Q1: Causes of Delayed Recovery After Anesthesia (5 marks)

I. Drug-Related Causes (Pharmacological) - Most Common

• Overdose or accumulation of inhalational agents (volatile agents: halothane, isoflurane, sevoflurane)
• Relative overdose in elderly, obese, or hypothermic patients (decreased MAC requirement)
• Prolonged N2O/narcotic technique

• Inadequate reversal of non-depolarising NMBAs (vecuronium, rocuronium, atracurium)
• Pseudocholinesterase deficiency - prolonged action of succinylcholine (genetic or acquired)
• Anticholinesterase overdose (neostigmine) causing phase II block
• Potentiating drugs: aminoglycosides, magnesium, furosemide

• Narcotic accumulation (morphine, fentanyl, pethidine)
• Especially pronounced in elderly, hepatic/renal impairment
• Presents as miosis, respiratory depression, somnolence

• Midazolam, diazepam - prolonged action in elderly, liver disease
• Synergistic sedation with other agents

• Preoperative sedatives (long-acting benzodiazepines at home)
• Acute alcohol intoxication - decreases barbiturate metabolism, acts as independent sedative
• Tricyclic antidepressants and anticholinergics augment scopolamine sedation
• Drugs decreasing MAC: methyldopa, reserpine, chronic opioid use
• Drugs decreasing hepatic blood flow (e.g., cimetidine) - reduce drug metabolism

II. Metabolic/Physiological Causes

III. Hypothermia

• Reduces MAC, potentiates drug effect
• Antagonises neostigmine-induced NMB reversal
• Slows hepatic drug metabolism
• Every 1°C drop in body temperature decreases MAC by approximately 5%

IV. Respiratory Causes

• Intraoperative hyperventilation - raises apnoeic threshold; PaCO2 must rise above threshold to re-stimulate respiration post-op
• Hypoxia (PaO2 <50 mmHg) - cerebral dysfunction
• Severe hypercapnia (PaCO2 >70 mmHg) - CO2 narcosis, alters consciousness

V. Central Nervous System (Neurological) Causes

• Cerebrovascular accident (stroke) - risk raised by severe intraoperative hypotension/hypertension, carotid disease, emboli
• Air embolism - paradoxical via right-to-left shunt (especially CHD)
• Cerebral hypoxia or oedema - from severe intraoperative hypotension
• Increased intracranial pressure - especially in neurosurgical patients
• Malignant hyperthermia - prolonged somnolence can follow, though not the typical presentation

VI. Patient-Related (Pre-existing Disease)

• Chronic hypertension - altered cerebral autoregulation, intolerant to hypotension
• Liver disease - reduced first-pass and hepatic drug metabolism; reduced albumin increases free drug
• Renal disease - decreased excretion
• Prior stroke or symptomatic carotid disease - increases CVA risk intraoperatively
• Elderly patients - reduced MAC, reduced protein binding, decreased metabolism

Diagnostic Approach & Management

1. Assess airway and oxygenation - maintain ventilation
2. Peripheral nerve stimulator - to differentiate paralysis from coma
3. Check glucose, ABG, electrolytes
4. Pupils - fixed dilated pupils suggest CNS catastrophe; miosis suggests opioid effect
5. Review drug chart - all agents given, home medications
6. Pharmacological reversal if indicated:
   • Naloxone 0.04-0.4 mg IV for opioid excess
   • Flumazenil 0.2 mg IV for benzodiazepine excess
   • Sugammadex/Neostigmine for residual NMB
   • Physostigmine for anticholinergic excess
7. CT scan if unresponsiveness is prolonged (exclude intracranial pathology)
8. Processed EEG (BIS) - low BIS can indicate residual anesthesia OR ischaemic brain injury

Source: Morgan and Mikhail's Clinical Anesthesiology, 7e, pp. 405-407

Q2: 30-Year-Old Multigravida with 2 Previous LSCSs for Elective LSCS - Placenta Previa with Risk of Accreta (10 marks)

A. PREOPERATIVE PREPARATION

1. Multidisciplinary Team (MDT) Conference

• Scheduled before 34 weeks gestation
• Team: Obstetrician/MFM specialist, Senior Anaesthesiologist, Gynaecological/Vascular/Urological surgeon, Interventional Radiologist, Neonatologist, Blood Bank, ICU team
• Agree on: date, time, location (main OT preferred), blood bank orders, escalation plan

