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Mechanical Ventilator - Comprehensive Guide (EMT/Paramedical Level)

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1. BASIC PHYSIOLOGY YOU MUST KNOW

Minute Volume (VE)

• Normal VE = 7-10 L/min
• VT = Alveolar Volume (VA) + Dead Space Volume (VDS)
• Anatomic dead space ~ 2.2 mL/kg lean body weight (trachea + larger airways)
• Alveolar ventilation determines PaCO2 (carbon dioxide in blood)

Key Pressure Concepts

• Stiffer lungs (low compliance) = higher pressures for same volume
• Bigger volume = higher pressure

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2. VENTILATOR SETTINGS (Key Parameters)

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3. VENTILATOR MODES

A. Spontaneous / Support Modes

• Patient triggers every breath; ventilator adds a set pressure boost
• Patient controls rate and tidal volume
• No set respiratory rate (backup apnea rate present)
• Used for: weaning, alert/breathing patients

• Constant positive pressure throughout breathing cycle
• Patient breathes spontaneously against set pressure
• Used for: sleep apnea, mild respiratory failure, post-extubation support

• IPAP (inspiratory) + EPAP (expiratory) pressures set separately
• IPAP initial: 10-25 cm H2O | EPAP initial: 5-8 cm H2O
• PSV = IPAP - EPAP (the support given to each breath)
• Titrate FiO2 to maintain SpO2 86-92%

B. Mandatory (Machine-Controlled) Modes

• Ventilator delivers a set tidal volume regardless of pressure needed
• Clinician sets: VT, RR, FiO2, PEEP, flow rate
• Advantage: guaranteed volume delivery
• Risk: pressure can spike if lungs become stiffer

• Ventilator delivers gas to a set pressure; volume varies with compliance
• Clinician sets: pressure target, RR, FiO2, PEEP
• Advantage: safer pressures, better for stiff lungs
• Risk: VT drops if compliance worsens

• Every patient breath triggers a full machine-assisted breath
• Machine also delivers breaths if patient doesn't breathe at set rate
• Most common initial mode in emergency/ICU
• Can be used with VCV (AC-VCV) or PCV (AC-PCV)

• Machine delivers set number of mandatory breaths synchronized with patient effort
• Patient can breathe spontaneously between mandatory breaths
• Add PSV on top of SIMV for patient comfort

C. Advanced/Specialty Modes

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4. INITIATING MECHANICAL VENTILATION

Indications

• Respiratory or cardiac arrest
• Coma / altered consciousness
• Refractory hypoxemia (SpO2 < 90% despite O2)
• Severe respiratory acidosis
• Paradoxical breathing / impending fatigue
• Failure of non-invasive ventilation

RSI (Rapid Sequence Intubation) for Asthma/Bronchospasm

• Ketamine 1-2 mg/kg - preferred induction (bronchodilator + sympathomimetic)
• Propofol 1.5-2 mg/kg - alternative (bronchodilator but can cause hypotension)
• Succinylcholine 1.5 mg/kg OR Rocuronium 1 mg/kg for paralysis
• Use large ETT ≥ 8.0 mm to allow suctioning and bronchoscopy

Initial Settings for a Standard Adult Patient

• RR: 10-14 | VT: 7-8 mL/kg IBW | PEEP: 5 | FiO2: 100%
• Flow rate: 60 L/min | Waveform: Decelerating

• RR: 12-16 | Pressure high: ~20 cm H2O | PEEP: 5 | FiO2: 100%
• Monitor VT obtained; adjust pressure to achieve ~7-8 mL/kg IBW

• Low-high approach: IPAP 10 / EPAP 5 / FiO2 100%, titrate IPAP up
• High-low approach: IPAP 20-25 / EPAP 5 / FiO2 100%, titrate down
• Reassess at 30 minutes; check ABG within 1-2 hours

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5. VENTILATOR EMERGENCIES & TROUBLESHOOTING

The DOPES Mnemonic (Sudden Deterioration on Ventilator)

Alarm Troubleshooting Table

Hypotension on Ventilator

1. Auto-PEEP / air trapping - most common in asthma/COPD
   • Action: Disconnect patient from ventilator for 20-30 seconds to allow full expiration; reduce RR; increase flow rate
2. Tension pneumothorax
   • Action: Needle decompression (2nd ICS MCL or 4th-5th ICS AAL)
3. Decreased venous return from high PEEP/hyperinflation
4. Volume depletion (give IV fluids)
5. Sedative-induced vasodilation

