Table 15-4: Evaluating Acid-Base Status in Respiratory Acidosis

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The Ratio and What It Means

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The Core Concept in Simple Terms

• When CO₂ rises (respiratory acidosis), H⁺ also rises - but by how much tells you the story:
  • Chronic respiratory acidosis: kidneys compensate over days, so H⁺ rises only a little per unit CO₂ (ratio ~0.3)
  • Acute respiratory acidosis: no kidney compensation yet, so H⁺ rises more per unit CO₂ (ratio ~0.8)
  • If H⁺ rises even more than 0.8 → something extra is adding acid = metabolic acidosis on top
  • If H⁺ rises less than 0.3 → something is neutralizing acid = metabolic alkalosis on top

How to Calculate ΔH⁺ and ΔPco₂ from ABG Values

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Step 1: Get Your Lab Values

• pH (to convert to H⁺)
• PaCO₂ (directly given in mmHg)

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Step 2: Convert pH → H⁺ Concentration

The Easy Clinical Formula (the "80 minus" rule):

H⁺ (nmol/L) ≈ 80 − (last two digits of pH)

This formula is accurate between pH 7.20 - 7.55. Outside this range, use the exact formula below.

Exact Formula (if needed):

H⁺ = 10^(9 − pH) nmol/L

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Step 3: Calculate ΔH⁺ and ΔPco₂

ΔH⁺ = Patient's H⁺ − 40

ΔPco₂ = Patient's PaCO₂ − 40

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Worked Example

• H⁺ = 80 − 24 = 56 nmol/L

• ΔH⁺ = 56 − 40 = 16
• ΔPco₂ = 60 − 40 = 20

• Ratio = 16 ÷ 20 = 0.8

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Quick Summary

1. Read pH and PaCO₂ from ABG
2. H⁺ = 80 − (last 2 digits of pH)
3. ΔH⁺ = H⁺ − 40
4. ΔPco₂ = PaCO₂ − 40
5. Ratio = ΔH⁺ ÷ ΔPco₂ → look up in table

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Sodium Bicarbonate (NaHCO₃) - Preparations & Dosing

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Part 1: Available Preparations

A. IV Injection (Bolus) Vials/Ampules

8.4% is the most commonly used in emergencies - each 50 mL ampule = 1 mEq/mL, easy to dose.

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B. IV Infusion (Drip) Bags

The 1.26% (isotonic) bag is safest for large-volume infusions - least risk of hypernatremia.

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C. Oral Tablets

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Part 2: Dosing by Clinical Situation

1. Metabolic Acidosis (General)

Dose (mEq) = 0.3 × Weight (kg) × Base Deficit

• Give half the calculated dose first, then recheck ABG
• Non-urgent: 2-5 mEq/kg IV over 4-8 hours
• Never correct fully in one go - risk of overshoot alkalosis

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2. Cardiac Arrest

• Initial bolus: 1 mEq/kg IV push (= 1 mL/kg of 8.4%)
• Repeat: 0.5 mEq/kg every 10 minutes
• Guided by ABG

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3. Wide QRS / Sodium Channel Blockade (e.g., TCA overdose, beta-blocker overdose)

• Bolus: 2-3 mEq/kg rapid IV over 1-2 minutes
  • 70 kg patient = 140-210 mEq = 3-4 ampules of 8.4%
• Goal: Narrow QRS to < 100-120 ms
• Maintenance infusion: 3 ampules NaHCO₃ + 1L D5W, run at twice maintenance rate
• Target serum pH: 7.50-7.55

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4. Hyperkalemia (Subacute)

• Use isotonic (1.26%) infusion - NOT hypertonic push
• Effect on K⁺ takes 4-6 hours (not useful acutely on its own)
• Hypertonic bolus does NOT lower K⁺ acutely and risks hypernatremia

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5. Urinary Alkalinization (e.g., salicylate toxicity, rhabdomyolysis)

• Add 150 mEq NaHCO₃ to 850 mL D5W (= 1L bag)
• Infuse to maintain urine pH > 7.5 and serum pH 7.50-7.55
• Monitor urine pH hourly

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Part 3: Key Practical Points

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ABG vs VBG - Complete Comparison

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Part 1: Basic Differences

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Part 2: How the Values Compare

The key rule: VBG tells you about acid-base status reliably. It tells you nothing about oxygenation - use pulse oximetry (SpO₂) for that instead.