2. Anaesthesiologist's Preoperative Assessment

• History: Number and nature of prior LSCSs, anaesthetic history (spinal/epidural complications), difficult airway history, symptoms of placenta accreta (pelvic pain, haematuria), cardiorespiratory reserve
• Examination: Airway (Mallampati, mouth opening, neck mobility - difficult airway more likely with oedema of pregnancy), BP, weight, back examination for neuraxial access
• Investigations:
  • FBC, coagulation profile (PT, aPTT, fibrinogen), group and screen/crossmatch
  • Type and crossmatch at least 4-6 units packed red blood cells (pRBCs) + FFP + platelets
  • Renal and liver function tests
  • MRI pelvis or Doppler ultrasound to confirm depth of invasion (accreta vs increta vs percreta)
  • Urology review if bladder invasion suspected (percreta)
  • Baseline ABG in symptomatic cases

3. Blood Bank Preparation & Massive Transfusion Protocol (MTP)

• Activate MTP proactively
• Cross-matched blood available in OT before knife-to-skin
• Cell salvage (intraoperative autotransfusion) - controversial in obstetrics but increasingly used with leucodepletion filter
• Recombinant Factor VIIa available as backup
• Tranexamic acid 1 g IV before incision (antifibrinolytic)
• Target: ratio of 1:1:1 (pRBC : FFP : Platelets)

4. Vascular Access & Monitoring

• Two large-bore peripheral IV cannulae (14-16G) minimum
• Arterial line (radial) - for continuous BP monitoring and frequent blood draws
• Central venous catheter - if peripheral access inadequate or for vasoactive drugs
• Standard monitors: SpO2, NIBP, ECG, temperature, urinary catheter (Foley) pre-op
• Consider pre-op interventional radiology for prophylactic balloon occlusion catheters (internal iliac or aortic) to reduce intraoperative blood loss - place before OT in IR suite

5. Bowel & Aspiration Prophylaxis

• Fasting as per guidelines (6 hours solid, 2 hours clear fluids)
• Antacid premedication: Sodium citrate 30 mL PO + Ranitidine 150 mg PO the night before and morning of surgery + Metoclopramide IV (to reduce aspiration risk - all obstetric patients are at risk)

6. Consent & Counselling

• Informed consent for anaesthesia including potential conversion from regional to general
• Consent for possible hysterectomy (caesarean hysterectomy), blood transfusion, ICU admission
• Counsel partner - he may be present during delivery under regional anaesthesia but may be asked to leave if conversion to GA occurs

B. ANAESTHETIC PLAN

Choice of Anaesthesia: Combined Spinal-Epidural (CSE) - Preferred

• CSE provides flexibility - spinal for initial surgical block for delivery, epidural extension for prolonged surgery (hysterectomy can take 2-4 hours)
• Allows patient to be awake for delivery (bonding, partner present)
• Reduces blood loss compared to GA (neuraxial anaesthesia reduces sympathetic tone and venous pooling)
• Epidural catheter can be used postoperatively for pain management
• A review of 350 cases of placenta previa found neuraxial anaesthesia associated with reduced blood loss and reduced transfusion compared to GA

• Position: sitting or lateral decubitus
• Level: L2-L3 or L3-L4 interspace
• Spinal component: Hyperbaric bupivacaine 0.5% (1.8-2.2 mL) + fentanyl 15-20 mcg ± preservative-free morphine 100-150 mcg (for post-op analgesia)
• Epidural catheter placed and tested before surgery commences
• Target block level T4-T6 for caesarean section

• Massive haemorrhage occurs (early conversion before airway oedema worsens)
• Coagulopathy develops (neuraxial top-up unsafe)
• Block inadequate for prolonged hysterectomy
• Patient haemodynamically unstable

General Anaesthesia Preparation (Standby):