Barotrauma

• Prevention: Keep plateau pressure < 30 cm H2O, tidal volume 6-8 mL/kg IBW
• Permissive hypercapnia (accept higher CO2 to keep pressures safe) - especially in asthma/ARDS

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6. LUNG-PROTECTIVE VENTILATION (Critical Concept)

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7. WEANING FROM VENTILATOR

When to Consider Weaning

• Underlying cause of respiratory failure is resolving
• Patient is awake, following commands
• Hemodynamically stable (minimal/no vasopressors)
• FiO2 ≤ 40-50% and PEEP ≤ 5-8 cm H2O
• Adequate oxygenation (SpO2 > 92% on low settings)

Rapid Shallow Breathing Index (RSBI) = f/VT

• Most validated weaning predictor
• Measure during spontaneous breathing (no pressure support)
• RSBI < 100 = likely successful weaning
• RSBI > 100 = rapid, shallow breathing = likely weaning failure

Spontaneous Breathing Trial (SBT)

• Place patient on T-piece, low-level CPAP (5 cm H2O), or minimal PSV (5-8)
• Duration: 30 minutes to 2 hours
• Monitor: RR, SpO2, heart rate, BP, work of breathing, mental status
• If patient tolerates → proceed to extubation
• If patient fails → resume full ventilatory support; identify and treat cause

4 Methods for Weaning Trial

1. T-tube trials - patient breathes through T-piece; oldest method
2. SIMV - gradually reduce mandatory breath rate
3. PSV - gradually reduce pressure support level
4. CPAP - patient breathes spontaneously at low PEEP

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8. QUICK REFERENCE - WAVEFORM SELECTION

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9. NON-INVASIVE VENTILATION (NIV/NPPV) KEY POINTS

• Best evidence: COPD exacerbation, cardiogenic pulmonary edema, immunocompromised patients
• Contraindications: respiratory arrest, inability to protect airway, excessive secretions, facial trauma, vomiting
• Mask fit is critical - check for leaks
• Reassess response within 30 minutes
• If no improvement or deterioration → intubate

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Summary - EMT/Paramedic Priority Actions

1. Any sudden deterioration - disconnect from ventilator, give BVM, use DOPES mnemonic
2. High pressure alarm - suction first, then bronchodilator, then check for pneumothorax
3. Low pressure alarm - check all circuit connections immediately
4. Hypotension - think auto-PEEP (disconnect briefly) or tension pneumothorax
5. Power failure - switch to BVM; never leave patient without ventilation
6. Always use ideal body weight for tidal volume calculations, not actual weight

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🫁 VENTILATOR - BASIC TO ADVANCED (All Modes)

Average Student-ku Pura'njukura Mathiri - Simple Tamil-English Mix

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FIRST THINGS FIRST - Ventilator Enna Panradhu?

"Patient-ku saaga vida maatom - machine breath kudukuthu"

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CORE CONCEPT - 3 PILLARS

VENTILATOR = VOLUME control + PRESSURE control + PATIENT effort

• Atha based-a MODES decide panrom.

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PART 1: BASIC SETTINGS (Settings Enna Enna Set Panurom?)

⚠️ IBW = Ideal Body Weight - Actual weight use panna KOODATHU. Height-based calculate panurom. Why? Heavy patient-ku lungs same size dhan. Actual weight use panna over-ventilation aagum.

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PART 2: VENTILATOR MODES - ONE BY ONE

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MODE 1: CONTROLLED VENTILATION (CV)

aka IMV - Intermittent Mandatory Ventilation

Machine FULL control. Patient zero effort. Machine decides everything.

Patient-ai auto-pilot plane-la vacha mathiri. Pilot (patient) hands off. Machine alone fly pannuthu.

• Machine-e decide pannum: Rate, Volume, everything
• Patient breathe panna try pannaluam - MACHINE LISTEN PANNADU
• Patient effort = 0 (or heavily sedated/paralyzed)

• General anesthesia (OT-la)
• Heavily sedated ICU patients
• Completely paralyzed patients (neuromuscular blockade)
• Apnea (breathe panna marakirkavan)

• VT, RR, FiO2, PEEP - all of it

• Doctor 100% control pannan

• Patient-ku heavy sedation MUST (otherwise patient fight pannan with machine - "dyssynchrony")
• Patient effort use aagadu = muscle waste aagum

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MODE 2: ASSIST-CONTROL (AC) Mode

MOST COMMON - Emergency-la Default Mode

Patient breath edukka try pannan - Machine help pannum. Patient breath edukka marandha - Machine alone pannum.

