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Modes of Mechanical Ventilation

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Basic / Standard Modes

1. Controlled Mechanical Ventilation (CMV)

• Volume-Controlled Ventilation (VCV): preset tidal volume, airway pressure is variable — used when CO₂ clearance and minute ventilation must be tightly controlled.
• Pressure-Controlled Ventilation (PCV): preset inspiratory pressure, tidal volume is variable — preferred when compliance is very poor and barotrauma risk is high.

In CMV, prolonged use promotes diaphragmatic atrophy and contractility dysfunction.

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2. Assist-Control Ventilation (AC)

• AC-VC (Volume Control): tidal volume set at 6–8 mL/kg predicted body weight; plateau pressure must be kept < 30 cm H₂O.
• AC-PC (Pressure Control): inspiratory pressure set; minute ventilation must be closely monitored because changes in compliance alter tidal volume.

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3. Synchronized Intermittent Mandatory Ventilation (SIMV)

• If sedation is deep or rate set high → functionally equivalent to VC
• If rate set low → functionally equivalent to PSV with occasional "sigh" breaths
• Useful for gradual weaning by reducing mandatory rate over time

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4. Pressure Support Ventilation (PSV)

• Requires the patient to have a reliable respiratory drive (apnea alarms mandatory)
• Reduces work of breathing
• Primary use: ventilator weaning and liberation
• Can be used stand-alone or embedded within SIMV

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5. Continuous Positive Airway Pressure (CPAP)

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Breath Types Within Modes

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The Pressure-Volume Curve and Safe Window

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Newer / Advanced Modes

6. Airway Pressure Release Ventilation (APRV)

• Tidal volume generated = difference between P-high and P-low
• Achieves high mean airway pressure → improved alveolar recruitment
• Hemodynamically well tolerated with minimal sedation
• Used in severe hypoxemia and ARDS
• A recent meta-analysis (nearly 19 years of data) suggested a mortality benefit in acute hypoxemic respiratory failure vs. conventional modes

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7. Bilevel Positive Airway Pressure Ventilation (BiLevel / DUOPAP)

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8. High-Frequency Oscillatory Ventilation (HFOV)

• Used as a salvage mode or bridge to ECMO in severe ARDS
• Physiologically appealing for lung protection in ARDS
• Clinical trials (e.g., OSCILLATE, OSCAR) have not shown survival benefit in adults; may even increase mortality in some subgroups
• May have benefit in pediatric ARDS

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9. Proportional Assist Ventilation (PAV) / Proportional Pressure Support (PPS)

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Key Concepts: Ventilator-Induced Lung Injury (VILI)

• Barotrauma — excess airway pressure
• Volutrauma — alveolar overdistension from large tidal volumes
• Atelectrauma — cyclic recruitment/derecruitment
• Biotrauma — inflammatory mediator release from injured lung

• Tidal volume: 6–8 mL/kg predicted body weight
• Plateau pressure: < 30 cm H₂O
• Monitor for auto-PEEP (measured via expiratory hold)

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Summary Comparison

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Mechanical Ventilation — Comprehensive Overview

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1. Objectives of Mechanical Ventilation

Increased work of breathing is the most common reason for intubation. It can be clinically assessed by palpating phasic sternomastoid contraction, tracheal tug, suprasternal recession, and intercostal recession — not by any single objective measurement threshold. — Fishman's Pulmonary Diseases

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2. Indications for Mechanical Ventilation

• Acute hypoxemic respiratory failure (PaO₂/FiO₂ < 200 in ARDS)
• Acute hypercapnic respiratory failure (respiratory acidosis with pH < 7.20–7.25)
• Apnea or impending respiratory arrest
• Airway protection (obtunded, GCS < 8, inability to protect airway from secretions/aspiration)
• Excessive work of breathing that is unsustainable
• Refractory status asthmaticus or status epilepticus

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3. Initial Ventilator Settings

Predicted body weight (not actual) drives VT settings because lung size correlates with height, not weight — critical in obese patients.

