Supine X-Ray in Small Bowel Obstruction (SBO)

Key Radiological Features on Supine AXR

1. Dilated Small Bowel Loops

• Calibre >3 cm (up to 3.5-5 cm) is considered dilated
• Loops lie centrally in the abdomen (vs large bowel which is peripheral)
• Multiple loops arranged in a ladder or step-ladder pattern

2. Valvulae Conniventes (Plicae Circulares)

• The hallmark of small bowel on X-ray
• These folds cross the entire width of the lumen (unlike haustra of the colon, which are incomplete)
• Regularly spaced, giving a "concertina" or "stack of coins" effect - most prominent in jejunum
• The ileum is largely featureless - smooth walls without prominent folds

3. Absent or Minimal Colonic Gas

• Gas is absent from the colon distal to the obstruction - this is an important differentiating feature
• Contrast to paralytic ileus, where both small and large bowel are dilated with gas throughout

4. String of Beads Sign

• Seen when dilated small bowel loops are almost completely fluid-filled (very little gas)
• Residual gas is trapped between valvulae conniventes, forming a line of small gas bubbles
• Virtually diagnostic of SBO when present
• Supine film may appear deceptively normal in fluid-filled SBO (the loops appear as soft-tissue density with no gas to outline them)

5. Normal-Appearing Film (Pitfall)

• In a proportion of SBO cases, the supine AXR is normal or equivocally abnormal
• This occurs when dilated loops are predominantly fluid-filled, with no gas to be seen
• A normal plain film does not exclude SBO - CT is mandatory if clinical suspicion persists

Summary Table: SBO vs LBO on Supine AXR

Important Additional Signs

• Hernia: Always inspect below the inguinal ligament on the supine film. A gas-filled loop passing into the inguinal region suggests a strangulated hernia as the cause (especially in women aged >60).
• Pneumobilia (Gallstone Ileus triad - Rigler's triad): SBO + pneumobilia + ectopic gallstone - present in ~38% of cases of gallstone ileus.
• Soft-tissue mass: May indicate intussusception. A "target sign" (concentric rings) is seen if the intussusception is orientated end-on.
• Fluid levels: More prominent on erect film; the number of fluid levels is proportional to the degree and site of obstruction (more levels = more distal obstruction). On supine alone, fluid levels are not visible.

Limitation of Supine AXR

• Identifying the transition point (dilated proximal loops + collapsed distal loops)
• Determining the aetiology (adhesions, hernia, mass, volvulus, intussusception)
• Detecting complications - strangulation, ischaemia, perforation, closed-loop obstruction

"A normal plain radiograph does not exclude an obstruction - if there is persistent concern, further imaging is indicated; CT is the modality of choice." - Bailey & Love's Surgery, 28th Ed.

How to Check Air-Fluid Levels in X-Ray for SBO

First, the Critical Point: Supine vs Erect

The Erect AXR - Step by Step

1. Air-fluid levels - sharp horizontal white lines at the gas-fluid interface within each bowel loop
2. Gas above (dark on X-ray) + fluid below (grey/white on X-ray) within the same loop
3. The levels are at different heights in different loops - this is the stepladder pattern, and is the classic SBO appearance

Specific Criteria for SBO on Erect AXR (Box 91.2, Sabiston)

"The most specific finding for SBO is the triad of: dilated small bowel loops (>3 cm), air-fluid levels on upright films, and absence of gas in the colon." - Schwartz's Principles of Surgery, 11th Ed.

What the "Stepladder Pattern" Means

• Multiple bowel loops, each with their own air-fluid level, but the levels are at progressively different heights across the film
• Looks like a staircase or ladder going across the abdomen
• The loops are arranged transversely, centrally, with their valvulae conniventes visible
• This pattern occurs because each loop has trapped a different amount of fluid/gas and they do not all empty into each other

Fluid Levels - What They Tell You

• Fluid levels appear later than gas shadows (gas accumulates first, then fluid catches up and separates)
• The number of fluid levels is directly proportional to:
  • The degree of obstruction (more complete = more levels)
  • The site of obstruction (more distal = more levels, as more loops are involved proximal to it)
• When fluid levels are very prominent and numerous, the obstruction is advanced
• Two fluid levels are normal in adults (one at duodenal cap, one at terminal ileum) - don't over-call