Auto-rickshaw back-a thallu pannum mathiri. Nee thallu pannan - auto thoothu varum. Nee thallu pannalana - auto dhan thallu pannikkum. But every breath = FULL support.

1. Patient inspiratory effort pannan (trigger) → Machine senses it → Full breath delivers
2. Patient effort illana → Machine set rate-la breath delivers
3. Every single breath = Full machine-assisted breath

AC - Volume Control (AC-VC)

• Machine guarantees fixed VT (e.g., 500 mL every time)
• Pressure varies based on lung stiffness
• You set: VT, RR, PEEP, FiO2, Flow rate
• You watch: Peak pressure, Plateau pressure

Problem: Lung stiff aana → Pressure increase aagum → Barotrauma risk

AC - Pressure Control (AC-PC)

• Machine guarantees fixed Pressure (e.g., 20 cm H2O every time)
• Volume varies based on lung compliance
• You set: Driving pressure, RR, PEEP, FiO2
• You watch: Tidal Volume, Minute ventilation

Problem: Lung compliance change aana → VT change aagum → Under/over-ventilation risk

• Newly intubated patients (ICU, ED)
• Critically ill, unstable patients
• Patients who need rest (full support needed)

• Minimum VE guaranteed
• Less sedation needed than pure controlled
• Patient can trigger extra breaths

• Patient fast breathe panan → Machine follows → Respiratory alkalosis (too much CO2 blown off)
• Asthma/COPD-la dangerous (air trapping = auto-PEEP)

AC-VC: RR 10-14, VT 7-8 mL/kg IBW, PEEP 5, FiO2 100%, Flow 60 L/min
AC-PC: RR 12-16, Pressure 20 cm H2O, PEEP 5, FiO2 100%

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MODE 3: SIMV - Synchronized Intermittent Mandatory Ventilation

Machine certain number of breaths kudukum. Middle-la patient sarakku breathe pannalaam - but alone (no help).

School-la attendance mathiri. Machine 10 breaths mandatory kudukum. Nee 5 extra breathe eduka want pana - edukalam, but without help. Own effort.

1. Clinician sets mandatory RR (e.g., 10/min)
2. Machine delivers those 10 breaths - synchronized with patient effort
3. Between those breaths → Patient can breathe spontaneously, but NO machine help (unless PSV added on top)

• SIMV-VC: Mandatory breaths = volume-targeted
• SIMV-PC: Mandatory breaths = pressure-targeted

Usually SIMV + PSV together use panurom. Why? Spontaneous breaths-ku support kudukka. Otherwise patient extra work pannum.

• Weaning process (slowly reducing machine support)
• Spontaneously breathing patients who need partial support

• Patient uses respiratory muscles = muscle strength maintain aaguthu
• Weaning tool as support slowly reduced

• Spontaneous breaths unsupported (unless PSV added) = patient works hard between mandatory breaths
• Complex monitoring needed

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MODE 4: PSV - Pressure Support Ventilation

Patient breathe pannanum - machine boost kudukum. Patient breathe pannalana - machine pannadu.

Bike-la accelerator pidi mathiri. Nee pedal pannanum dhan - machine power add pannum. But pedal pannalana - nothing happens. 100% patient effort needed to start.

1. Patient makes inspiratory effort (trigger)
2. Machine adds a preset pressure boost (e.g., PS 10)
3. Breath ends when flow drops below threshold (~25% of peak)
4. Patient controls: Rate, I-time, Flow demand
5. Machine only controls: Pressure support level + PEEP + FiO2

• No set respiratory rate
• Backup apnea rate present in modern ventilators (safety net)
• VT depends on PS level + patient effort + lung compliance

• Awake, breathing patients who need help
• Weaning from ventilator
• BiPAP (non-invasive) = essentially PSV

• Patient comfort - breathes naturally
• Respiratory muscles stay active
• Best weaning mode

• Patient apnea aana - no backup (danger! - that's why backup rate set panrom)
• VT not guaranteed

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MODE 5: CPAP - Continuous Positive Airway Pressure

Constant pressure maintain panrom. Patient tannoda breathe pannum. Machine breath kudukadu - just "open door" kudukuthu.