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4. Ventilator-Induced Lung Injury (VILI)

• Plateau pressure (measured by inspiratory hold): target < 30 cmH₂O
• Driving pressure = Plateau pressure − PEEP: target < 15 cmH₂O (emerging evidence)
• Auto-PEEP (measured by expiratory hold): reflects breath stacking/air trapping

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5. PEEP — Titration and Rationale

• Increasing mean airway pressure
• Recruiting collapsed alveoli
• Reducing intrapulmonary shunt

1. ARDS Network FiO₂–PEEP table — empirical combinations of FiO₂ and PEEP levels
2. Pressure-volume curve — set PEEP just above the lower inflection point
3. Esophageal pressure monitoring — estimates transpulmonary pressure to individualize PEEP; however, a recent phase 2 trial showed no benefit over empirical high PEEP-FiO₂ titration

High PEEP must be balanced against hemodynamic compromise: elevated intrathoracic pressure reduces venous return and cardiac output.

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6. MV in ARDS — Special Strategies

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7. MV in Obstructive Disease (COPD / Asthma)

• Permissive hypercapnia — accept higher PaCO₂ to allow longer expiratory time
• Low respiratory rate (10–14/min) and long expiratory time (I:E ≥ 1:3 or 1:4)
• Minimize PEEP in asthma (air trapping already generates intrinsic PEEP)
• Extrinsic PEEP set at ~75–85% of measured auto-PEEP in COPD to offset triggering effort
• Deep sedation ± paralysis during most severe phase of asthma
• Non-invasive positive pressure ventilation (NIPPV/BiPAP) is the preferred initial approach in COPD exacerbations — reduces need for intubation and mortality

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8. Non-Invasive Ventilation (NIV) vs. Invasive MV

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9. Liberation from Mechanical Ventilation (Weaning)

Step 1: Daily Readiness Assessment

Step 2: Weaning Predictor — Rapid Shallow Breathing Index (RSBI)

• Measured during unaided spontaneous breathing (not with PS or CPAP, which falsely lower it)
• RSBI < 100 → likely to tolerate extubation (high sensitivity ≥ 0.90)
• RSBI > 100 → rapid shallow breathing pattern predicts failure

RSBI is a screening test (high sensitivity), not a confirmatory test. A positive screen should prompt an SBT, not immediate extubation. — Fishman's Pulmonary Diseases

Step 3: Spontaneous Breathing Trial (SBT)

• Comfortable, no marked anxiety or dyspnea
• RR < 35/min
• SaO₂ > 90%
• SBP 90–180 mmHg
• HR change < 20% from baseline

Step 4: Extubation Decision

• Ability to follow commands
• Adequate cough and ability to manage secretions
• No excessive secretion burden

• COPD patients — extubate to NIPPV → reduced mortality and HAP
• Chronic hypercapnic respiratory failure — NIPPV reduces reintubation rates
• General low-risk patients — HFNC reduces reintubation vs. conventional O₂ at 48–72h

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10. Failure to Wean — Causes

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11. Complications of Mechanical Ventilation

Pulmonary

• VILI (barotrauma, volutrauma, atelectrauma, biotrauma)
• Pneumothorax, pneumomediastinum
• Ventilator-Associated Pneumonia (VAP)

VAP — Key Facts

• Incidence: up to 15% of mechanically ventilated patients
• Mortality: ~50% when it occurs
• Defined as pneumonia ≥ 48h after intubation
• Common pathogens: S. aureus, P. aeruginosa, gram-negative enteric rods
• Empiric treatment: IV β-lactam (pip-tazo, cefepime, or ceftazidime) ± vancomycin/linezolid (for MRSA risk) or carbapenem (MDR gram-negative risk)
• Treatment duration: 7 days (shorter is non-inferior)

VAP Prevention Bundle

Extrapulmonary Complications

• GI stress ulcers/bleeding → PPI or H₂ blocker prophylaxis
• DVT/PE → pharmacological ± mechanical prophylaxis
• Delirium and sleep disruption → minimize sedation, early mobility, day-night cycle
• ICU-acquired weakness (diaphragm and peripheral muscle atrophy) → minimize NMB, early physiotherapy