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Part 3: The Conversion Formula

Estimated ABG pH  = VBG pH + 0.05
Estimated ABG PCO₂ = VBG PCO₂ − 5 mmHg

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Part 4: When to Use Each

✅ VBG is sufficient for:

• Metabolic acidosis monitoring (DKA, renal failure)
• Screening for hypercarbia (if VBG PCO₂ normal → arterial PCO₂ almost certainly normal)
• COPD exacerbation (acid-base assessment)
• Electrolyte monitoring (Na⁺, K⁺, Cl⁻, iCa²⁺)
• Lactate measurement
• Serial monitoring (reduces repeated arterial stabs)

❌ ABG is required when:

• Oxygenation status is the primary concern (PaO₂ needed)
• Respiratory failure assessment (exact PaCO₂ matters)
• VBG PCO₂ is elevated (> 45 mmHg) - need the exact arterial value
• Major trauma
• Titrating mechanical ventilation
• VBG shows abnormal result and exact arterial values are needed to guide management

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Part 5: Quick Memory Aid

VBG = ACID-BASE ✅ + OXYGENATION ❌
ABG = ACID-BASE ✅ + OXYGENATION ✅

If VBG is normal → ABG is almost certainly normal
If VBG is abnormal → may need ABG to confirm and quantify

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1. WPW + AF is a trap - the wide irregular rhythm above 200 bpm needs procainamide or DC cardioversion, not AV nodal blockers
2. The 48-hour rule governs cardioversion safety without prior anticoagulation
3. ESC 2024 now recommends starting DOACs in the ED - don't defer to outpatient
4. 80%+ of AF patients can be safely discharged from the ED after adequate rate/rhythm control
5. The new AF-CARE framework shifts focus toward managing underlying comorbidities (weight, OSA, hypertension, alcohol) as equal pillars alongside anticoagulation and rate/rhythm control

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Pulmonary Tuberculosis - Full Clinical Note with Latest WHO 2025 Updates

• WHO Consolidated Guidelines Module 4 (April 2025) - DS-TB & DR-TB
• WHO Module 6 (October 2025) - TB & Comorbidities (Undernutrition)
• ATS/CDC/ERS/IDSA 2025 Clinical Practice Guideline (AJRCCM, January 2025)
• WHO Global TB Report 2024

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1. EPIDEMIOLOGY & BURDEN

• TB remains the leading infectious disease killer globally (second only to COVID-19 in recent years)
• 10.8 million new TB cases reported in 2023 (WHO Global TB Report 2024)
• ~10% of all TB cases are drug-resistant (MDR/RR-TB)
• High-burden countries: India, Indonesia, China, Philippines, Pakistan, Nigeria, Bangladesh, South Africa
• Undernutrition is the single largest modifiable risk factor (new WHO 2025 focus)

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2. ETIOLOGY & MICROBIOLOGY

• Causative agent: Mycobacterium tuberculosis (aerobic, slow-growing, acid-fast bacillus)
• Transmission: airborne - droplet nuclei (1-5 µm) from infectious pulmonary/laryngeal TB
• Infectiousness depends on: cavitary disease, AFB smear positivity, cough frequency, inadequate ventilation
• Only ~10% of infected immunocompetent individuals develop active disease over a lifetime (90% maintain latent infection)

Risk Factors for Progression from Latent to Active TB

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3. PATHOPHYSIOLOGY

Primary Infection

1. Bacilli inhaled → deposited in alveoli → engulfed by alveolar macrophages
2. Initially uncontrolled replication → spread to hilar lymph nodes (forming Ghon complex)
3. Immune response (2-8 weeks): T-cell mediated → granuloma formation
4. Granuloma = organized collection of activated macrophages, epithelioid cells, Langhans giant cells, lymphocytes
5. Central caseous necrosis occurs in granuloma - this is the hallmark of TB
6. In most: granuloma contains infection → latent TB infection (LTBI)

