Instrumental Diagnostic Methods of the Gastrointestinal Tract

1. Plain and Contrast Radiography (X-Ray)

Plain Abdominal X-Ray

Barium Studies

• Sensitivity for moderate-to-severe esophagitis: 79–100%; mild esophagitis is usually missed
• Spontaneous reflux of barium into the proximal esophagus is highly specific for GER, but not sensitive
• Provocative maneuvers (leg lifting, coughing, Valsalva, water siphon) improve sensitivity

• Polyps < 6 mm: detected in 32% of cases
• 6–10 mm: 53%

• 10 mm: 48%

— Sleisenger and Fordtran's Gastrointestinal and Liver Disease

2. Esophagogastroduodenoscopy (EGD / Upper GI Endoscopy)

1. Diagnostic — defining the source of GI bleeding, staging GI cancers, obtaining samples for histology or culture
2. Therapeutic — hemostasis, lesion resection, remodeling of the GI tract (obesity management, creating passages between lumens)
3. Combined modality — with interventional radiology or surgery

Instrument

Preparation

• Fasting for at least 6 hours prior to the procedure
• Patients with achalasia (POEM): clear liquids for 2 days
• Anticoagulants and antiplatelet agents should be discontinued in advance (see table below)
• Sedation: typically deep sedation with propofol in the USA; alternatively midazolam + fentanyl; or no sedation in selected patients

Scope of Examination

• Posterior pharynx, epiglottis, upper esophageal sphincter, larynx, vallecula
• Entire esophagus and stomach
• Duodenal bulb and first and second portions of the duodenum

Endoscopic Images — Upper GI Abnormalities

3. Colonoscopy

Indications

• Colorectal cancer (CRC) screening and surveillance
• Evaluation of lower GI bleeding, change in bowel habits, or iron deficiency anemia
• Diagnosis and monitoring of inflammatory bowel disease (IBD)
• Polypectomy (definitive therapeutic removal)
• Biopsy of suspicious lesions

Technique

• Requires bowel preparation (oral laxatives; polyethylene glycol-based solutions)
• Performed under sedation (propofol preferred; or midazolam + fentanyl)
• Anticoagulants/antiplatelet agents stopped before therapeutic procedures
• Cecum and terminal ileum are intubated; examination of the entire colon is performed
• Polypectomy uses wire snares; flat polyps may be removed by submucosal injection of hypotonic fluid followed by snare

Limitations

• Fails to reach the cecum in up to 10% of cases
• May miss small lesions located at flexures or behind folds
• Requires sedation and bowel preparation (more costly and invasive than FOBT/FIT)
• Post-polypectomy bleeding risk

Colonoscopy Findings

4. Rectosigmoidoscopy (Sigmoidoscopy / Proctoscopy)

Rigid Sigmoidoscopy

Flexible Sigmoidoscopy

Role in Current Practice

• Evaluation of distal symptoms (rectal bleeding, urgency, tenesmus)
• Surveillance in left-sided IBD
• Confirming perianal/rectal pathology (hemorrhoids, proctitis, polyps, fistulas, intussusception)

Proctoscopy

— Sleisenger and Fordtran's Gastrointestinal and Liver Disease; Berek & Novak's Gynecology

5. Gastric Juice Examination

Physiology of Gastric Secretion

• Basal conditions — circadian pattern, highest at night, lowest in the morning; regulated by cholinergic (vagal) and histamine input
• Stimulated conditions — three phases:
  • Cephalic phase: sight, smell, and taste of food → vagal stimulation
  • Gastric phase: nutrients/amino acids → G cell → gastrin → parietal cell activation; stomach distension also stimulates gastrin release
  • Intestinal phase: food entering the intestine → luminal distension and nutrient assimilation

Gastric Juice Analysis (Clinical Use)

— Harrison's Principles of Internal Medicine, 22e; Mulholland & Greenfield's Surgery