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12. Hemodynamic Effects of Positive Pressure Ventilation

• Increases intrathoracic pressure → reduces venous return → reduces preload → may drop cardiac output
• High PEEP exaggerates this effect
• RV afterload may increase (alveolar overdistension compresses alveolar vessels)
• Clinically important in patients with hypovolemia or RV dysfunction — always assess fluid responsiveness when hemodynamic instability develops on the ventilator

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Tourniquet Reperfusion Injury in Regional Anaesthesia

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1. Background — Why Tourniquets Are Used

• Create a bloodless surgical field (limb exsanguination with Esmarch/Martin bandage)
• Confine local anaesthetic to the limb in Intravenous Regional Anaesthesia (IVRA / Bier block)
• Reduce intraoperative blood loss in procedures like Total Knee Arthroplasty (TKA)

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2. The Core Problem — Ischemia-Reperfusion Injury (IRI)

"Prolonged distal ischemia can result in venous congestion or muscle damage and reperfusion injury after release." — Sabiston Textbook of Surgery

"When blood flow is restored to ischemic tissues, cells that might otherwise recover are damaged by new processes set in motion during reperfusion." — Robbins & Cotran Pathologic Basis of Disease

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3. Mechanisms of Reperfusion Injury

A. Reactive Oxygen Species (ROS) / Oxidative Stress

• During ischemia, xanthine dehydrogenase is converted to xanthine oxidase in endothelial cells
• On reperfusion, oxygen returns → xanthine oxidase uses it to generate superoxide (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (·OH)
• Leukocytes recruited to the ischemic zone are a major additional source of ROS via NADPH oxidase
• Cellular antioxidant defenses (superoxide dismutase, catalase, glutathione) are depleted during ischemia, so cells are hypersensitive to ROS on reflow
• ROS cause: lipid peroxidation of cell membranes, DNA strand breaks, protein oxidation, mitochondrial dysfunction

B. Intracellular Calcium Overload

• Ischemia impairs Na⁺/K⁺-ATPase and Na⁺/Ca²⁺ exchanger → intracellular Na⁺ rises → intracellular Ca²⁺ accumulates
• On reperfusion, ROS further damage the sarcoplasmic reticulum and cell membranes → Ca²⁺ influx is amplified
• High intracellular Ca²⁺ → opens the mitochondrial permeability transition pore (mPTP) → ATP depletion → cell death
• In muscle: pathological Ca²⁺ accumulation → contraction band necrosis

C. Neutrophil-Mediated Inflammation

• Ischemic cells release DAMPs (damage-associated molecular patterns) and cytokines
• Endothelial cells upregulate adhesion molecules (ICAM-1, selectins) → neutrophils adhere and extravasate
• Infiltrating neutrophils release ROS (respiratory burst), proteases, and elastase → amplify tissue damage
• Complement activation (via IgM antibodies deposited in ischemic tissue) further fuels the inflammatory cascade

D. Endothelial Dysfunction

• Reperfused endothelium loses its anti-thrombotic, vasodilatory properties
• Reduced nitric oxide (NO) production → vasoconstriction, platelet aggregation
• Increased endothelin-1 → further vasoconstriction → "no-reflow" phenomenon in microcirculation
• Pro-thrombotic state → microvascular thrombosis → secondary ischemia despite macrovascular patency

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4. Tissues Affected in the Limb

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5. Systemic Effects at Tourniquet Release

"After tourniquet release, mean arterial blood pressure decreases significantly, partly owing to the release of metabolites from the ischemic limb into the circulation and the decrease in peripheral vascular resistance." — Miller's Anesthesia, 10e

• Sympathetic block from neuraxial/peripheral RA already attenuates compensatory vasoconstriction
• Vasodilation from RA + vasodilation from tourniquet release can combine to produce profound hypotension
• Opioid adjuncts to spinal/epidural anaesthesia partially attenuate tourniquet pain but not the hemodynamic response

