• End-tidal carbon dioxide (ETCO2) monitoring is a noninvasive technique which will measure the partial pressure or maximal concentration of Co2 at the end of an exhaled breath which is expressed as percentage or mmHg of Co2.
  • The normal value of ETCO2 is 5% - 6%, which is equivalent to 34 mmHg - 45mmHg.
  • Co2 reflects cardiac output and pulmonary blood flow as gas is carried out by the venous system to the right side of the heart and then pumped to the lungs by the right ventricles. 
  • When Co2 diffuses out of the lungs into the exhaled air, a device called capnometer, measure the partial pressure or maximal concentration of Co2 at the end of exhalation. ETCO2 is more important because when it decreases rapidly, it suggests an abrupt change in pulmonary perfusion due to increased alveolar dead space. 

What is capnography?

  • Capnography is the monitoring of the concentration or partial pressure of CO2 in respiratory gas. The graphical representation of ETCO2 is shown a waveform format and it is known as capnogram.
  • CO2 monitoring provides information regarding 
  • Pulmonary function,
  • Indirectly as cardiac f

A normal waveform has different phases –


  1. Phase 1 (inspiratory baseline) reflects inspired gas, which is normally with low CO2
  2. Phase 2 (expiratory upstroke) is the transition between VDana, Which does not participate in gas exchange and alveolar gas from respiratory bronchioles and alveoli. (VDana=anatomic dead space )
  3. Phase 3 is the alveolar plateau, where the last of the alveolar gas is sampled. This is normally the PETCO2.
  4. Phase 0 reflecting the respiratory downstroke, and the beginning of inspiration
Other features 
  • Normal range 35- 45 mmHg / 5 – 6 %
  • The alpha (α)angle is the transition from phase 2 – phase 3
  • The beta() angle is the transition from phase 3 – phase1(the start of inspiration)
  • An additional phase 4 (terminal upstroke before phase 0) may be seen in pregnancy 

Types of capnography monitor


Mainstream    
  • it measures respiratory gas directly from the airway 
  • A sensor located on the airway adaptor at the hub of the endotracheal tube.
  • Accurate, less response time.
  • Heavy, contaminated easily with secretion.
  • Configured for intubated patients.
Side’s stream
  • It measures respiratory gas via nasal /nasal - oral cannula
  • Aspirating a small sample from exhaled breath through the cannula tubing to a sensor located inside the monitor
  • Lightweight, slow response time, not contaminated easily
  • Configured for intubated and not – intubated patients.

Indication of ETCO2 in an emergency room

  • Cardiopulmonary resuscitation – after 20 minutes of high-quality CPR, ETCO2 is consistently below 10mmHg indicating poor prognosis.
  • ETCO2 is a reliable indicator with a high prognostic value in determining the CPR outcome
  • airway assessment- ETCO2 Can be used as a confirmation tool for post endotracheal intubation. An ideal value to be obtained after 6 breaths.
  • Procedural sedation and analgesia(PSA) – during PSA, after administrating IV sedation patient may undergo some kind of respiratory distress like apnea Which can be easily detected by ETCO2
  • Pulmonary disease – (pulmonary thromboembolism) - ETCO2 reading is significantly lower than normal range due to the reduction of pulmonary perfusion and increased alveolar dead space 
  • Also other diseases like heart failure, shock, metabolic disorder, DKA.

Flat ETCO2 trace

  • Ventilator disconnection
  • Oesophageal intubation
  • Capnograph not connected to a circuit
  • Respiratory or cardiac arrest
  • Capnography obstruction

A sudden drop in ETCO2

  • Kinked ET tube
  • Ventilator defective
  • CO2 analyzer defective

Sudden increases in ETCO2

  • ROSC during cardiac arrest
  • Correction of ET tube obstruction


Increased ETCO2
Decreased ETCO2
Alveolar ventilation
  • Hypoventilation
  • Bronchial intubation 
  • Partial airway obstruction
  • COPD


  • Hyperventilation
  • Apnoea
  • Total airway obstruction
  • Accidental tracheal extubation
  • metabolism
    • Fever
    • Sodium bicarbonate
    • Tourniquet release


  • hypothermia
  • Pulmonary perfusion
    • increased cardiac output
    • increased blood pressure


  • hypovolemia
  • decreased cardiac output
  • hypotension
  • cardiac arrest
  • errors
    • faulty ventilation
    • water samples in an adapter
    • exhausted CO2 absorber


  • circuit disconnection
  • sampling tube leak
  • leak around ET tube