Reactivation (Post-Primary TB)

• Latent bacilli survive inside granulomas for years/decades
• Disruption of immune control → bacilli reactivate → progressive disease
• Characteristic location: apical and posterior segments of upper lobes (high O₂ tension)
• Cavity formation: caseous material liquefies → drains into bronchus → cavity + airborne transmission

Key Immunology Concepts

• TST (Mantoux) and IGRA measure T-cell sensitization to TB antigens - positive = prior exposure
• Cavity = most infectious (highest bacillary load + direct bronchial communication)
• HIV coinfection: CD4 <200 → atypical presentation, no cavity, lower lobe involvement, smear-negative but culture-positive

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4. CLINICAL PRESENTATION

Pulmonary TB (most common form)

• Persistent cough >2-3 weeks (productive or dry)
• Fever (low-grade, afternoon)
• Night sweats
• Weight loss / anorexia
• Fatigue
• Hemoptysis (advanced/cavitary disease)
• Note: up to 25% of culture-confirmed TB do not report cough

• Often non-specific
• Post-tussive rales in upper zones
• Amphoric (hollow) breath sounds = cavity
• Advanced: signs of consolidation, pleural effusion

Special Presentations

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5. DIAGNOSIS

Step 1: Suspect TB - Who to Test

• Cough >2 weeks + fever/sweats/weight loss
• Hemoptysis
• Abnormal CXR (upper lobe infiltrate, cavity, fibrosis)
• Known HIV + any respiratory symptoms
• Contact with confirmed TB case
• Migrant from high-burden country

Step 2: Chest Radiography

CT chest is more sensitive than plain CXR, especially in HIV or subtle disease.

Step 3: Microbiological Diagnosis

Step 4: Immunological Tests

TST/IGRA = positive means prior exposure / LTBI - DOES NOT diagnose active TB. Active TB requires microbiological confirmation.

Diagnostic Algorithm (WHO-recommended)

Symptoms of TB (cough >2wks, fever, weight loss, night sweats)
        ↓
Chest X-ray
        ↓
Xpert MTB/RIF (first-line WHO diagnostic tool)
    ├─ MTB detected, RIF resistant → MDR-TB workup → LPA/DST
    ├─ MTB detected, RIF sensitive → DS-TB treatment
    └─ MTB not detected → sputum culture + clinical evaluation
                          ├─ Culture positive → TB treatment
                          └─ Culture negative + clinical improvement on treatment → culture-negative TB

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6. TREATMENT - LATEST WHO & ATS/CDC 2025 UPDATES

A. Drug-Susceptible TB (DS-TB)

Standard 6-Month Regimen (remains an alternative option - WHO 2022/2025)

🆕 NEW 4-Month Regimen (WHO 2022 / ATS/CDC/ERS/IDSA 2025 - Conditionally Recommended)

• Drug-susceptible pulmonary TB confirmed
• NOT for: confirmed/suspected MDR-TB, HIV on efavirenz-based ART (drug interaction with rifapentine), pregnancy (insufficient data), extrapulmonary TB (evidence limited)

Always give pyridoxine (B6) 25-50 mg/day with isoniazid to prevent peripheral neuropathy.

🆕 4-Month Regimen for Children (Non-Severe TB, 3 months - 16 years)

• WHO now recommends the shorter 4-month regimen for children with non-severe TB (no suspicion of MDR/RR-TB)
• Non-severe = peripheral lymphadenopathy, uncomplicated pulmonary disease without cavitation

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B. Drug-Resistant TB (DR-TB)

Definitions

🆕 BPaLM Regimen - NOW Preferred for MDR/RR-TB (WHO 2022 onwards, reinforced 2025)

If fluoroquinolone-resistant: use BPaL (without moxifloxacin) - same duration

Grouping of DR-TB Drugs (WHO Priority)

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C. Latent TB Infection (LTBI) - Preventive Therapy

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7. WHO 2025 NEW UPDATES - SUMMARY