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6. Tourniquet Pain Under Regional Anaesthesia

• Develops after ~60 minutes of inflation
• Mechanism: unmyelinated C fibers (carrying dull aching pain) are resistant to or "outlast" the block on slow-conducting nerve fibers; as the neuraxial/peripheral block recedes, C fibers become unblocked
• Under spinal anaesthesia: a dense sacral/lumbar block may cover surgery but C fibers eventually breakthrough
• Under peripheral nerve block: some C-fiber pathways may escape blockade
• Management:
  • Add opioids to spinal/epidural (e.g., fentanyl, morphine) — reduces tourniquet pain
  • Double cuff technique in IVRA: inflate distal cuff over anaesthetized skin, then deflate proximal cuff — delays pain
  • Ketamine sub-dissociative doses IV
  • Limit tourniquet time to < 60–90 minutes where possible

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7. Specific Context: IVRA (Bier Block) and Reperfusion

• Local anaesthetic (typically lidocaine 0.5%, 3 mg/kg; or prilocaine in UK/Europe) is retained by the tourniquet
• If the cuff deflates before 30 minutes, a bolus of LA enters the systemic circulation → LAST (seizures, cardiovascular collapse)
• After 45 minutes, much of the LA is protein-bound or redistributed → risk greatly reduced
• On deflation: cyclic deflation (inflate-deflate-inflate for 5-second cycles) reduces peak plasma LA concentration

• Ischemic metabolites (lactic acid, K⁺, CO₂)
• Any unbound LA in the limb vasculature
• Pro-inflammatory cytokines

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8. Nerve Injury — Tourniquet Neuropraxia

• Nerve injury from tourniquet is attributable to the combined effects of ischemia and mechanical compression at the cuff site
• The radial nerve at the spiral groove (upper arm) and the common peroneal nerve at the fibular neck (lower limb) are most vulnerable
• Clinically: post-tourniquet syndrome — weakness, paralysis, paresthesia in the distribution of the compressed nerve
• Usually neuropraxia (demyelination without axon loss) → recovers in days to weeks
• Prolonged inflation > 2 hours, excessive pressure, or poor cuff placement → axonotmesis (wallerian degeneration) → slower recovery

"Nerve injury after tourniquet inflation has been attributed to the combined effects of ischemia and mechanical trauma. When prolonged tourniquet inflations are required, deflating the tourniquet for 30 minutes of reperfusion may reduce neural ischemia." — Miller's Anesthesia, 10e

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9. Special Populations — Higher Risk of IRI

"In patients with known preexisting sciatic neuropraxias, neuropathic pain, and vascular disease in the operative leg, the operation can be performed without a tourniquet." — Miller's Anesthesia, 10e

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10. Prevention and Mitigation Strategies

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Summary

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Spinal Needles in Anaesthesia

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Overview

1. A tightly fitting, removable stylet that completely occludes the lumen — to prevent tracking epithelial cells into the subarachnoid space
2. A hub for syringe attachment and for feeling needle advancement
3. A shaft of varying length (standard ~90 mm for adults) and gauge (16–29 G)
4. A tip design — the most important variable in clinical practice

"The most important characteristics of a spinal needle are the shape of the tip and the needle diameter." — Miller's Anesthesia, 10e

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Classification by Tip Design

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Diagram — Tip Designs

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Scanning Electron Micrograph — Actual Needle Tips

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Individual Needle Types

1. Quincke Needle (Cutting, End-Injection)

• Tip: Sharp cutting bevel, angled ~45°
• Injection: From the end of the needle (bevel)
• Available gauges: 20–29 G
• CSF flow: Fast and reliable — easy to confirm placement
• PDPH risk: Highest — the sharp bevel cuts dural fibres longitudinally, leaving a slit that CSF leaks through
• Key clinical rule: Orient bevel parallel to the longitudinal axis of the spine (parallel to dural fibres) — this parts fibres rather than cutting across them, reducing PDPH incidence
• Advantages: Reliable, inexpensive, excellent tactile feedback
• Disadvantages: Higher PDPH, not preferred as first choice for elective spinal