6. Stool Examination (Coprogram)

Collection

Macroscopic Examination

Microscopic Examination

• A stool aliquot is mixed with 95% ethanol + Sudan III stain
• Neutral fats appear as large orange or red droplets
• ≥60 stained droplets per high-power field (HPF) = steatorrhea
• Procedure is repeated with acetic acid + heat to convert soaps/fatty acids → droplets; up to 100 droplets/HPF after this step is normal
• Patients with pancreatic steatorrhea show greater increases in fatty acids and soaps

• Stool + eosin in 10% ethanol
• Rectangular fibers with clearly evident cross-striations

• 10 fibers/HPF suggests maldigestion or hypermotility

• Small fleck of mucus/liquid stool + methylene blue → differential count
• PMN-predominant: infectious colitis (Salmonella, Campylobacter, Shigella), IBD
• Mononuclear: viral gastroenteritis, early amebiasis
• Fecal calprotectin is a modern alternative marker of intestinal inflammation (used in IBD screening)

Chemical / Immunologic Examination

— Goldman-Cecil Medicine; Henry's Clinical Diagnosis and Management by Laboratory Methods

Coprogram Summary Table

• Goldman-Cecil Medicine International Edition, 2-Volume Set (Chapter 120 — GI Endoscopy; Chapter on Stool Examination)
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease (Barium Esophagogram, Barium Enema, Colonoscopy, Sigmoidoscopy)
• Henry's Clinical Diagnosis and Management by Laboratory Methods (Stool Collection and Examination)
• Harrison's Principles of Internal Medicine, 22e (Gastric Acid Secretion Physiology)

Hypoxia

Definition

Hypoxia may be thought of as the functional equivalent of ischemia. It refers to a lack of or misuse of oxygen at the tissue level. Hypoxemia refers to the blood's oxygen content — when a patient's PaO₂ and SaO₂ are low, the patient is hypoxemic. — Fishman's Pulmonary Diseases and Disorders

Classification — Four Traditional Types

The four categories are: (1) hypoxemia, in which the PO₂ of the arterial blood is reduced; (2) anemic hypoxia, in which the arterial PO₂ is normal but the amount of hemoglobin available to carry O₂ is reduced; (3) ischemic or stagnant hypoxia, in which the blood flow to a tissue is so low that adequate O₂ is not delivered despite a normal PO₂ and Hb concentration; and (4) histotoxic hypoxia, in which the amount of O₂ delivered is adequate but, because of a toxic agent, the tissue cells cannot make use of the O₂ supplied. — Ganong's Review of Medical Physiology, 26th Edition

Causes of Hypoxemia (Type 1 — Hypoxic Hypoxia)

1. Reduced Inspired O₂ Tension (↓FiO₂)

• High altitude (↓barometric pressure → ↓PO₂)
• Confined spaces, smoke-filled rooms

2. Alveolar Hypoventilation

• ↓PAO₂ and resultant ↓PaO₂, with accompanying hypercapnia
• Examples: narcotic/sedative overdose (↓central drive), obesity-hypoventilation syndrome, obstructive sleep apnea, neuromuscular disease, abdominal compartment syndrome

3. Diffusion Impairment

• Increased barrier across alveolar-capillary membrane → detected as ↓DLco
• Examples: pulmonary fibrosis, interstitial pneumonias
• Often coexists with V̇/Q̇ mismatch; most prominent during exercise or at altitude

4. Ventilation-Perfusion (V̇/Q̇) Mismatch

• Focal areas of diminished ventilation with preserved perfusion
• Most common mechanism in clinical practice
• Examples: COPD, asthma, interstitial edema, pneumonia
• Responds to supplemental oxygen (FiO₂ titration corrects hypoxemia)
• Reflex hypoxic pulmonary vasoconstriction (HPV) reduces mismatch; widespread HPV → pulmonary hypertension → cor pulmonale

5. Right-to-Left Shunt

• Fraction of pulmonary blood bypasses ventilated alveoli
• Examples: alveolar filling (cardiogenic edema, pneumonia, drowning, atelectasis), anatomic shunts (ASD, pulmonary AV malformations)
• Key feature: hypoxemia persists despite 100% FiO₂ (when shunt fraction >20–25%)