Module 4 Update (April 15, 2025)

1. 4-month DS-TB regimen (2HPZM/2HPM) now formally included alongside the 6-month standard
2. Updated evidence for BPaLM as preferred MDR-TB regimen with refined dosing guidance
3. New guidance on treatment monitoring - culture conversion at 2 months remains key milestone
4. Strengthened recommendations for DST before treatment for all patients

Module 6 Update (October 7, 2025) - TB & Undernutrition

1. Nutritional assessment and counselling mandatory for ALL people with TB and household contacts
2. Nutritional interventions recommended for all TB patients with undernutrition (regardless of age, drug resistance, pregnancy, or severity)
3. Food assistance to household contacts in food-insecure settings to prevent TB (supported by RATIONS trial evidence)
4. Operational handbook released to support country-level implementation

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8. MONITORING & FOLLOW-UP

Drug-induced Hepatotoxicity (DILI) - Threshold for stopping

Stop hepatotoxic anti-TB drugs if: ALT >3x ULN with symptoms OR ALT >5x ULN without symptoms

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9. INFECTION CONTROL

• Airborne precautions: N95 respirator for healthcare workers (not surgical mask)
• Negative pressure isolation rooms for hospitalized patients
• Patient education: cover cough, natural ventilation
• Continue isolation until: 3 consecutive negative sputum smears (after starting treatment)
• TB is notifiable in virtually all countries

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10. TB SPECIAL POPULATIONS

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11. END TB STRATEGY TARGETS (2030)

• 90% reduction in TB deaths (vs 2015)
• 80% reduction in TB incidence rate
• No TB-affected families facing catastrophic costs
• Key 2027 milestone: license at least one new TB vaccine

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The 5 Biggest 2026 Changes to Remember

1. Tenecteplase = Alteplase (Class 1) - Single bolus injection makes it much faster and easier to use. Many centers are already switching.
2. "Disabling vs Non-disabling" replaces NIHSS as the IVT trigger - A patient with NIHSS 2 but dominant-hand weakness that prevents their job SHOULD get IVT. A patient with NIHSS 4 but no functional impact should NOT get IVT (give DAPT instead).
3. Extended IVT window to 4.5-9 hours with CTP or DWI-FLAIR mismatch on imaging - important for rural areas and wake-up strokes.
4. EVT for large cores (ASPECTS 3-5) is now Class 1 - previously was controversial. More patients now qualify for thrombectomy even with larger infarcts on arrival.
5. Don't aggressively lower BP after reperfusion - targeting <140 mmHg post-reperfusion was found to harm outcomes. Permissive hypertension up to 180/105 is acceptable.

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Triage Systems for the Emergency Physician - Complete Reference

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PART 1: ED (ROUTINE) TRIAGE SYSTEMS

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1. Emergency Severity Index (ESI) 🇺🇸

Step 1: Does the patient need IMMEDIATE life-saving intervention? → Level 1
Step 2: Is this a HIGH-RISK situation / confused / severe distress? → Level 2
Step 3: How many RESOURCES will this patient need?
         ≥2 resources → Level 3 (also check vital signs - abnormals bump to Level 2)
         1 resource → Level 4
         0 resources → Level 5

• HR >100 or <50 | RR >20 | SpO₂ <92% | Temperature >38.5°C | Altered mentation

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2. Manchester Triage System (MTS) 🇬🇧

1. Select the presenting complaint flowchart (52 flowcharts e.g., chest pain, headache, breathing difficulty)
2. Work through discriminators in priority order (e.g., airway compromise → shock → extreme pain)
3. Assign level based on the first positive discriminator encountered

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3. Canadian Triage and Acuity Scale (CTAS) 🇨🇦

• Only system with mandatory reassessment intervals at each level
• Uses modifiers: first-order (chief complaint-based) and second-order (pain, mechanism of injury)
• Pediatric version: PaedsCTAS (accounts for age-adjusted vitals, pediatric-specific presentations)
• Highest reliability among all systems (κ = 0.7-0.95)

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4. Australasian Triage Scale (ATS) 🇦🇺

• Specifies initial assessment time (not just time to physician)
• Explicitly states duration of each assessment
• Includes mental health and social triage criteria

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5. South African Triage Scale (SATS) 🇿🇦

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HEAD-TO-HEAD COMPARISON TABLE

No single triage system has proven clearly superior to the others - selection depends on setting, training, and local validation.

