSPIRATORY FLOW PATTERNS ⭐⭐⭐

You should recognize the waveform AND what it means.

Square / Constant Flow

FLOW

┌──────────┐

│ │

─────┘ └─────

Flow stays constant throughout inspiration.

Commonly associated with:

Volume Control

Important:

* VT = set

* Flow = set

* Pressure varies

Decelerating Flow

FLOW

|\

| \

| \

| \

─────| \────────

High flow initially → progressively decreases.

Classic for:

Pressure Control

and

Pressure Support

Pressure-controlled modes commonly produce a decelerating inspiratory flow waveform.

NBRC MEMORY

Volume Control → Square FLOW

Pressure Control → Square PRESSURE + Decelerating FLOW

That’s a very important distinction.

⸻

17. PRESSURE CONTROL (PC) ⭐⭐⭐

What do YOU set?

Think:

PC = I control PRESSURE, not volume.

Common settings:

* Inspiratory pressure / pressure control

* RR

* Inspiratory time

* PEEP

* FiO₂

* Trigger/sensitivity as appropriate

What varies?

🚨 TIDAL VOLUME

VT depends on:

* Compliance

* Airway resistance

* Set pressure

* Inspiratory time

* Patient effort

Pressure control selects an inspiratory pressure rather than a target VT, so changing compliance or resistance can change delivered VT.

PC Breath

Time-triggered or patient-triggered

Pressure-limited

Time-cycled

PC Graphics

Pressure

┌──────────┐

│ │

______│ │______

Flow:

|\

| \

| \

_______| \________

Square pressure + decelerating flow

NBRC Scenario

Patient is on PC.

Yesterday:

VT = 500 mL

Today:

VT = 300 mL

Something changed.

Think:

• ↓ Compliance

• ↑ Airway resistance

• ↓ Patient effort

• Secretions/bronchospasm

• Pneumothorax

• Pulmonary edema/ARDS progression

🚨 In pressure control, don’t assume VT stays constant.

18. PRESSURE SUPPORT (PSV) ⭐⭐⭐

This is one of the areas I would study particularly well.

PSV = spontaneous breathing mode

Patient must initiate breaths.

PSV is characterized by patient-triggered, pressure-limited, flow-cycled breaths.

What do YOU set?

• Pressure support

• PEEP

• FiO₂

• Trigger/sensitivity

• Rise time

• Cycling criterion on ventilators that allow adjustment

What does PATIENT control?

Patient largely determines:

• RR

• Inspiratory time

• VT

• Minute ventilation

Therefore:

🚨 PSV does NOT guarantee VT or minute ventilation.

No mandatory breaths are provided in pure PSV.

Cycling

PSV = FLOW-CYCLED

A commonly used cycling threshold is when inspiratory flow decreases to approximately 25% of peak inspiratory flow, although this can be adjustable depending on the ventilator.

Increasing PS does what?

↑ PS → usually ↑ VT → ↓ WOB

Decreasing PS:

↓ PS → patient performs more work

Therefore PSV is useful for:

• Supporting spontaneous breathing

• Overcoming artificial-airway/circuit resistance

• Weaning/liberation assessment

PSV GraphicS

pressure

/──────\

______/ \______

Flow:

/\

/ \

_____/ \________

19. PC vs PSV — KNOW THIS TABLE ⭐⭐⭐

Pressure Control

Pressure Support

Pressure targeted

✅

✅

VT guaranteed

❌

❌

Mandatory breaths possible

✅

❌ pure PSV

Patient can trigger

✅

✅

Time triggered possible

✅

❌

Cycling

TIME

FLOW

Inspiratory flow

Decelerating

Decelerating

RR can be set

✅

❌ pure PSV

Ti set directly

✅

Usually ❌

Common use

Full/partial ventilatory support

Spontaneous support/weaning

MEMORY

PC = TIME ends inspiration.

PS = FLOW ends inspiration.

20. VENTILATOR GRAPHICS / WAVEFORMS ⭐⭐⭐

Your professor specifically mentioned graphics, so I would know these patterns.

There are three major scalars:

Pressure – Time

Flow – Time

Volume – Time

Flow curves can reveal respiratory mechanics, patient effort, mode/settings, and patient-ventilator asynchrony.

FLOW DOES NOT RETURN TO ZERO BEFORE NEXT BREATH

🚨 AUTO-PEEP / AIR TRAPPING

Inspiration

/\

___/ \____

\

\____

---------------0

↑

next breath starts

before zero

Think:

* COPD/asthma

* RR too high

* Ti too long

* Flow too low

* Not enough expiratory time

Possible correction:

↓ RR

and/or

↑ inspiratory flow → ↓ Ti → ↑ Te

⸻

SCOOPED EXPIRATORY FLOW-VOLUME LOOP

Think:

OBSTRUCTION

Examples:

* COPD

* Asthma

* Bronchospasm

⸻

PRESSURE-TIME WAVEFORM: PATIENT TRYING TO TRIGGER

A dip/deflection in pressure without a delivered breath may mean:

Trigger sensitivity problem / ineffective triggering

Possible causes include:

* Auto-PEEP

* Weak patient effort

* Trigger setting not sensitive enough

⸻

FLOW STARVATION

Patient wants more inspiratory flow than the ventilator provides.