• Rapid Sequence Induction (RSI):
  • Pre-oxygenate 3-5 minutes
  • Induction: Ketamine 1-2 mg/kg IV (preferred in haemodynamically unstable patient; maintains BP) or Thiopentone/Propofol if stable
  • Cricoid pressure applied
  • Succinylcholine 1.5 mg/kg IV OR Rocuronium 1.2 mg/kg IV (with sugammadex available)
  • ETT with cuff - confirmed by capnography
  • Difficult airway trolley immediately available (video laryngoscope, bougies, supraglottic airway, surgical airway kit)
• Maintenance: Isoflurane/Sevoflurane in O2/air; reduce volatile concentration after delivery (high concentrations cause uterine relaxation, worsen haemorrhage)
• Post-delivery: Nitrous oxide can be added, opioids (fentanyl/morphine), TIVA if needed

C. ANTICIPATED PERIOPERATIVE COMPLICATIONS & MANAGEMENT

1. Massive Haemorrhage (Primary Risk)

• Activate Massive Transfusion Protocol immediately
• Uterotonic agents after delivery of baby:
  • Oxytocin infusion 20-40 units in 500 mL NS (never rapid IV bolus - causes hypotension)
  • Methylergonovine (Methergine) 0.2 mg IM (avoid in hypertension)
  • Carboprost (Hemabate) 0.25 mg IM (avoid in asthma)
  • Misoprostol 800 mcg rectal
• If uterine atony unresponsive: bimanual compression, intrauterine balloon tamponade (Bakri balloon)
• Caesarean hysterectomy - definitive treatment for accreta; plan and consent pre-operatively
• Inflate pre-placed internal iliac/aortic balloon catheters (IR) to reduce flow
• Transfuse in 1:1:1 ratio (pRBC:FFP:Platelets); target Hb >8 g/dL, fibrinogen >2 g/L, platelets >75×10⁹/L
• Tranexamic acid (antifibrinolytic): 1 g IV, repeat if haemorrhage ongoing
• Recombinant Factor VIIa 90 mcg/kg IV as last resort
• Intraoperative cell salvage (with leucodepletion filter)

2. Disseminated Intravascular Coagulation (DIC)

• Serial coagulation monitoring (POC TEG/ROTEM if available)
• FFP 10-15 mL/kg for prolonged PT/aPTT
• Cryoprecipitate for fibrinogen <1.5 g/L
• Platelets for count <50-75 × 10⁹/L
• Treat underlying cause

3. Haemodynamic Instability / Hypotension

4. Difficult/Failed Airway

5. Amniotic Fluid Embolism (AFE)

6. Bladder/Ureteric Injury

7. Neonatal Depression

• Neonatology/paediatrics team scrubbed and present
• Neonatal resuscitation equipment ready
• Risk of prematurity (delivery ~34-36 weeks in planned accreta cases)

8. Postoperative Care

• Planned ICU admission post-surgery
• Continue monitoring for delayed haemorrhage (drains, urine output, serial Hb)
• DVT prophylaxis once haemostasis confirmed (LMWH)
• Epidural catheter for multimodal analgesia (if placed and haemostasis/coagulation satisfactory)
• Psychological support (counselling about hysterectomy, loss of fertility)

Summary Table: Anaesthetic Plan at a Glance

Sources: Creasy & Resnik's Maternal-Fetal Medicine, pp. 1732-1733; Morgan & Mikhail's Clinical Anesthesiology, 7e, pp. 405-407

Measurement of FRC, DLCO, RV, and Closing Capacity (CC)

First: Understanding the Lung Volume Map

TLC = IRV + TV + ERV + RV
FRC = ERV + RV
CC  = CV + RV

• FRC = volume in the lungs at end of a normal quiet expiration (tidal exhalation), where inspiratory and expiratory muscles are both relaxed. Normal ~2.5 L.
• RV = volume remaining after a maximal forced exhalation. Normal ~1.2 L.
• DLCO = rate of CO transfer across the alveolar-capillary membrane.
• CC (Closing Capacity) = lung volume at which dependent small airways start to close. CC = Closing Volume (CV) + RV.