Normal AXR Does Not Exclude SBO

• If dilated loops are predominantly fluid-filled (no gas), the supine film may look completely normal - described as a "gasless abdomen" or "pseudotumor sign"
• Even the erect film may show no air-fluid levels if loops contain only fluid
• Plain films miss SBO in up to 20-33% of cases
• CT is mandatory when clinical suspicion remains despite a normal or equivocal X-ray

Three-View Approach for Suspected SBO

1. Supine AXR - assess bowel gas pattern, loop size, valvulae conniventes, absent colonic gas
2. Erect AXR - look for air-fluid levels, stepladder pattern, free air under diaphragm
3. Erect CXR - look for free air under diaphragm (perforation)

"Free air in the peritoneal cavity, reflecting perforation of a viscus, is best diagnosed on upright or lateral views, and is easily missed on the supine view." - Mulholland & Greenfield's Surgery

Air-Fluid Levels in SBO - Bailey & Love's 28th Edition

The Direct Answer: ERECT Radiograph

"Small bowel fluid levels may be seen on an erect abdominal radiograph"

"Fluid levels seen radiologically appear later than gas shadows as it takes time for gas and fluid to separate. These are most prominent on an erect abdominal radiograph or cross-sectional imaging."

What the Two Views Show (Bailey & Love)

• On supine: you see the gas pattern and valvulae conniventes (the "concertina/ladder" folds of jejunum)
• No fluid levels are visible - gas and fluid have not separated from the viewing angle

• On erect: fluid settles to the bottom, gas rises - you see a sharp horizontal white line (the interface) with dark gas above it
• Multiple such levels in different loops = the stepladder pattern

Bailey & Love's Key Points on Fluid Levels (p. 1405)

Summary (Bailey & Love's, verbatim logic)

• Supine AXR → shows dilated loops, valvulae conniventes, absent colonic gas
• Erect AXR → shows fluid levels (gas above, fluid below = stepladder pattern)
• Fluid levels appear after gas shadows, so an early SBO may show gas but no fluid levels yet
• A normal plain film does not exclude SBO

Why Fluid Levels Are Seen in X-Ray of SBO

Part 1: Why Fluid Accumulates in the Obstructed Bowel

Gas (Dark component on X-ray)

"There is a significant overgrowth of both aerobic and anaerobic organisms, resulting in considerable gas production. Following the reabsorption of oxygen and carbon dioxide, the majority is made up of nitrogen (90%) and hydrogen sulphide."

• Bailey & Love's, p. 1397

Fluid (White/grey component on X-ray)

"Fluid is made up of the various digestive juices:

• Saliva: 500 mL/24h
• Bile: 500 mL/24h
• Pancreatic secretions: 500 mL/24h
• Gastric secretions: 1 litre/24h

This accumulates in the gut lumen as absorption by the obstructed gut is retarded."

• Bailey & Love's, p. 1397

• Reduced oral intake
• Defective intestinal absorption
• Losses from vomiting
• Sequestration in the bowel lumen (the "third space")
• Transudation into the peritoneal cavity

Part 2: Why This Creates a Visible Fluid Level on X-Ray

• Gas and fluid physically separate within each distended loop
• At the interface, there is an abrupt change from fluid density (white) below to gas density (black) above
• The X-ray beam passes horizontally across this sharp horizontal interface
• This creates the characteristic bright white horizontal line = the fluid level

• Gas and fluid are still both present, but the interface is now parallel to the X-ray beam (both beam and interface are horizontal)
• The beam cannot "cut across" the interface → no fluid level is visible
• You only see the gas outline of the loops

Part 3: Why Fluid Levels Appear LATER Than Gas Shadows (Bailey & Love)

"Fluid levels seen radiologically appear later than gas shadows as it takes time for gas and fluid to separate."

• Bailey & Love's Summary Box 78.9, p. 1405

• Early SBO → mostly gas pattern visible on supine, no fluid levels yet on erect
• Established SBO → gas and fluid have separated → fluid levels now visible on erect
• Advanced/paralytic SBO → fluid levels become more conspicuous and more numerous

"During the obstructive process, fluid levels become more conspicuous and more numerous when paralysis has occurred. When fluid levels are pronounced, the obstruction is advanced."