Balloon slightly inflate vacha mathiri lungs vaikkurom. Deflate aagaama. Patient tannoda breathe pannum through that open balloon.

• Same pressure (positive) maintained throughout - both inhale AND exhale
• No inspiratory pressure boost (unlike PSV)
• Patient does ALL the work
• Machine just maintains the set PEEP level

• Weaning (SBT - Spontaneous Breathing Trial)
• Sleep apnea (non-invasive CPAP at home)
• Mild respiratory failure
• Post-extubation support
• RSBI test pannuvom - CPAP 5 + PS 0 la 3 minutes vaippom

CPAP = Constant pressure, no boost during inhalation
PSV  = Pressure BOOST added during each inhalation

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MODE 6: BiPAP - Bilevel Positive Airway Pressure

Inhale-la higher pressure, Exhale-la lower pressure. Non-invasive (mask, no tube).

Door-a open panna help panrom (IPAP), then slightly close but not full close (EPAP). Two levels of pressure.

• IPAP = Inspiratory Positive Airway Pressure (higher level)
• EPAP = Expiratory Positive Airway Pressure (lower level)
• Pressure Support = IPAP - EPAP (actual support given)

Low-High Approach: IPAP 10 / EPAP 5 / FiO2 100%
High-Low Approach: IPAP 20-25 / EPAP 5 / FiO2 100%

EPAP 8 if: Morbid obesity or auto-PEEP

• Hypercapnia (CO2 high) → Increase IPAP (increase PS)
• Hypoxia (O2 low) → Increase EPAP + FiO2
• Fast RR / dyspnea → Increase IPAP

• COPD exacerbation (BEST evidence)
• Cardiogenic pulmonary edema (BEST evidence)
• Immunocompromised patients (avoid intubation)
• Obesity hypoventilation

• Respiratory arrest
• Vomiting / aspiration risk
• Cannot protect airway
• Facial trauma
• Excessive secretions

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MODE 7: APRV - Airway Pressure Release Ventilation

(Advanced Mode)

Lungs-ai HIGH pressure-la hold pannurom (oxygenation). Brief-a LOW pressure-la release panrom (CO2 out). Patient throughout breathe pannalaam.

Dam-la water (air) hold panrom. Short-a gate open panurom - flush aaguthu (CO2 out). Gate close - dam full again (oxygenation).

• P-high = High pressure level (oxygenation) - e.g., 25-35 cm H2O
• P-low = Low pressure (usually 0) - brief release
• T-high = Time at high pressure (4-6 seconds)
• T-low = Time at low pressure (0.2-0.8 sec for restrictive, 0.8-1.5 sec for obstructive)

• Patient can breathe spontaneously throughout cycle
• Most time at P-high → oxygenation
• Brief release to P-low → CO2 clearance
• Less sedation needed (unlike inverse ratio ventilation)

• Severe ARDS (refractory hypoxemia)
• Patients who failed conventional modes
• Open lung ventilation strategy

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MODE 8: PRVC / Volume Control Plus

(Dual/Hybrid Mode - Advanced)

Machine guarantee-a oru volume kudukum, but pressure adjust panudu - lungs protect pannuvadha.

Smart AC mathiri - temperature target fix. But power automatically adjust pannum based on room condition.

• Closed-loop system
• Each breath-ku compliance calculate pannum
• Next breath-la pressure adjust pannum to achieve target VT
• Volume guaranteed + Pressure kept as low as possible

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QUICK COMPARISON TABLE - All Modes at a Glance

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PART 3: PATIENT CONDITION → CORRECT MODE

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PART 4: WEANING STEP-BY-STEP

• Patient awake, following commands ✓
• Hemodynamically stable ✓
• Cause of intubation resolving ✓
• FiO2 ≤ 40-50% ✓
• PEEP ≤ 5-8 cm H2O ✓
• RSBI < 105 ✓ (RR ÷ VT in Liters → e.g., 20 breaths / 0.5 L = 40 = good)

• Put patient on CPAP 5 / PS 0 for 3 minutes
• Calculate RSBI = Respiratory Rate ÷ Tidal Volume (L)
• RSBI < 105 = likely can wean
• RSBI > 105 = 95% chance of failure - don't extubate