1. MEASUREMENT OF FRC

Method 1 - Body Plethysmography (Body Box) - Gold Standard

1. The patient sits inside the sealed airtight box and breathes normally to allow tidal breathing.
2. At end-expiration (i.e., at FRC), a shutter in the mouthpiece is closed.
3. The patient makes panting inspiratory efforts against the closed shutter.
4. As the diaphragm contracts:
   • Thoracic volume increases → alveolar pressure decreases (below atmospheric)
   • The increase in thoracic volume compresses the air inside the box → box pressure rises
5. The relationship between the change in mouth pressure (≈ alveolar pressure, ΔPalv) and the change in box pressure (ΔPbox, which reflects change in thoracic volume, ΔV) allows FRC to be calculated:

FRC = ΔV / (ΔPalv / Patm)

• Measures all intrathoracic gas, including trapped (non-communicating) gas behind closed airways
• Therefore gives the largest value of FRC among the three methods
• Most accurate in obstructive lung disease

• Expensive, bulky equipment
• Requires patient cooperation (panting)
• May overestimate FRC if abdominal gas is compressed and decompressed during panting maneuvers
• Claustrophobia

Method 2 - Helium Dilution (Closed Circuit)

1. The patient breathes quietly until end-expiration (at FRC).
2. At this point, the patient is connected to the closed circuit.
3. The patient continues to breathe within this closed circuit. Helium from the circuit mixes with and is diluted by the gas in the lungs.
4. Breathing continues until helium concentration equilibrates (same concentration throughout the circuit + lungs).
5. Final helium concentration (C2) is measured.

C1 × V1 = C2 × (V1 + FRC)

FRC = [(C1 × V1) / C2] - V1

• Simple, inexpensive, widely available
• Good for normal lungs

• Underestimates FRC in patients with significant airway obstruction (e.g., emphysema, COPD) because gas trapped behind closed airways does not equilibrate with the helium circuit - so "non-communicating" gas is missed
• Takes longer (up to 10 minutes) in obstructive disease
• Other insoluble gases (e.g., sulfur hexafluoride, SF6) can be used by the same principle

Method 3 - Nitrogen Washout (Open Circuit)

1. At end-expiration (FRC), the patient is switched to breathe 100% oxygen.
2. As the patient breathes O2, the nitrogen is progressively exhaled and "washed out."
3. The expired gas is continuously sampled and N2 concentration is measured.
4. The test ends when expired N2 falls to <1.5% (approximately 7 minutes).
5. The total volume of nitrogen exhaled is calculated by integrating the N2 concentration × expired volume over the test period.

FRC = Total volume of N2 exhaled / 0.80

• Does not require a closed circuit or special inert gas
• Modern ventilators have built-in software to estimate FRC using this technique at the bedside (valuable in ICU)

• Like helium dilution, underestimates FRC in obstructive disease (non-communicating trapped gas is not washed out)
• Patient must breathe from 100% O2 source (some discomfort, risk of N2 narcosis reversal)
• The test is slow in severe obstruction

Method 4 - Imaging

• CT scan of the chest can estimate FRC with reasonable accuracy by measuring gas volume.
• MRI (especially hyperpolarised gas MRI) can measure regional FRC.
• CT may overestimate FRC because it measures all intrathoracic gas (including non-ventilated regions).
• Used mainly in research and in specific clinical situations.

Comparison of FRC Methods

2. MEASUREMENT OF RESIDUAL VOLUME (RV)

Method 1 - Derivation from FRC

RV = FRC - ERV

• Measure FRC by any of the above methods.
• Measure ERV by spirometry (normal ~1.2 L).
• RV = FRC - ERV (normal RV ~1.2 L)

Method 2 - Derivation from TLC

RV = TLC - VC

• TLC (Total Lung Capacity) = measured by plethysmography or dilution
• VC (Vital Capacity) = easily measured by spirometry

Method 3 - Body Plethysmography (Direct)

• Obstructive disease (COPD, emphysema): RV ↑↑ due to air trapping
• RV/TLC ratio >35% suggests hyperinflation
• RV/TLC ratio >40% suggests significant air trapping

3. MEASUREMENT OF DLCO (Diffusing Capacity for Carbon Monoxide)

Principle

• Surface area of membrane available for gas exchange
• Pressure gradient (alveolar CO partial pressure minus blood CO partial pressure)
• Solubility and molecular weight of the gas
• Thickness of the alveolar-capillary membrane

Standard Method - Single-Breath DLCO (Krogh Technique) - Most Widely Used

1. Patient exhales completely to RV.
2. Patient inhales rapidly to TLC a gas mixture containing:
   • 0.3% CO (tracer gas)
   • 10% Helium (inert tracer to measure alveolar volume and dilution)
   • 21% O2
   • Balance: N2
3. Patient holds breath for exactly 10 seconds (breath-hold at TLC).
4. Patient exhales rapidly; the first 750 mL is discarded (to clear anatomical dead space).
5. An alveolar sample of exhaled gas is collected and analysed for CO and He concentrations.