• Bailey & Love's, p. 1405

Summary Diagram (concept)

ERECT AXR - single obstructed loop:

    ┌───────────────┐
    │  GAS (dark)   │  ← nitrogen, hydrogen sulphide, swallowed air
    ├───────────────┤  ← FLUID LEVEL (sharp white horizontal line)
    │ FLUID (white) │  ← pooled digestive secretions (up to 2.5L/day)
    └───────────────┘

Multiple loops → multiple levels at different heights = STEPLADDER PATTERN

55-Year-Old Intoxicated Man in ED - Hypertensive and Diabetic

The First and Most Important Rule

"Drunk" is a diagnosis of exclusion.

Step-by-Step Emergency Management

STEP 1 - Immediate Bedside Assessment (First 2 Minutes)

• Is he maintaining his airway? Listen for gurgling (aspiration risk)
• Position him in the recovery position if GCS is reduced
• Have suction ready - aspiration of vomit is the leading cause of death in alcohol intoxication

• RR, SpO2 - give supplemental O2 if SpO2 <94%

• "The most important goals in treatment of acute alcohol intoxication are to prevent severe respiratory depression and aspiration of vomitus." - Katzung's Pharmacology, 16th Ed.

• BP, HR, IV access - establish two large-bore IV lines
• In a hypertensive patient: is this chronic or acute hypertension from pain/agitation?
• ECG - rule out arrhythmia

• GCS score (document it)
• Pupils - equal, reactive? Unequal pupils = herniation/stroke until proven otherwise
• Focal neurological signs? Facial droop, arm drift, speech?

• Look for injuries - lacerations, bruising, signs of fall or assault
• Check for Medic-Alert bracelet

STEP 2 - Bedside Tests (Do IMMEDIATELY, Simultaneously)

If CBG is <70 mg/dL (3.9 mmol/L): give 25g dextrose IV (50 mL of D50W or 125 mL D20W) immediately - do not wait for labs.

STEP 3 - Empirical "Coma Cocktail" (If Altered/Unconscious)

"Thiamine is given to protect against Wernicke-Korsakoff syndrome." - Katzung's Pharmacology, 16th Ed.

STEP 4 - Blood Investigations

• Blood glucose (confirm bedside test)
• Full blood count (infection, anaemia)
• Renal function + electrolytes (dehydration, uraemia)
• Liver function tests (alcoholic liver disease)
• Blood alcohol level (BAL)
• Arterial blood gas (ABG) - metabolic acidosis? Anion gap? (methanol/ethylene glycol)
• Coagulation profile (liver disease, DIC)
• Serum osmolality (osmol gap for toxic alcohols)
• Troponin (chest pain, hypertensive patient)
• HbA1c (baseline diabetes control)

Blood alcohol level: symptoms correlate with level:

• 20-100 mg/dL: mild - euphoria, mild incoordination
• 100-200 mg/dL: moderate - ataxia, slurred speech, obvious intoxication

• 300 mg/dL: stupor, coma

• 400 mg/dL: potentially fatal

• Goldman-Cecil Medicine, 26th Ed.

STEP 5 - Imaging

• CT head (non-contrast): Mandatory in this patient given:
  • Age >50
  • Hypertension (risk of haemorrhagic stroke)
  • Any focal neurology
  • Uncertain mechanism (could have fallen and hit head)
  • GCS <13 or not improving as expected

  • "Traumatic brain injury complicated by intoxication is prone to misinterpretation because of uncertainty as to the main cause of stupor or coma." - Adams & Victor's Neurology, 12th Ed.

• CXR: Aspiration pneumonia, cardiomegaly (hypertensive heart disease)

STEP 6 - Specific Management of the Hypertension

• Do not aggressively lower BP acutely unless hypertensive emergency is confirmed (BP >180/120 + end organ damage: encephalopathy, chest pain, papilloedema, oliguria)
• Agitation from intoxication will itself raise BP - it may resolve with sedation
• If he is a known hypertensive on medication, has he taken his drugs today? Rebound hypertension from missed doses is common
• Target BP reduction: no more than 25% in first hour if hypertensive emergency

STEP 7 - Blood Sugar Management

• If he is a known diabetic on insulin/OHAs: check when he last ate and last took medication
• Alcohol causes hypoglycaemia by inhibiting hepatic gluconeogenesis - especially dangerous in fasting or malnourished state
• Alcohol also masks hypoglycaemic awareness
• Maintain CBG monitoring every 1-2 hours while he remains altered