• CPAP 5 + PS 6-8 OR T-piece
• Watch 30-120 minutes
• STOP if: RR > 35, SpO2 < 90%, HR > 140, BP > 180 or < 90, agitation, distress

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PART 5: EMERGENCY TROUBLESHOOTING

DOPES Mnemonic (Patient deteriorates on ventilator)

Alarm Table

Hypotension on Vent - Think This Order:

1. Auto-PEEP - disconnect briefly (most common in COPD/asthma)
2. Tension pneumothorax - needle decompress
3. High PEEP - reduce PEEP
4. Volume depletion - IV fluids
5. Sedation-induced - reduce sedation

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EXAM SUMMARY BOX

MOST COMMON INITIAL MODE    → AC-VC
BEST MODE FOR WEANING       → PSV → CPAP
BEST NON-INVASIVE           → BiPAP
SEVERE ARDS MODE            → AC-VC (low VT) or APRV
PARALYZED PATIENT ONLY      → Controlled (CV/IMV)
SMART HYBRID MODE           → PRVC

RSBI < 105   = EXTUBATE ready
RSBI > 105   = 95% fail - don't extubate
Plateau P    < 30 cm H2O always
Tidal Vol    = 6-8 mL/kg IBW (never actual weight)

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🫁 ANATOMY & PHYSIOLOGY OF VENTILATION

Basic to Advanced - Average Student Purinja Mathiri

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PART 1: BIG PICTURE - Enna Nadakuradhu?

ATMOSPHERE
    ↓  (Air breathe in)
CONDUCTING AIRWAYS  (Transport panurom - NO gas exchange)
    ↓
ALVEOLI  (Gas exchange nadakudu - O2 in, CO2 out)
    ↓
BLOOD → HEART → WHOLE BODY
    ↑  (CO2 collect panni tirupi lungs-ku)
CELLS

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PART 2: ANATOMY - Respiratory System Structure

TWO ZONES - Conducting Zone vs Respiratory Zone

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ZONE 1: CONDUCTING ZONE (Air Highway)

NOSE / MOUTH
    ↓
NASOPHARYNX / OROPHARYNX
    ↓
LARYNX (Voice box - epiglottis here)
    ↓
TRACHEA (Windpipe)
    ↓
RIGHT MAIN BRONCHUS + LEFT MAIN BRONCHUS (Carina-la split)
    ↓
BRONCHI (increasingly smaller)
    ↓
BRONCHIOLES
    ↓
TERMINAL BRONCHIOLES  ← End of conducting zone

🔹 Trachea

• Length: 10-13 cm
• C-shaped cartilaginous rings (horseshoe shape) - front + sides protected; back = membranous wall
• Cricoid cartilage = narrowest part (adults): Men 17 mm, Women 13 mm
• Splits at CARINA → Right + Left main bronchus

🔹 Right vs Left Main Bronchus

⚠️ Clinical Pearl: ETT accidentally deep-a push panna → Right side-la goes (right mainstem intubation). Left lung ventilate aagadu. Left-side breath sounds disappear.

🔹 Airway Generations

• Trachea = Generation 0
• Right + Left bronchi = Generation 1
• Total 23 generations exist
• Each generation = smaller tubes, but MORE number = total area INCREASES

🔹 Airway Wall Layers

Cartilage = structural support. Bronchioles-la cartilage illai → Smooth muscle alone controls diameter → Bronchospasm possible here!

🔹 Smooth Muscle Control

• Sympathetic (β2 receptors): Bronchodilation - airways OPEN
  • Adrenaline, Salbutamol (albuterol) work here
• Parasympathetic (Muscarinic receptors): Bronchoconstriction - airways CLOSE
  • Atropine, Ipratropium block this

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ZONE 2: RESPIRATORY ZONE (Gas Exchange Zone)

RESPIRATORY BRONCHIOLES (some alveoli here)
    ↓
ALVEOLAR DUCTS (fully lined with alveoli)
    ↓
ALVEOLAR SACS
    ↓
ALVEOLI ← Gas exchange happens here!

🔹 Alveoli - The Star of the Show

• Each lung: ~300 million alveoli (total body = 600 million)
• Size: ~200 micrometers diameter each
• Total surface area: ~70 square meters (size of a tennis court!)
• Walls: Ultra thin → allows rapid O2/CO2 diffusion
• Lined with Type I pneumocytes (gas exchange) + Type II pneumocytes (surfactant production)

🔹 Surfactant - Why Important?