• The He dilution tells us the alveolar volume (VA) at which gas exchange occurred.
• The difference between CO inhaled and CO exhaled gives the amount of CO transferred.

DLCO = (Rate of CO uptake) / (Mean alveolar CO partial pressure) DLCO = [VA × ln(FiCO/FeCO)] / (PB × breath-hold time)

• VA = alveolar volume (from He dilution)
• FiCO / FeCO = inspired/expired CO fractions
• PB = barometric pressure

Other DLCO Methods (Less Common)

Factors Affecting DLCO

• Interstitial lung disease (ILD/IPF) - thickened membrane
• Emphysema - destroyed alveolar walls, reduced surface area
• Pulmonary embolism - reduced capillary perfusion
• Anaemia - reduced Hb available to bind CO
• Pneumonectomy / lobectomy

• Polycythaemia (more Hb)
• Left-to-right intracardiac shunts (increased pulmonary blood flow)
• Pulmonary haemorrhage (alveolar red cells absorb CO)
• Exercise (increased recruitment of pulmonary capillaries)
• Asthma (mildly increased)

4. MEASUREMENT OF CLOSING CAPACITY (CC)

Definitions

• Closing Volume (CV): The volume of gas exhaled from the point where dependent small airways (which lack cartilaginous support) begin to close, down to RV.
• Closing Capacity (CC): CC = CV + RV. The total lung volume at which dependent airway closure begins.
• Small airways in dependent lung zones close first because:
  • Gravity reduces the lung volume in dependent areas (diaphragmatic effect)
  • These airways rely on radial traction from elastic recoil of surrounding parenchyma to stay open; at low lung volumes this traction is lost

Method 1 - Single-Breath Nitrogen Washout (Fowler / Resident Gas Technique)

1. Patient exhales fully to RV (maximally).
2. Patient then inhales a single breath of 100% O2 all the way to TLC.
   • The O2 preferentially enters dependent (basal) alveoli first (they are more compliant at low volumes)
   • Apical alveoli already contain the residual N2 from the previous breath; they receive less O2
   • Result: after the O2 breath, basal alveoli have a lower N2 concentration than apical alveoli
3. The patient exhales slowly and steadily from TLC to RV, while expired N2 concentration is continuously measured.

• The onset of Phase IV = Closing Volume begins (dependent airways start to close; only N2-rich apical alveoli continue to contribute exhaled gas → N2 concentration suddenly rises steeply).
• The volume exhaled from the Phase III-IV junction down to RV = Closing Volume (CV).
• CC = CV + RV

• CV: ~400 mL
• CV/VC: ~8%
• CC: ~1900 mL
• CC/FRC: ~30%

Method 2 - Bolus (Resident Gas) Technique

1. Patient exhales to RV.
2. Patient inhales a small bolus of 100% O2 (usually 500 mL) at the very start of the inhalation from RV.
3. This bolus enters dependent airways first (where O2 replaces N2).
4. The remainder of the breath to TLC is with air (not 100% O2).
5. Patient then exhales slowly from TLC to RV with continuous N2 monitoring.

• Phase IV onset is identified similarly as in the single-breath technique.
• Because only a bolus of O2 was given, the phase IV rise is sharper and easier to identify.