STEP 8 - Supportive Care

• IV fluids: Normal saline if dehydrated/vomiting (avoid dextrose until glucose known)
• Electrolytes: Correct hypokalaemia (alcohol + vomiting depletes K+)
• Antiemetics: Metoclopramide or ondansetron (aspiration risk)
• Positioning: Recovery position if not fully alert
• NPO until fully awake
• Monitoring: Continuous SpO2, BP, HR, GCS every 30 minutes
• Safety: Secure IV lines; agitated patients may self-harm

STEP 9 - Watch for Alcohol Withdrawal (If He Stops Drinking)

• Withdrawal begins 6-8 hours after last drink
• Seizures peak at 12-24 hours
• Delirium tremens (DTs) at 48-72 hours - potentially fatal
• Treat with benzodiazepines (diazepam or lorazepam) using CIWA-Ar score

• "Alcohol withdrawal is one of the most common causes of seizures in adults." - Katzung's, 16th Ed.

• Note: In a hypertensive diabetic, benzodiazepines lower BP and are preferred over antipsychotics for withdrawal

When to Consider Hemodialysis

"The use of hemodialysis is often considered in comatose patients with extremely high blood alcohol concentrations (>500 mg/dL), particularly if accompanied by acidosis, and in those who have concurrently ingested methanol or ethylene glycol." - Adams & Victor's Neurology, 12th Ed.

Summary Priorities at a Glance

1. AIRWAY - recovery position, suction ready
2. GLUCOSE - check immediately, treat if low + give THIAMINE first
3. NEURO - GCS, pupils, focal signs → CT head
4. BP - manage carefully, don't over-treat
5. BLOODS - glucose, renal, LFT, ABG, BAL, osmol gap
6. MONITOR - for withdrawal starting 6h later

How to Check ST Segment Elevation and Depression on ECG

Step 1 - Know the ECG Anatomy First

"The J point is the junction between the end of the QRS complex and the beginning of the ST segment." - Harrison's, 22nd Ed.

Step 2 - Find the Isoelectric Baseline

"A line drawn horizontally from the PQ junction denotes the isoelectric line."

• Pfenninger & Fowler's Procedures for Primary Care

• Look at the TP segment (the flat line between the T wave and the next P wave) - this is the truest flat baseline
• Alternatively use the PQ/PR junction (the flat part just before the QRS begins)
• Draw an imaginary horizontal line across the entire complex at this level
• This horizontal line = 0 reference level for ST measurement

  P                    T
 /\                   /\
/  \    _____________/  \
     \  |             ST |
      \_| J              |____
PQ ----↑ isoelectric line ----

Step 3 - Identify the J Point

• The J point is where the QRS complex ends (the S wave finishes) and the ST segment begins
• It is the "elbow" or "notch" where the steeply sloping QRS transitions into the flatter ST segment
• This is the point where you start measuring ST deviation

Step 4 - Measure ST Deviation

"ST segment deviation (depression or elevation) should be measured at the J-point (i.e., ST zero, also known as the beginning of the ST segment or end of the QRS complex). Deviation is measured up or down from the level of the PQ junction."

• Each small box = 1 mm = 0.1 mV (at standard calibration: 10 mm = 1 mV)
• Measure vertically from the isoelectric line to the J point

                  ↑ ST elevated
___isoelectric____J__________  = normal (J point ON the line)

___isoelectric________________
                  J            = ST elevated (J point ABOVE the line)
                  ↑ measure this distance

                  J            = ST depressed (J point BELOW the line)
___isoelectric________________
                  ↓ measure this distance

Step 5 - Check 80ms After the J Point Too

• Count 2 small boxes (2 mm = 40ms each) to the right of the J point
• Measure the ST height at this point against the isoelectric line
• This is the point used by computerised ECG analysis and for exercise stress test interpretation

"Most experts consider the study positive only if the slope for 80 msec after the J-point is horizontal or downsloping." - Pfenninger & Fowler's

Criteria: What Counts as Significant?

ST Elevation (STEMI criteria)

ST Depression (ischaemia criteria)

"A positive EET is considered when 1 mm of ST segment depression occurs in at least three beats in a row, in more than one lead."