Alveoli = bubbles. Bubbles want to collapse (surface tension). Surfactant is soap - reduces surface tension, prevents collapse.

Pressure = 2 × Surface tension / Radius

• Small alveolus → surfactant MORE concentrated → tension LOW → won't collapse
• Large alveolus → surfactant LESS concentrated → tension HIGH → won't over-expand
• Net effect: All alveoli stay stable size

⚠️ Why PEEP is needed on ventilator? Surfactant loss (ARDS, prematurity) or low lung volume → alveoli collapse → PEEP open pannuthu alveoli-ai.

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PART 3: PLEURA - Two Layers

LUNG
  ↓ (hugs the lung)
VISCERAL PLEURA
  [Intrapleural space - thin fluid layer, negative pressure]
PARIETAL PLEURA
  ↓ (lines chest wall)
CHEST WALL

• This negative pressure keeps lungs expanded
• Lung-ku collapse aagathu pull pannuthu

⚠️ Pneumothorax: Air enters pleural space → negative pressure lost → lung collapses. Tension PTX: Pressure builds → mediastinum shifts → cardiac output drops → Emergency!

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PART 4: MUSCLES OF BREATHING

Inspiration Muscles (Active process - energy use pannurom)

• Diaphragm (75% of work) - dome shape → contracts → flattens → chest volume increases
  • Moves down 1.5 to 7 cm during breathing
  • Nerve supply: Phrenic nerve (C3, C4, C5) - "C3, 4, 5 keeps the diaphragm alive"

• External intercostal muscles - ribs up + out → chest expands
• Sternocleidomastoid (SCM) - elevates rib cage (distress-la use)
• Scalene muscles - prevents upper rib from collapsing inward
• Pectoralis muscles (arms fixed vacha) - chest expansion assist

⚠️ Accessory muscle use = respiratory distress sign! Patient-ku SCM, intercostal retractions visible = working hard to breathe. Intubation consider panum time.

Expiration Muscles (Normally PASSIVE - no energy needed!)

• Abdominal muscles (rectus abdominis, obliques) - diaphragm-ai up push pannuthu
• Internal intercostal muscles - ribs down + in

Ventilator pearl: Ventilator expiration phase = 100% passive. Machine just opens expiratory valve → elastic recoil pushes air out. If expiration time not enough → air trap → auto-PEEP.

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PART 5: MECHANICS OF BREATHING - How Air Moves

Boyle's Law Basis

Pressure × Volume = Constant Volume increase → Pressure decrease → Air flows IN Volume decrease → Pressure increase → Air flows OUT

Normal Inspiration

Brain says "breathe" → Phrenic nerve → Diaphragm contracts
    → Thoracic cavity volume INCREASES
    → Intrapleural pressure becomes MORE negative (-5 → -8 cm H2O)
    → Lungs expand (pulled by suction)
    → Intra-alveolar pressure drops BELOW atmospheric
    → Air flows IN (nose → alveoli)

Normal Expiration

Diaphragm relaxes
    → Thoracic cavity volume DECREASES
    → Elastic recoil of lungs kicks in
    → Intra-alveolar pressure RISES above atmospheric
    → Air flows OUT
    → (Passive - no muscle work!)

On Ventilator - REVERSED Mechanics!

This is why ventilator is called Positive Pressure Ventilation - opposite of natural breathing!

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PART 6: LUNG VOLUMES (Numbers to Know)

FRC = Resting lung volume. PEEP increases FRC → keeps alveoli open. RV = Never goes out. Even after maximum forced exhale, 1.2 L remains.

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PART 7: DEAD SPACE - Important Concept

Types:

• Nose, trachea, bronchi, bronchioles (conducting zone)
• Air here doesn't reach alveoli for exchange
• Rule: ~2.2 mL/kg lean body weight
• Easy memory: 1 mL per pound body weight (150 lb → ~150 mL)

• Alveoli that are ventilated but NOT perfused (no blood flow)
• Normally zero in healthy lungs
• Increases in: pulmonary embolism, low cardiac output

• Healthy person: ~150 mL (mostly anatomical)
• Sick patient: Can be much higher

Formula for Alveolar Ventilation:

VA (Alveolar ventilation) = (VT - VD) × Rate

Example:
VT = 500 mL, Dead space = 150 mL, Rate = 14
VA = (500 - 150) × 14 = 350 × 14 = 4900 mL/min

Ventilator impact: ETT adds dead space (tube itself). Tracheostomy reduces dead space (shorter path to lungs) → easier weaning.