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PART 2: MASS CASUALTY INCIDENT (MCI) TRIAGE SYSTEMS

Universal MCI Triage Color Categories

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1. START (Simple Triage And Rapid Treatment) 🇺🇸

Can the patient WALK?
    YES → GREEN (minimal/walking wounded)
    NO ↓
Is patient BREATHING?
    NO → Open airway → still not breathing? → BLACK (deceased)
         Breathing after opening? → RED (immediate)
    YES ↓
Respiratory rate?
    >30/min → RED (immediate)
    <10/min → RED (immediate)
    10-29/min ↓
Perfusion: Radial pulse or capillary refill >2 sec?
    No pulse / CRT >2 sec → RED (immediate) + hemorrhage control
    Pulse present / CRT ≤2 sec ↓
Mental status: Can follow simple commands?
    NO → RED (immediate)
    YES → YELLOW (delayed)

1. Open airway (head-tilt/chin-lift)
2. Direct pressure on obvious external hemorrhage

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2. SALT (Sort, Assess, Lifesaving interventions, Treatment/Transport) 🇺🇸

"If you can hear me and can walk, move to [safe area]" → GREEN
Assess remaining in priority: Still > Waving > Those who walked but couldn't relocate

Rosen's Emergency Medicine recommends START until more evidence on SALT accumulates.

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3. JumpSTART (Pediatric MCI Triage)

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4. CareFlight Triage

• Performed best overall across all ages in multi-tool evaluation
• Simpler than JumpSTART for pediatric victims

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5. RAMP (Rapid Assessment of Mentation and Pulse)

• Faster than START (assesses only 2 parameters)
• Results largely similar to START in final categorization
• Suitable when time is extremely limited

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PART 3: SPECIAL SITUATIONS IN TRIAGE

CBRN / Hazmat Triage

• Decontamination BEFORE standard triage
• Antidote administration (e.g., atropine for organophosphate) is a lifesaving intervention during SALT
• PPE for triage personnel mandatory

Pediatric Considerations (Routine ED)

• All major ED systems have pediatric versions (ESI, CTAS, MTS, ATS)
• No single pediatric ED triage system proven superior
• Pediatric Triage Tape (PTT): Length-based criteria for infants/toddlers in MCI settings

Geriatric Triage

• Both ESI and MTS show limited sensitivity in elderly patients with atypical presentations (PLOS ONE, 2025)
• ESI performs better than MTS for discriminating urgency in older adults
• Current consensus: triage systems need revision to account for age-related vulnerability and non-specific presentations

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QUICK REFERENCE SUMMARY

ROUTINE ED TRIAGE (most to least reliable):
CTAS > ESI > MTS > ATS > SATS

MCI/DISASTER TRIAGE:
Adults: START (most validated) or SALT (US national standard)
Children: JumpSTART (pediatric modification of START)

KEY PRINCIPLE DIFFERENCES:
ED triage   = save the sickest INDIVIDUAL
MCI triage  = save the MOST people with available resources

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• ESI (USA) - acuity + resource utilization
• MTS / Manchester (UK/Europe) - symptom flowcharts
• CTAS (Canada) - most reliable, mandatory reassessment intervals
• ATS (Australia) - time-based, nationally mandated
• SATS (South Africa) - TEWS scoring, resource-limited settings

• START - adult field triage (RPM assessment, 30-60 sec/patient)
• SALT - US national standard (Sort→Assess→Lifesaving interventions), adds Gray category
• JumpSTART - pediatric modification of START
• CareFlight - best overall performance in pediatric evaluations
• RAMP - rapid 2-parameter assessment (mentation + pulse)

• CBRN/Hazmat triage
• Geriatric triage limitations
• Pediatric ED triage systems

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What's inside the guide: