2006 OPEN FORUM Abstracts
COMPARISION OF ETCO2 WITH CAPILLARY BLOOD GAS PCO2
Denise Willis, RRT-NPS, Robert Warren, MD.
Arkansas
Children's Hospital,
Little Rock
,
AR.
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.