Mathematical Arterialisation of Capillary Blood for Blood Gas Analysis in Critical Ill Patients
Mathematical Arterialisation of Capillary Blood for Blood Gas Analysis in Critical Ill Patients
The aim of the study is to compare capillary blood gas analysis compensated by v-TAC software
 (aCBGE, aCBGF) to arterial blood gas analysis (ABG) in terms of pH, pCO2 and pO2 and the
 clinical usefulness of this method compared to the gold standard of ABG.
In clinical practice blood gas analysis is an essential tool for monitoring respiratory
 status. The gold-standard method is arterial blood gas analysis (ABG) of blood from the
 patient's radial or femoral artery. An alternative to arterial sampling commonly used is
 arterialized capillary blood gas analysis from the earlobe (CBGE). Though CBGE is less
 invasive and can be performed by non-medical staff, it is less useful in the acute setting
 because an adequate vasodilatation is needed which typically lasts for at least 10 minutes
 and the quality of results tends to be operator dependable. Different trials have evaluated
 the agreement between ABG and CBGE. Whereas a close agreement between ABG and CBGE has been
 found for evaluating pH and the partial pressure of carbon dioxide (PCO2), several trials
 showed considerable variations for the partial pressure of oxygen (PO2) and that CBGE tends
 to underestimate PO2. Similar findings apply for capillary blood gas analysis from the
 fingertip (CBGF) which is even less accurate compared to CBGE in estimating PO2. Recently a
 method has been developed to calculate ABG values mathematically from peripheral venous
 blood, supplemented with oxygen saturation (SpO2) measurement by pulse oximetry, by use of
 venous-to-arterial conversion (v-TAC) software (OBI Medical, Denmark). The principle of the
 method is a mathematical transformation of venous blood gas analysis (VBG) values to
 arterialised values (aVBG) by simulating the transport of blood back through the tissue (6).
 This approach leads to a clinically acceptable agreement between ABG and aVBG for pH and
 PCO2. For PO2 the limits of agreement of aVBG are similar to those of PO2 from CBGE for
 values below 75 mmHg and with mean bias close to zero. However, it has not been evaluated
 previously whether v-TAC software can be used to calculate ABG values from CBGE (aCBGE) or
 CBGF (aCBGF). As CBGE and CBGF values are much closer to ABG values, compared to VBG values,
 the agreement between aCBGE/aCBGF and ABG in terms of pH, PCO2 and pO2 might be better
 compared to CBG without mathematical arterialisation by v-TAC.
Inclusion Criteria:

 -  Admission to the intensive / intermediate care or weaning unit at the department of
 pneumology and intensive care medicine of the university hospital RWTH Aachen

 -  Arterial line inserted to monitor the patient's blood pressure or respiratory status

 -  Male or female aged ≥ 18 years

 -  Written informed consent prior to study participation

 -  The subject is willing and able to follow the procedures outlined in the protocol

 Exclusion Criteria:

 -  Hemodynamic instability

 -  Impaired perfusion of peripheral limbs e.g. due to severe heart failure

 -  Pregnant and lactating females. Pregnancy will be ruled out in females of childbearing
 age by pregnancy test.

 -  Patient has been committed to an institution by legal or regulatory order

 -  The subject is mentally or legally incapacitated