• Pfenninger & Fowler's

Step 6 - Assess the Shape of the ST Segment

Quick Summary - Step by Step

1. Find the PQ junction → draw your isoelectric line
2. Find the J point → where QRS ends, ST begins
3. Measure vertically at J point vs isoelectric line:
      - J point ABOVE line → ST ELEVATION
      - J point BELOW line → ST DEPRESSION
4. Also measure at J+80ms (2 small boxes right of J point)
5. Note the SHAPE (concave/convex/horizontal/downsloping)
6. Check ≥2 contiguous leads showing the same change
7. Apply threshold: ≥1 mm limb leads / ≥2 mm chest leads for STEMI

ST Elevation and Depression - Real ECG Examples

1. ST Segment Morphology - STEMI vs Benign vs Pericarditis

• Convex / straight ST elevation → STEMI until proven otherwise
• Concave ("smiley face") ST elevation → more likely pericarditis or benign early repolarization

2. Hyperacute T waves → Evolving to ST Elevation (STEMI)

3. Anterior Wall STEMI (V1-V4)

4. Anterolateral STEMI (V1-V6 + I + aVL)

5. Inferior STEMI with Reciprocal ST Depression

6. ST Elevation in aVR + Widespread ST Depression (Left Main Occlusion)

7. Wellens Syndrome - Deep T Wave Inversions → Progression to Anterior STEMI

Summary Table - Which Leads Show What

Teaching This ECG - Inferior STEMI with Reciprocal Changes

Step 1 - Rate and Rhythm

• Rate: ~60 bpm (R-R interval approximately 5 large boxes; 300 ÷ 5 = 60)
• Rhythm: Normal sinus rhythm - P wave before every QRS, regular spacing
• PR interval: Normal (~160-180 ms, ~4 small boxes)

Step 2 - Axis

• Lead I: Low amplitude but positive (upright QRS)
• Lead aVF: Strongly positive (tall R waves)
• Axis = Normal (~+60° to +90°)

Step 3 - The KEY Finding - ST Changes

ST ELEVATION (Inferior leads: II, III, aVF)

Normal ST:   ___QRS___ST____T___
                      ↑ flat, at baseline

This ECG:    ___QRS___ST⬆____T___
                      ↑ ST is lifted ABOVE the isoelectric line

• In lead II: ST segment is clearly elevated above the PR segment baseline, with a rounded elevated ST merging into a tall T wave
• In lead III: ST elevation even more prominent - the ST rises steeply after the QRS
• In lead aVF: Marked ST elevation with upward-sloping morphology

The inferior wall of the heart is supplied by the Right Coronary Artery (RCA) in ~90% of people. This ECG tells you the RCA is occluded.

ST DEPRESSION (Reciprocal changes: I and aVL)

Normal ST:   ___QRS___ST____T___
                      ↑ at baseline

This ECG:    ___QRS___ST⬇____T___
                      ↓ ST dips BELOW the isoelectric line

• In lead I: The ST segment drops below the PR baseline after the QRS
• In lead aVL: Deep ST depression - the most obvious reciprocal change on this ECG

Think of it like this: leads I/aVL "look at" the opposite side of the heart from II/III/aVF. When one side is elevated, the other side is depressed - like two sides of the same coin.

Step 4 - Lead by Lead Summary Table

Step 5 - Why III > II Matters

If ST elevation in lead III > ST elevation in lead II, combined with ST depression in aVL → 90% sensitive for Right Coronary Artery (RCA) occlusion

Step 6 - What to Look For Next (Clinical Implications)

Right Ventricular Infarction

• If elevation in III > II → order right-sided leads (V4R)
• ST elevation in V4R = RV infarction - critical because these patients must NOT receive nitrates (will drop BP fatally)

Posterior Extension

• Check if V1-V3 show ST depression with tall R waves → suggests posterior wall involvement
• Treat with posterior leads V7-V9

Step 7 - The Complete Diagnosis

"12-lead ECG showing normal sinus rhythm at 60 bpm. Normal axis. There is ST segment elevation in the inferior leads II, III, and aVF, consistent with an acute inferior STEMI. There is reciprocal ST depression in leads I and aVL. This pattern is consistent with occlusion of the right coronary artery. Right-sided leads should be obtained to rule out right ventricular infarction. The patient requires immediate reperfusion therapy."

The "Mirror Test" - A Quick Memory Trick

Lead III:  ↑ ST elevation (actual infarction zone)
Lead aVL:  ↓ ST depression (mirror image)