Hypercapnia
Definition
PaCO2 >6.3 kPa (50 mmHg).
- Presentation:
Raised end-tidal CO2 or PaCO2.
*Immediate management:
1-Increase inspired O2 to maintain SpO2 >95%.
2-Check/increase minute ventilation. If patient is breathing spontaneously, then exclude excessive depth of anaesthesia and consider starting assisted ventilation.
3-Compare measured inspired/expired tidal volumes for evidence of circuit leaks leading to reduced Vt. Check ETT or LMA cuff pressure.
4-Examine the capnography trace to exclude rebreathing of CO2. If detected, increase fresh gas fl ow or change CO2 absorbent in circle system.
5-Check for disconnections within the breathing system that increase dead space (e.g. internal limb of a Bain circuit).
6-Ensure expiratory valves of circle system are not sticking.
7-If safe functioning of the circuit is in doubt, change to alternative means of assisted ventilation. Remember to maintain adequate anaesthesia.
8-Examine patient for signs of inadequate anaesthesia and deepen if necessary.
*Subsequent management
If PaCO2/ETCO2 continues to rise, exclude malignant hyperthermia or thyroid storm.
*Investigations
ABGs to confirm elevated PaCO2, and check for base defi cit suggestive of metabolic problem. K + and CK if MH suspected.
*Risk factors
Increased CO2 production—including pyrexia, sepsis, malignant hyperthermia, neuroleptic malignant syndrome, serotonin syndrome, reperfusion injury, thyroid storm.
Decreased CO2 excretion—respiratory depression, bronchospasm, inadequate minute volume during IPPV, inappropriate or faulty breathing system, partial airway obstruction, ineffective breathing during SV, excessive dead space.
Increased CO2 delivery to the lungs—abdominal insufflation with CO2, capnothorax.
Rebreathed CO2—exhausted soda lime, inadequate FGF in partial rebreathing system, circuit valve fault, CO2 in the fresh gas mixture.
*Exclusions
Malignant hyperthermia.
A degree of hypoventilation is common under anaesthesia with SV.
*Paediatric implications
Small increases in equipment dead space may signifi cantly compromise the elimination of expired carbon dioxide.
*Special considerations
The vast majority of cases of intraoperative hypercapnia is not clinically significant in the absence of dysrhythmias or raised intracranial pressure, and specific treatment is not needed.
The toxic child/adolescent with sepsis may have a very high metabolic rate and raised CO2. If MH needs to be ruled out, investigate as above