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EDAIC Part I (European Diploma in Anaesthesiology)

15.1 Interpreting Investigations: ECG, Chest Radiograph, Lung Function and Blood Tests

Q: A patient's flow-volume loop shows flattening of both the inspiratory and expiratory limbs with a plateau of peak flow. What is the most likely diagnosis?

Fixed upper airway obstruction such as tracheal stenosis. Flattening of both limbs indicates fixed upper airway obstruction, such as tracheal stenosis, because the narrowing limits flow in both directions regardless of transmural pressure. Variable extrathoracic obstruction (for example vocal cord palsy) flattens the inspiratory limb, not the expiratory limb. Emphysema gives a scooped expiratory limb, and fibrosis produces a small loop of normal shape.

Key Takeaways

On the ECG, a QRS duration above 120 ms indicates bundle branch block or ventricular conduction delay, and a corrected QT above about 450 ms in men or 460-470 ms in women is prolonged.
An FEV1/FVC ratio below 0.7 (or below the lower limit of normal) indicates airflow obstruction, whereas a restrictive defect shows reduced total lung capacity with a normal or high ratio.
A flat inspiratory and expiratory limb on the flow-volume loop suggests fixed upper airway obstruction; flattening of only the inspiratory limb suggests variable extrathoracic obstruction.
Diffusing capacity (DLCO) falls in emphysema, interstitial lung disease and pulmonary vascular disease but is preserved or high in asthma and pulmonary haemorrhage.
In an arterial blood gas, check pH, then the primary change in $P_a\text{CO}_2$ or bicarbonate, then compensation, the anion gap and the alveolar-arterial oxygen gradient.

Last updated: October 2026

15.1 Interpreting Investigations: ECG, Chest Radiograph, Lung Function and Blood Tests

Why Investigations Matter

The preoperative assessment and intensive care questions in EDAIC Paper B often present an ECG description, radiograph findings, lung function numbers or a blood gas. A systematic approach avoids missing a second abnormality.

The 12-Lead ECG

Systematic Approach

Rate: 300 divided by the number of large squares between R waves (at 25 mm/s).
Rhythm: P wave before every QRS? Regular or irregular?
Axis: normal axis is about -30° to +90°. Left axis deviation (lead I positive, lead II negative) suggests left anterior fascicular block; right axis deviation suggests right ventricular strain or left posterior fascicular block.
Intervals:

Interval	Normal	Abnormal meaning
PR	120-200 ms	Above 200 ms: first-degree AV block; below 120 ms: pre-excitation (with delta wave in Wolff-Parkinson-White)
QRS	Below 120 ms	120 ms or more: bundle branch block, ventricular rhythm, hyperkalaemia, tricyclic toxicity
QTc	Up to about 450 ms (men), 460-470 ms (women)	Prolonged: hypokalaemia, hypomagnesaemia, hypocalcaemia, drugs (ondansetron, droperidol, antipsychotics, macrolides), congenital long QT

ST segments and T waves: ST elevation in contiguous leads suggests acute infarction; widespread saddle-shaped elevation with PR depression suggests pericarditis.

Patterns That Change Management

Finding	Significance
Left bundle branch block	Underlying structural heart disease; new LBBB with symptoms suggests acute ischaemia; inserting a pulmonary artery catheter risks complete block if right bundle is injured
Bifascicular block (RBBB plus left anterior fascicular block) with first-degree block	Risk of complete heart block is low but external pacing should be available
Second-degree Mobitz II or complete heart block	Needs pacing before elective surgery
Peaked T waves, wide QRS, absent P waves	Hyperkalaemia; give calcium immediately
U waves, flat T waves	Hypokalaemia
Short QT	Hypercalcaemia
Right ventricular strain, S1Q3T3, sinus tachycardia	Pulmonary embolism (sinus tachycardia is the commonest finding)
Left ventricular hypertrophy (Sokolow-Lyon: S in V1 plus R in V5 or V6 above 35 mm)	Hypertension, aortic stenosis

Territories of Ischaemia

Inferior: II, III, aVF (right coronary artery usually).
Anterior and septal: V1-V4 (left anterior descending).
Lateral: I, aVL, V5-V6 (circumflex or diagonal branches).
Posterior: ST depression with tall R waves in V1-V3.

The Chest Radiograph

Check patient details, projection (PA or AP), rotation (clavicles equidistant from the spinous processes), inspiration (about 6 anterior ribs above the diaphragm) and penetration.

A  Airway: trachea central, carina, tube tip 3-7 cm above the carina in adults
B  Breathing: lung fields, pleura, costophrenic angles, pneumothorax
C  Circulation: heart size (cardiothoracic ratio below 0.5 on a PA film), mediastinum
D  Diaphragm: free air under the diaphragm, hemidiaphragm level
E  Everything else: bones, soft tissues, lines, drains, pacemakers

Central venous catheter tip: lower superior vena cava, at or just above the level of the carina.
Nasogastric tube: below the diaphragm, in the left upper quadrant, after following the midline.
Pulmonary oedema: upper lobe venous diversion, Kerley B lines, bat-wing shadowing, effusions.
Anterior mediastinal mass with tracheal compression warns of airway collapse on induction.
Pneumothorax on a supine film may show only a deep sulcus sign.