Method 3 - Using Other Tracer Gases

Clinical Significance of CC

Summary Comparison Table

Sources: Miller's Anesthesia 10e, pp. 1303-1304; Morgan & Mikhail's Clinical Anesthesiology 7e, pp. 925-927; Murray & Nadel's Textbook of Respiratory Medicine, p. 253; Fishman's Pulmonary Diseases and Disorders, Appendix B

Heparin-Induced Thrombocytopenia (HIT) - DNB Theory Answer (5 Marks)

Definition

Types

Incidence

• Unfractionated heparin (UFH) > Low molecular weight heparin (LMWH)
• Surgical patients > Medical patients
• Orthopedic patients on prophylactic UFH: ~5%
• Medical patients on therapeutic UFH: ~0.5-1%
• After cardiopulmonary bypass: seroconversion 20-50%, but clinical HIT only 1-3%

Pathophysiology (Mechanism of HIT Type II)

1. Heparin enters the circulation and displaces platelet factor 4 (PF4) from endothelial cell surfaces, raising plasma PF4 levels 15-30 fold. PF4 is a positively charged protein stored in platelet alpha-granules.
2. Heparin (negatively charged polyanion) binds to PF4 (positively charged) to form PF4-heparin complexes (neoantigens) - these are the immunogenic target.
3. The immune system generates IgG antibodies against these PF4-heparin complexes.
4. IgG-PF4-heparin complexes bind to FcγRIIA receptors on the platelet surface → platelet activation → release of highly procoagulant platelet microparticles + more PF4 → positive feedback loop.
5. Simultaneously, the complexes activate monocytes and endothelial cells → tissue factor expression → massive thrombin generation → prothrombotic state.
6. Result: Platelet consumption (thrombocytopenia) + hypercoagulability (thrombosis) - the paradox of HIT.

Key paradox: Despite falling platelets, HIT is a THROMBOTIC disorder, not a bleeding disorder. Never give platelet transfusions in HIT.

Clinical Features

• Thrombocytopenia: Platelet count falls to <100×10⁹/L, or >50% drop from baseline. Severe thrombocytopenia (<15×10⁹/L) is uncommon.
• Timing: Platelet fall 5-14 days after starting heparin (in heparin-naive patients); within hours in re-exposed patients (previous heparin within 30-100 days - "rapid onset HIT" due to pre-formed circulating antibodies).
• Thrombosis: ~50% of untreated HIT patients develop thrombosis:
  • Venous: DVT, pulmonary embolism (most common)
  • Arterial: Stroke, acute MI, limb ischemia, mesenteric ischemia
  • Skin necrosis at injection sites
  • Acute systemic reaction after IV UFH bolus (fever, chills, hypotension, dyspnoea)

Diagnosis

Step 1: Clinical Scoring - The 4T Score (Pretest Probability)

• 6-8: High probability → Stop heparin immediately, start alternative anticoagulant, test
• 4-5: Intermediate → Test, consider stopping heparin
• ≤3: Low probability (<2% chance) → Testing not indicated; continue heparin

Step 2: Laboratory Confirmation

• PF4-Heparin ELISA: Detects IgG/IgM/IgA antibodies to PF4-heparin complex
  • Sensitivity: ~98%, Specificity: ~74-89%
  • Higher optical density (OD) = stronger positivity = higher risk of thrombosis
  • IgG-specific ELISA improves specificity
  • Heparin inhibition step (adding 100 IU/mL heparin) neutralises true HIT antibodies → fall in OD confirms heparin-dependent antibody

• Serotonin Release Assay (SRA) - Gold Standard:
  • Washed donor platelets pre-loaded with ¹⁴C-labelled serotonin are incubated with patient serum at low (0.1-0.3 IU/mL) and high (100 IU/mL) heparin concentrations
  • True HIT: serotonin release at LOW heparin dose but NOT at high dose (high dose neutralises antibodies)
  • Sensitivity: ~100%, Specificity: ~95%
  • Limitation: requires radioactive material, technically demanding, limited to reference labs
• Heparin-Induced Platelet Activation Assay (HIPA):
  • Similar principle to SRA; uses platelet aggregometry instead of radiolabelling
  • Good specificity but lower sensitivity than SRA

Management

Step 1 - Stop all Heparin Immediately

• Stop UFH, LMWH, heparin flushes, heparin-coated catheters
• Do NOT give platelets (will fuel thrombosis)
• Do NOT switch to LMWH (cross-reacts with HIT antibodies in 80% of cases)

Step 2 - Start Alternative (Non-Heparin) Anticoagulation Immediately

Step 3 - Transition to Warfarin (After Platelet Recovery)

