Capnometry


Clinical Aspects

Bhavani Shankar Kodali MD
  

Capnometry

Various factors result in either increased or decreased/absent PETCO2.

PETCO2 increased

 CO2 output Pulmonary perfusion
 Alveolar Ventilation Technical errorsMachine faults
 
Fever
Malignant hyperpyrexia
Sodium bicarbonate
Tourniquet release
Venous CO2 embolism
 Increased cardiac output

Increased blood pressure
 
Hypoventilation
Bronchial intubation
Partial airway obstruction
Rebreathing
 
Exhausted CO2 absorber
Inadequate fresh gas flows
Leaks in breathing system
Faulty ventilator
Faulty valves

PETCO2 decreased

 CO2 output
 Pulmonary perfusion Alveolar VentilationTechnical errors Machine faults
 Hypothermia
 
Reduced cardiac output
Hypotension
Hypovolemia
Pulmonary embolism
Cardiac arrest

 Hyperventilation
Apnea
Total airway obstruction
Partial airway obstruction
Accidental tracheal extubation


Circuit disconnection
Sampling tube leak
Malfunction of ventilator

The table above shows various factors that influence PETCO2 measurements at the lips. However, it is strongly recommended that capnometry be used always in conjunction with capnography as the latter, besides furnishing  diagnostic and therapeutic information, is the only way to confirm the adequacy of CO2 sampling. Inadequate CO2 sampling leads to falsely low PETCO2 values which may be erroneously interpreted.

References:

1. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review article. Canadian J Anaesth 1992;39:6:617-32.

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