2006 OPEN FORUM Abstracts
COMPARISION OF ETCO2 WITH CAPILLARY BLOOD GAS PCO2
Denise Willis, RRT-NPS, Robert Warren, MD.
Background: The capillary blood gas (CBG) is standard of care for routine monitoring and evaluation of ventilation in children with chronic pulmonary problems. ABG's are not preferred for routine analysis in the stable child due to difficulty in obtaining a sample and associated pain. Although CBG's tend to be less painful than arterial sticks, many children still become apprehensive and upset with needle sticks and blood draws. An upset or combative child may exhibit altered CO2 due to increased minute ventilation. ETCO2 is a noninvasive method for evaluating ventilation and can be measured by nasal cannula or tracheostomy tube. ETCO2 is routinely monitored in our pulmonary clinic on all mechanically ventilated children and occasionally in the non-ventilated child. A review of the literature reveals an extensive evaluation of ETCO2 monitoring in various settings. However it has not been specifically compared to the CBG PCO2 in the outpatient clinical setting in a wide variety of diagnoses or in both ventilated and non-ventilated children.
Hypothesis: If sufficient correlation is observed between the CO2 measurements, the need for a routine CBG could be reduced. However, underestimation of ETCO2 may occur due to obstructive lung disease due to increased dead space ventilation. Anxiety producing hyperventilation may also have an effect on PCO2.
Methods: The BCI 8200 Capnometer was used for this project. It was sent to the manufacturer for preventative maintenance prior to beginning the study. Routine calibration was performed per manufacturer specifications. Any child requiring a CBG in the pulmonary clinic, regardless of diagnosis, was approached for study participation. A baseline ETCO2 was measured in a steady state prior to placement of the CBG warmer. A second ETCO2 was measured simultaneously as blood was being drawn into the capillary tube.
Results: Consent was obtained for 18 individuals. Their ages ranged from 4 months to 21 years. Diagnoses included 6 with cerebral palsy (CP), 6 with neuromuscular disease (NMD), 4 children had bronchopulmonary dysplasia (BPD) and 2 had restrictive lung disease (RLD) due to thoracic cage deformity. Thirty-three percent required assisted ventilation and 28% had a tracheostomy. Thirty-nine percent of all children had either a baseline or simultaneous ETCO2 within 2-3 mmHg of PCO2. One or both ETCO2 values were within 2-3 mmHg of PCO2 in 66% of NMD, 50% of RLD, 33% of CP and 0% of BPD. The difference between the two ETCO2 values in all children averaged 6 mmHg. The average difference between baseline ETCO2 and PCO2 overall was 7 mmHg. By diagnosis that difference was 8 mmHg in CP, 6 mmHg for NMD, 8 mmHg in BPD and 5 mmHg in RLD. The difference between simultaneous ETCO2 and PCO2 averaged 10 mmHg overall. By diagnosis the difference was 13 mmHg for CP, 6 mmHg for NMD, 15 mmHg in BPD and 8 mmHg in RLD. Sixty-seven percent of all children were anxious appearing, had an increased respiratory rate or were crying during the CBG.
Conclusions: ETCO2 obtained in a steady state may be comparable to PCO2 in those with no underlying lung disease such as NMD unless bronchiectasis is present. Anxiety may directly affect minute ventilation and therefore alter ETCO2 and PCO2. Alteration of CO2 due to hyperventilation would be observed more rapidly with ETCO2 than a CBG. Data collection is currently ongoing and additional data will be presented.