Describe Capnography in detail.

Principle

Capnography is the continuous measurement and waveform display of carbon dioxide in the respiratory gases, with the end-tidal CO₂ (ETCO₂) approximating the arterial CO₂. It is one of the most useful monitors, confirming ventilation and much more, and is mandatory whenever the airway is instrumented.

The normal capnograph: a flat inspiratory baseline, a rapid expiratory upstroke, an alveolar plateau whose peak is the end-tidal CO₂, then a sharp downstroke as inspiration begins.

What It Confirms

A normal capnograph trace confirms tracheal tube placement (a sustained trace — ‘no trace = wrong place’), adequate ventilation, and (indirectly) circulation (CO₂ must be delivered to the lungs). It detects disconnection, obstruction, hypoventilation and apnoea immediately, and is invaluable in CPR (a rise in ETCO₂ signals return of circulation).

Abnormal Traces

The shape and level are informative. A rising ETCO₂ occurs with hypoventilation, rebreathing, or malignant hyperthermia (and CO₂ insufflation). A falling/absent ETCO₂ occurs with hyperventilation, disconnection/obstruction, oesophageal intubation (no sustained trace), or a fall in cardiac output/cardiac arrest or pulmonary embolism. A sloping (‘shark-fin’) plateau suggests obstruction/bronchospasm.

Central Role in Safety

Capnography has become perhaps the single most valuable safety monitor because a normal trace simultaneously reassures on three fronts — the tube is in the trachea, the lungs are being ventilated, and the circulation is carrying CO₂ to them — while any deviation gives an almost instantaneous warning. It is mandatory whenever a tracheal tube or supraglottic airway is used, in theatre, in the intensive care unit and during transfer, and it is central to the safe conduct of anaesthesia and to resuscitation. Its great strength over pulse oximetry is immediacy: a disconnection or oesophageal intubation produces an instant change in the capnograph long before the oxygen saturation would fall.

Capnography in Emergencies

The value of capnography extends well beyond routine anaesthesia into emergency and critical care, where it has become a key tool: during cardiopulmonary resuscitation it confirms tracheal tube placement, gives feedback on the quality of chest compressions (better compressions raise the end-tidal CO₂ by improving pulmonary blood flow), and provides one of the earliest signs of the return of spontaneous circulation, seen as an abrupt rise in the end-tidal CO₂. It is equally valuable for monitoring any sedated or ventilated patient, detecting apnoea or airway obstruction during procedural sedation long before oxygen desaturation would occur.

• Capnography: continuous CO₂ waveform; ETCO₂ approximates arterial CO₂; mandatory with an instrumented airway.
• Confirms tube placement (sustained trace), ventilation, and circulation; instant warning of disconnection/apnoea.
• Rising ETCO₂: hypoventilation, rebreathing, malignant hyperthermia.
• Absent/falling: disconnection, obstruction, oesophageal tube, cardiac arrest/low output, PE.
• Shark-fin plateau: airway obstruction/bronchospasm; ETCO₂ rise = ROSC marker in CPR.