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PART 8: GAS EXCHANGE - O2 in, CO2 out

Where it happens: Alveolar-Capillary Membrane

1. Surfactant layer
2. Alveolar epithelium (Type I cells)
3. Basement membrane
4. Capillary endothelium

Partial Pressures - The Driving Force

The Golden Relationship - Alveolar Ventilation & CO2

PaCO2 = VCO2 × K / VA

Alveolar ventilation ↑ → PaCO2 ↓ (blows off CO2)
Alveolar ventilation ↓ → PaCO2 ↑ (CO2 builds up = hypercapnia)

This is why ventilator RR and VT control CO2!

• CO2 high (acidosis) → Increase RR or VT → blow off more CO2
• CO2 low (alkalosis) → Decrease RR or VT → retain more CO2

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PART 9: COMPLIANCE - Stiffness of Lungs

Compliance = Change in Volume / Change in Pressure

CL (Lung) = Normal: 150-200 mL/cm H2O
CW (Chest wall) = Normal: 200 mL/cm H2O
Total = 100 mL/cm H2O

New balloon = high compliance (easy to blow) Old balloon = low compliance (hard to blow, stiff)

• Low compliance → plateau pressure rises → barotrauma risk
• That's why we monitor plateau pressure < 30 cm H2O

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PART 10: AIRWAY RESISTANCE

• Bronchospasm (asthma - smooth muscle contracts)
• Secretions / mucus plug
• Mucosal edema (allergic, infection)
• Low lung volume (airway collapse)

Peak Pressure - Plateau Pressure = Peak-Plateau Gradient Normal < 4 cm H2O HIGH gradient (>4) = increased resistance (bronchospasm, secretion)

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PART 11: CONTROL OF BREATHING - Brain's Role

Three Brain Centers

CEREBRAL CORTEX
→ VOLUNTARY breathing (you decide when to breathe, speak, hold breath)

PONS (Pneumotaxic center)
→ Fine-tunes rhythm, controls respiratory rate

MEDULLA OBLONGATA (Main automatic center)
→ AUTOMATIC breathing (you don't think about it - happens 24/7)
→ DRG (Dorsal Respiratory Group) - Inspiration
→ VRG (Ventral Respiratory Group) - Expiration + Apneustic center

Chemical Control - What triggers breathing?

• Detect CO2 / H+ in cerebrospinal fluid (CSF)
• CO2 crosses blood-brain barrier → forms H+ → stimulates breathing
• Main driver in normal people

• Carotid body - most important (IX nerve)
• Aortic body (X nerve)
• Detect: Low O2 (< 60 mmHg), High CO2, Low pH
• Backup system

Important: HYPOXIC DRIVE in COPD!

• They ALWAYS have high CO2 (used to it - brain becomes desensitized)
• Their main trigger = LOW O2 (hypoxic drive)
• If you give too much O2 → PO2 rises → hypoxic drive removed → patient stops breathing!
• Therefore: COPD patients → Target SpO2 88-92% (not 100%)

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PART 12: V/Q RATIO - Ventilation/Perfusion

• Both air and blood present → gas exchange perfect

Why PEEP helps in ARDS? PEEP opens collapsed alveoli → V/Q improves → O2 improves

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SUMMARY CHART - All Key Numbers

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FLOW CHART: Ventilation to Ventilator (How it connects)

PROBLEM: Patient can't breathe normally
         ↓
Why? Choose one:
         ↓
MUSCLES weak?         LUNGS stiff?          AIRWAY blocked?
(ALS, GBS,            (ARDS, Pulm edema,    (Asthma, COPD,
 Myasthenia)           Fibrosis)             Secretions)
         ↓                 ↓                      ↓
Diaphragm fails      Compliance LOW         Resistance HIGH
         ↓                 ↓                      ↓
No neg. pressure    Need high pressure      Need flow + time
to breathe          to open stiff lungs     for expiration
         ↓                 ↓                      ↓
VENTILATOR solves by POSITIVE PRESSURE
- Pushes air in (overcomes muscle failure)
- PEEP keeps alveoli open (overcomes stiffness)
- High flow rate + long expiration time (overcomes resistance)