• Begin warfarin only after platelet count has recovered to >150×10⁹/L
• Overlap with alternative anticoagulant for at least 5 days
• Target INR 2-3; continue for minimum 3 months if thrombosis confirmed

Step 4 - Special Situation: Cardiac Surgery Requiring CPB

• Defer elective surgery until antibody titres undetectable (usually >90 days)
• If urgent: Bivalirudin is the anticoagulant of choice for CPB in HIT
• Alternative: Plasmapheresis to remove antibodies before using heparin

Summary of Key Points for Exam

Sources: Henry's Clinical Diagnosis and Management by Laboratory Methods; Tietz Textbook of Laboratory Medicine 7e; Miller's Anesthesia 10e, p. 7449-7450

Chronological Changes in Stored Blood (Storage Lesion)

Background - Preservative Solutions and Shelf Life

• Citrate - anticoagulant; chelates (binds) Ca²⁺ preventing clotting
• Phosphate - buffer; maintains pH
• Dextrose - energy source for RBC glycolysis; maintains ATP
• Adenine - substrate for ATP resynthesis; extends RBC viability
• Cold storage (1-6°C) - reduces glycolytic rate ~40-fold compared to body temperature

The "Storage Lesion" - Overview

Chronological Changes - Day by Day

EARLY CHANGES (Days 1-7)

1. pH Falls (Acidosis Develops Rapidly)

• RBCs continue anaerobic glycolysis in storage, converting dextrose → lactate + H⁺
• Plasma pH falls from 7.4 → ~7.0-6.8 within the first week
• By day 35 (CPDA-1 blood), pH can reach as low as 6.7-6.5
• Acidosis contributes to hyperkalemia (H⁺-K⁺ exchange at cell membrane)

2. ATP Begins to Fall

• ATP is consumed by membrane ion pumps and RBC metabolic processes
• Falls progressively; rapid decline after week 2
• Critically important: ATP is required for:
  • RBC membrane deformability (biconcave shape)
  • Na⁺-K⁺ ATPase pump function
  • Maintaining RBC viability post-transfusion

3. Potassium (K⁺) Leaks Out - Immediate

• Hypothermia inhibits the Na⁺-K⁺-ATPase pump
• K⁺ leaks from cells into plasma; Na⁺ enters cells
• Plasma K⁺ rises from ~4 mEq/L to >20-30 mEq/L in old blood
• Total K⁺ load is modest (plasma volume only ~70 mL in a PRBC unit) but becomes clinically significant with:
  • Massive transfusion
  • Neonatal transfusion
  • Patients with pre-existing hyperkalemia or renal failure

4. Sodium Increases Intracellularly

• Reciprocal to K⁺ loss: Na⁺ enters RBCs
• Contributes to RBC swelling and loss of biconcave shape

INTERMEDIATE CHANGES (Days 7-21)

5. 2,3-DPG Falls Sharply

• 2,3-Diphosphoglycerate (2,3-DPG) is a key allosteric regulator of haemoglobin-oxygen affinity
• In acidic storage conditions, 2,3-DPG is rapidly consumed (acidosis inhibits its synthesis)
• Falls to near-zero levels by day 14 in CPDA-1 blood

• Hb-O₂ dissociation curve shifts LEFT (increased Hb-O₂ affinity; Hb holds on to O₂)
• P₅₀ falls from normal 26-27 mmHg to <10 mmHg in severely stored blood
• RBCs can load O₂ in the lungs but cannot release it to tissues - so despite raising the Hb level, tissue oxygenation is paradoxically impaired immediately after transfusion
• Recovery of 2,3-DPG: Returns to ~50% of normal within 3-8 hours; fully normal within 24-48 hours after transfusion (once RBCs enter normothermic, normoxic circulation)

6. Increased Osmotic Fragility

• Progressive lipid peroxidation and protein oxidation damage the RBC membrane
• Osmotic fragility increases - cells lyse more easily in hypotonic conditions
• Some cells undergo spontaneous haemolysis within the storage bag
• Free plasma haemoglobin (free Hb) rises progressively
• Free Hb scavenges nitric oxide (NO) → vasoconstriction, endothelial dysfunction