— Fishman's Pulmonary Diseases and Disorders; Costanzo Physiology, 7th Edition

Causes Summary Table

Cellular Response to Hypoxia — Hypoxia-Inducible Factors (HIFs)

Mechanism

• HIFs are heterodimers of HIF-α and HIF-β subunits
• Under normoxia: HIF-1α is hydroxylated (prolyl and asparaginyl hydroxylases require O₂) → recognized by VHL protein → ubiquitinated and rapidly degraded
• Under hypoxia: hydroxylases are inactive → HIF-1α accumulates → dimerizes with HIF-1β → complex translocates to nucleus → binds hypoxia response elements (HREs) on target genes

Genes Activated by HIF-1

HIFs are found in virtually all oxygen-breathing species. The HIFs serve as a "master switch" that permits the body to respond appropriately to hypoxia. — Guyton and Hall Textbook of Medical Physiology

Clinical Manifestations

Acute Hypoxia

Chronic Hypoxia

• Polycythemia (↑ erythropoietin → ↑ red cell mass)
• Right heart strain → cor pulmonale
• Finger clubbing (chronic tissue ischemia)
• Peripheral cyanosis

EEG in Severe Hypoxia (Neurological)

• Alpha coma: monorhythmic alpha-frequency EEG in comatose patient; unreactive to stimuli; frontal predominance — poor prognosis
• Burst suppression: bursts of mixed-frequency activity with intervening flat periods
• Periodic pattern: generalized spikes recurring at ~1–2/second; often with myoclonic jerks

— Ganong's Review of Medical Physiology; Bradley and Daroff's Neurology in Clinical Practice; Fishman's Pulmonary Diseases and Disorders

Physiological Responses and Compensation

Immediate (seconds–minutes)

• Chemoreceptor (carotid body) activation → ↑ ventilation
• ↑ Heart rate and cardiac output
• Sympathoadrenal stimulation

Short-term (hours–days)

• Hypoxic pulmonary vasoconstriction (HPV) — redirects blood from poorly ventilated to well-ventilated lung regions
• HIF-1 pathway activation

Long-term Acclimatization (days–weeks)

• ↑ Erythropoietin → ↑ red cell production → polycythemia → ↑ O₂-carrying capacity
• ↑ 2,3-DPG in erythrocytes → right shift of Hb-O₂ dissociation curve → ↑ O₂ unloading to tissues
• ↑ Capillary density (angiogenesis via VEGF)
• ↑ Respiratory alkalosis (hyperventilation-driven)

Effects of Altitude

Oxygen Therapy

Indications (Acute)

• Documented hypoxemia: PaO₂ <60 mmHg or SaO₂ <90%
• Acute situation where hypoxemia is suspected (respiratory distress)
• Severe trauma, severe acute anemia, acute MI with hypoxemia
• Low cardiac output with metabolic acidosis
• Hypotension (systolic BP <100 mmHg)

Targets

• SaO₂ 92–96% is reasonable for most hypoxemic patients at sea level with normal baseline lung function
• Higher targets may be appropriate in profound anemia (maximize O₂ content before transfusion)
• In COPD with CO₂ retention: lower target (SaO₂ 88–92%) to avoid suppressing hypoxic drive

Response to Supplemental O₂ by Mechanism

• Ganong's Review of Medical Physiology, 26th Edition — Classification and types of hypoxia; effects on brain; effects of altitude
• Guyton and Hall Textbook of Medical Physiology — HIF master switch; acclimatization
• Fishman's Pulmonary Diseases and Disorders — Pathophysiology of arterial hypoxemia; clinical manifestations; oxygen therapy
• Costanzo Physiology, 7th Edition — Causes of hypoxia table; CO/cyanide mechanisms
• Medical Physiology (Boron & Boulpaep) — HIF-1α molecular mechanism and hydroxylation
• Bradley and Daroff's Neurology in Clinical Practice — EEG patterns in hypoxic brain injury