Pulmonary Function Tests

Spirometry

Pattern	FEV1	FVC	FEV1/FVC	TLC
Obstructive (asthma, COPD)	Reduced	Normal or reduced	Below 0.7 (or below the lower limit of normal)	Normal or increased (hyperinflation)
Restrictive (fibrosis, chest wall disease, obesity, neuromuscular disease)	Reduced	Reduced	Normal or increased	Reduced

Bronchodilator reversibility (an increase in FEV1 of at least 12% and 200 mL) supports asthma.
COPD severity (GOLD) is graded by post-bronchodilator FEV1: at least 80% predicted (mild), 50-79% (moderate), 30-49% (severe), below 30% (very severe).
A vital capacity below about 15-20 mL/kg in neuromuscular disease warns of respiratory failure.

Diffusing Capacity (Transfer Factor)

The DLCO measures uptake of a small amount of carbon monoxide.

Reduced: emphysema, interstitial lung disease, pulmonary vascular disease and pulmonary hypertension, anaemia.
Normal or increased: asthma, pulmonary haemorrhage (blood in alveoli takes up carbon monoxide), polycythaemia, left-to-right shunt.

Flow-Volume Loops

Pattern	Appearance	Example
Obstructive (intrathoracic airways)	Scooped (concave) expiratory limb	Asthma, COPD
Fixed upper airway obstruction	Flattened inspiratory and expiratory limbs	Tracheal stenosis, goitre with fixed narrowing
Variable extrathoracic obstruction	Flattened inspiratory limb only	Vocal cord palsy, laryngeal tumour
Variable intrathoracic obstruction	Flattened expiratory limb only	Tracheomalacia, intrathoracic tumour
Restrictive	Small loop with normal shape	Fibrosis

Arterial Blood Gas Interpretation

1. Oxygenation: P_aO2 relative to F_IO2; calculate the A-a gradient
2. pH: acidaemia (<7.35) or alkalaemia (>7.45)
3. Primary process: P_aCO2 (respiratory) or HCO3- / base excess (metabolic)
4. Compensation: expected or not (Winter's formula for metabolic acidosis)
5. Anion gap: Na+ - (Cl- + HCO3-), normal about 8-12 mmol/L; correct for albumin
6. Delta ratio if the anion gap is raised: look for a second process

Alveolar gas equation: $P_A\text{O}_2 = F_I\text{O}_2 (P_B - P_{H_2O}) - \frac{P_a\text{CO}_2}{RQ}$ . Breathing air at sea level, $P_A\text{O}_2$ is about 13.3 kPa; a normal A-a gradient is below about 2 kPa in young adults and rises with age.
Winter's formula: expected $P_a\text{CO}_2$ (mmHg) $= 1.5 \times [\text{HCO}_3^-] + 8 \pm 2$ .

Routine Blood Tests

Result	Possible cause	Anaesthetic relevance
Microcytic anaemia	Iron deficiency	Treat with intravenous iron before major elective surgery
Macrocytosis	Alcohol, B12 or folate deficiency, liver disease	Consider nitrous oxide avoidance in B12 deficiency
Thrombocytopenia	Many causes	Below $80 \times 10^9/\text{L}$ is commonly regarded as a contraindication to epidural catheter placement
Raised urea with normal creatinine	Dehydration, gastrointestinal bleeding, high protein intake	Assess volume status
Hyponatraemia	SIADH, diuretics, hypotonic fluids	Correct chronic hyponatraemia slowly (no more than about 8-10 mmol/L in 24 hours)
Raised HbA1c	Poor diabetic control	Values above about 69 mmol/mol (8.5%) prompt optimisation before elective surgery (UK guidance)

Test Your Knowledge

A patient's flow-volume loop shows flattening of both the inspiratory and expiratory limbs with a plateau of peak flow. What is the most likely diagnosis?

Fixed upper airway obstruction such as tracheal stenosis

Variable extrathoracic obstruction such as unilateral vocal cord palsy, which flattens only the expiratory limb

Emphysema, which produces a scooped expiratory limb with preserved peak flow

Pulmonary fibrosis, which produces a small loop of normal shape

Test Your Knowledge

Which condition typically increases the diffusing capacity for carbon monoxide (DLCO)?

Emphysema, because of destruction of alveolar walls

Idiopathic pulmonary fibrosis, because of a thickened alveolar membrane

Pulmonary haemorrhage, because blood in the alveoli takes up carbon monoxide

Severe anaemia, because of the reduced haemoglobin available to bind carbon monoxide

Test Your Knowledge

A patient has a QRS duration of 150 ms, peaked T waves and absent P waves on the ECG. What is the most appropriate immediate treatment?

Amiodarone 300 mg intravenously for ventricular tachycardia

Insulin with glucose alone, then repeat the ECG and potassium in 1 hour

Magnesium sulphate 2 g intravenously for torsades de pointes

Intravenous calcium to stabilise the myocardium, then insulin with glucose

Sections you finish are checked off in the contents.

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