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2. Whitacre Needle (Pencil-Point, Side-Injection)

• Tip: Solid conical pencil-point — separates rather than cuts dural fibres
• Injection: Side port (small circular or oval aperture just proximal to the tip)
• Available gauges: 22–27 G (most commonly 25–27 G)
• CSF flow: Slightly slower than Quincke of equivalent gauge (due to side port)
• PDPH risk: Markedly lower than Quincke — dural fibres are separated and recoil closed after withdrawal
• Advantages: Low PDPH, good tactile feedback
• Disadvantages: Slightly higher failure rate at very fine gauges; aspiration may be needed to confirm CSF in ≤25 G

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3. Sprotte Needle (Pencil-Point, Long Side-Opening)

• Tip: Solid conical pencil-point (similar to Whitacre)
• Injection: Long elongated oval side port — significantly larger than Whitacre
• Available gauges: 24–29 G
• CSF flow: Fastest among pencil-point needles due to the large side opening
• PDPH risk: Low (similar to Whitacre)
• Unique risk: If the tip is subarachnoid (CSF flows freely) but the proximal part of the long port is not yet past the dura, part of the injected dose is delivered outside the subarachnoid space → failed or patchy block
• Advantages: Excellent CSF flow, low PDPH
• Disadvantages: Larger side port → risk of incomplete intrathecal injection

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4. Pencan Needle

• Similar pencil-point design to Whitacre/Sprotte
• Belongs to the non-cutting group
• Available in 25–27 G
• Used in combined spinal-epidural (CSE) kits

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5. Pitkin Needle (Cutting)

• Cutting tip (similar to Quincke in concept)
• Less commonly used in modern practice
• Listed among cutting-tip needles in Miller's Anesthesia

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Combined Spinal-Epidural (CSE) Setup

• The Tuohy (epidural) needle is sited in the epidural space first (loss of resistance technique)
• A long fine-gauge pencil-point spinal needle is then passed through the Tuohy lumen, penetrating the dura ("needle through needle")
• Intrathecal injection is made → spinal needle removed → epidural catheter threaded
• The spinal needle must protrude 2–3 mm beyond the Tuohy tip to reach the subarachnoid space

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Gauge and PDPH — Key Data

"The use of small-gauge needles reduces the incidence of post-dural puncture headache from 40% with a 22G needle to less than 2% with a 29G needle. The probability of PDPH or procedure failure is reportedly lowest with 26G atraumatic needles." — Miller's Anesthesia, 10e

• Lower PDPH
• But higher procedural failure rate — weaker tactile feedback, slower CSF flow (aspiration needed to confirm), needle deflects in tissue

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The Introducer Needle

• Inserted first into subcutaneous/interspinous tissues (not beyond ligamentum flavum)
• Acts as a guide and stiffener for the finer spinal needle
• Prevents contamination of the spinal needle tip with skin flora/epidermal cells
• Note: The introducer itself has a bevelled tip → if it accidentally enters the subarachnoid space in a thin patient, orient bevel parallel to the spine axis to minimise PDPH

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Summary Comparison Table

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Clinical Tips

1. Bevel orientation for Quincke: Always insert with bevel parallel to the spine's long axis — reduces PDPH by parting rather than severing dural fibres (longitudinal orientation)
2. Confirm CSF with fine gauges: With 25G and smaller, remove stylet after each "pop"; active aspiration may be needed
3. The Sprotte "partial injection" trap: Ensure the long side port is fully through the dura before injecting
4. NR-Fit (non-Luer) connectors: Newer spinal kits use non-standard connectors to prevent accidental intrathecal injection of wrong drugs — rely on the correct drug being drawn up into the non-Luer syringe
5. Sterility is paramount: Wear a mask — Streptococcus viridans (oral commensal) is a leading cause of post-spinal bacterial meningitis; full aseptic technique including hand wash, gloves, drape, and chlorhexidine-alcohol skin prep is mandatory

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