7. Loss of Nitric Oxide (NO)

• NO is a potent vasodilator and inhibitor of platelet aggregation
• Stored RBCs progressively lose NO and its precursors (S-nitrosothiol, SNO-Hb)
• Low NO in stored blood contributes to vasoconstriction and microcirculatory obstruction after transfusion

8. Methaemoglobin Accumulates

• Oxidative stress converts oxyhaemoglobin → methaemoglobin (MetHb)
• MetHb cannot carry oxygen
• Normally <1% of Hb; rises progressively in stored blood

LATE CHANGES (Days 21-42)

9. ATP Critically Depleted

• ATP falls to <50% of baseline
• RBC membrane loses flexibility and biconcave shape is lost
• Cells become spherocytic and rigid

10. RBC Deformability Severely Impaired

• Normal RBCs (7.5 μm diameter) must deform to pass through capillaries (3-5 μm)
• Loss of ATP → spectrin-actin cytoskeletal dysfunction → RBCs become rigid spherocytes
• Rigid RBCs:
  • Cannot navigate microcirculation → micro-occlusive events
  • Are rapidly cleared by the reticuloendothelial system (spleen, liver) within hours of transfusion
  • May be defective in delivering O₂ to cells even if viable

11. Progressive Haemolysis Increases

• Free plasma Hb rises; haemolysis index worsens
• Free Hb: scavenges NO, promotes oxidative injury, causes endothelial dysfunction
• Released haem: pro-inflammatory, promotes lipid peroxidation

12. Microparticle Formation

• Damaged RBC membranes bleb off → RBC-derived microparticles (RMPs)
• These are procoagulant (express phosphatidylserine)
• May contribute to post-transfusion thrombosis and inflammation

13. Clotting Factors Progressively Lost

• In whole blood (not PRBCs): Factors V and VIII are labile
• Factor VIII falls to ~50% by day 1, and ~30% by day 21
• Factor V falls similarly
• By 21-35 days of storage, whole blood is essentially devoid of functional Factors V and VIII
• Note: FFP should be given separately in massive transfusion

14. Platelet and White Cell Viability Lost (Whole Blood)

• Platelets: non-functional within 24-48 hours of storage at 4°C (platelets require room temperature storage at 20-24°C for up to 5-7 days)
• WBCs: Dead or non-functional within days → release cytokines and proteases into storage medium (contributing to febrile non-haemolytic transfusion reactions and TRALI)
• Leukoreduction (removing WBCs before storage) is now standard to prevent these effects

15. Supernatant Changes (Plasma Accumulation)

• Accumulation of:
  • Lactate ↑↑
  • K⁺ ↑↑ (>20-30 mEq/L)
  • Free Hb ↑↑
  • Inflammatory cytokines (IL-1, IL-6, IL-8, TNF-α from degranulating WBCs)
  • Lipids from membrane breakdown

Summary Table: Chronological Changes

Clinical Implications of Stored Blood Changes

1. Massive Transfusion (>10 units in 24 hours)

2. Neonatal Transfusion

• Use fresh blood (<7 days old) to minimize K⁺ load and maximise 2,3-DPG
• Even small K⁺ loads can cause fatal cardiac arrhythmias in neonates (small blood volume)
• Irradiated, CMV-negative, leukoreduced blood preferred

3. Emerging Solutions to Storage Lesion

• Rejuvenation solutions (inosine-based): Can restore ATP and 2,3-DPG in stored RBCs before transfusion - promising in animal models; clinical trials ongoing
• Leukoreduction (universal now): Removes WBCs before storage → reduces cytokine accumulation, TRALI risk, febrile reactions
• Pathogen inactivation systems: UV light + psoralen treatment of blood
• AS-7 (newer additive): Extends shelf life to 56 days with better preservation

Quick Recall Mnemonic: "PAD K MF"

• P - pH falls (acidosis)
• A - ATP falls
• D - 2,3-DPG falls (left shift of ODC)
• K - K⁺ rises (hyperkalemia)
• M - Microparticles form; MetHb rises
• F - Factors V and VIII fall; Free Hb rises (haemolysis)

Source: Miller's Anesthesia 10e, pp. 6572-6578 (Biochemical Changes in Stored Blood & Changes in Oxygen Transport)