Bhavani Shankar Kodali MD

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

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 Ventilation	Technical 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.