Think:

Patient demand > ventilator flow

Possible clues:

* Increased WOB

* Patient appears uncomfortable

* Pressure waveform may show inward scooping during inspiration

Possible response in VC:

↑ inspiratory flow

Waveform assessment is specifically useful for identifying patient-ventilator asynchrony and guiding setting changes.

⸻

21. HIGH PRESSURE ALARM — GRAPHICS + TROUBLESHOOTING ⭐⭐⭐

First:

ASSESS THE PATIENT.

Then determine:

PIP ↑ but Pplat SAME

➡ AIRWAY RESISTANCE PROBLEM

Think:

R = Resistance

* Secretions

* Bronchospasm

* Kinked ETT

* Biting tube

* Water in tubing

PIP ↑ AND Pplat ↑

➡ COMPLIANCE PROBLEM

Think:

* ARDS

* Pulmonary edema

* Atelectasis

* Pneumothorax

* Abdominal distention

Memory

PIP only = PIPE problem

PIP + PLAT = LUNG problem

⸻

22. LOW PRESSURE ALARM

Think:

LEAK / DISCONNECTION

Possible causes:

* Circuit disconnected

* ETT cuff leak

* Extubation

* Loose connection

* Large system leak

⸻

23. ADVERSE EFFECTS OF POSITIVE-PRESSURE VENTILATION (PPV) ⭐⭐⭐

This is another area I would definitely add because your professor specifically requested it.

CARDIOVASCULAR

Positive pressure increases intrathoracic pressure.

That can cause:

↓ Venous return → ↓ preload → ↓ cardiac output → hypotension

MEMORY

More pressure in chest → less blood back to heart.

High PEEP can make this more pronounced.

⸻

BAROTRAUMA

Excessive pressure can cause alveolar injury/rupture.

Think:

* Pneumothorax

* Pneumomediastinum

* Subcutaneous emphysema

⸻

VOLUTRAUMA

Excessive VT / overdistention

Can damage alveoli.

This is why lung-protective VT matters.

⸻

ATELECTRAUMA

Repeated:

Alveolar opening → collapse → opening → collapse

causes injury.

PEEP helps prevent repeated collapse when appropriately applied.

⸻

VENTILATOR-INDUCED LUNG INJURY

Think combination of:

* Barotrauma

* Volutrauma

* Atelectrauma

* Biotrauma/inflammatory injury

This is why current ventilator assessment emphasizes VT, plateau pressure, PEEP/auto-PEEP and driving pressure.

⸻

AUTO-PEEP / DYNAMIC HYPERINFLATION

Especially important with:

COPD / asthma

Causes:

* RR too high

* Insufficient expiratory time

* Low inspiratory flow causing long Ti

* Airway obstruction

Consequences:

* Air trapping

* ↑ WOB

* Hypotension

* Difficulty triggering ventilator

* Possible barotrauma

⸻

⭐ THE PAGE I WOULD MEMORIZE BEFORE YOUR EXAM

FLOW

* ↑ Flow → ↓ Ti → ↑ Te

* COPD → increase flow / allow long exhalation

* VC → commonly square flow

* PC/PS → decelerating flow

PRESSURE CONTROL

* Pressure = set

* VT = variable

* Flow = variable/decelerating

* Time-cycled

PRESSURE SUPPORT

* Patient-triggered

* Pressure-limited

* Flow-cycled

* VT/RR/VE depend substantially on patient + mechanics

* ↑ PS → usually ↑ VT and ↓ WOB

GRAPHICS

* Expiratory flow doesn’t reach zero → AUTO-PEEP

* PIP ↑ / Pplat same → ↑ resistance

* PIP ↑ / Pplat ↑ → ↓ compliance

* Low pressure → leak/disconnection

* Scooped expiratory loop → obstruction

PPV COMPLICATIONS

* ↓ venous return

* ↓ cardiac output

* Hypotension

* Barotrauma

* Volutrauma

* Atelectrauma

* Auto-PEEP/dynamic hyperinflation

* Ventilator-induced lung injury

The biggest new material I would concentrate on from what your professor specifically asked you about is flow, PC vs PS, recognizing waveform abnormalities, and adverse effects of positive pressure. Those fit extremely well with scenario-style questions where you’re shown a ventilator problem and asked what setting to change.