Proxima. pco 2 and Calculated values for HCO 3 - and Base Excess on Proxima

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1 Operating principles The Proxima Sensor is fabricated as a single multi-analyte chip that carries an array of individual sensors, each of which measures a different analyte. pco 2 is measured by direct potentiometry. Calculated parameters Calculated parameters bicarbonate (HCO 3 - ) and base excess (BE) are calculated after the values obtained from ph and pco 2. The bicarbonate ion is the main buffer substance in the body and plays a key role in maintaining the ph value in the blood. The Proxima System calculates the value of standard bicarbonate from the current analyte readings. This refers to the bicarbonate content of plasma which would be present in blood equilibrated to a pco 2 of 40 mmhg and po 2 of 100 mmhg at 37 ºC. The following equation is used to calculate the standard bicarbonate: [HCO 3 - ] std = x A + (((A 2.9) 2 x ( x thb))/1.000) where A = BE(B) (0.2 x thb x (100 - O 2 SATest))/100 BE (B) is calculated assuming 100% oxygen saturation. chb used in this calculation is 15 g/dl. O 2 sat is calculated from: O 2 SAT (est)=((n 4-15*N *N *N)/(N 4-15*N N *N+2.4*10 6 ))*100 where N= po 2 *10 (0.48*(pH(37)-7.4) *BE(B)) Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2

2 Base excess is a calculated parameter and allows the calculation of the buffer quantity that needs to be infused in a patient with impaired acid-base balance. Actual Base Excess is the concentration of titratable base when the blood is titrated with a strong base or acid to a plasma ph of 7.40 at a pco 2 of 40 mmhg and 37 ºC at the actual oxygen saturation. The Proxima System calculates the value of base excess from the current analyte readings. The Base excess calculated in the Proxima System is the BE of the blood. In addition to the parameters HCO 3 - and ph value, the base excess of the blood takes into account the buffer effect of the blood. BE b = ( *tHb)*((HCO 3 - act-24.8)+(( *thb)*(ph(37ºc)-7.40))) Where thb is an entered value of 15 g/dl. Measuring and Reference ranges Test /Abbreviation Units* Reportable range Partial Pressure Carbon Dioxide / PCO 2 Bicarbonate/ HCO 3 - mmhg (2) kpa mmol/l (2) Base Excess/ BE mmol/l (-29.9)- (+29.9) Reference range (arterial) (-2)-(+3) (2) *The Proxima System can be configured with any of these units To convert pco 2 results from mmhg to kpa, multiply the mmhg value by Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2

3 Each institution should establish its own reference range for diagnostic evaluation of patient results. It is recommended to use reference ranges for the population being tested. Clinical significance Carbon dioxide is a significant end product of metabolic processes in the body and is excreted as gaseous carbon dioxide during respiration and as bicarbonate ions by the kidneys. Carbon dioxide reacts with water to form carbonic acid, which in turn dissociates to form bicarbonate and hydrogen ions. CO 2 + H 2 O H 2 CO 3 H + HCO 3 - The bicarbonate/carbonic acid buffer system is the most important buffer keeping blood plasma in a narrow range around ph 7.4. Measurement of pco 2 together with ph is an important component in the diagnosis of acid-base disorders. PCO 2 is the respiratory component of acid-base balance and is a measure of the pressure of carbon dioxide dissolved in blood. Elevated pco 2 readings are a primary indication of respiratory acidosis or a compensatory response to a metabolic alkalosis (the respiratory system is the cause). Common causes of respiratory acidosis could be any condition that impairs gas exchange or lung ventilation (chronic bronchitis, cystic fibrosis, emphysema and pulmonary oedema), obstructive lung disease, head trauma, oversedation or inappropriate settings for mechanical ventilation. Low pco 2 levels are a primary indication of a respiratory alkalosis or a compensatory response to a metabolic acidosis (the respiratory system is not the cause but it compensating) Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2

4 Common causes of respiratory alkalosis could be hypoxemia, acute anxiety, asthma, early stages of congestive obstruction airway disease, low haemoglobin level, brain tumour or injury, respiratory stimulant drugs or excessive mechanical ventilation. HCO 3 - (bicarbonate) is the most abundant buffer in the blood plasma and is an indicator of the buffering capacity of blood. Regulated primarily by the kidneys, HCO 3 - is the metabolic component of acid-base balance. Changes of the HCO 3 - concentration in connection with ph values are used for the determination of whether an acidosis or alkalosis of metabolic origin is present. An increased level of HCO 3 - may be due to a metabolic alkalosis or a compensatory response in respiratory acidosis. Decreased levels of HCO 3 - are seen in metabolic acidosis and as a compensatory mechanism in respiratory alkalosis. Causes of primary metabolic acidosis are ketoacidosis, lactate acidosis (hypoxia) and diarrhoea. A negative base excess indicates the presence of metabolic acidosis as acid would need to be subtracted to return blood ph to normal and a positive base excess indicates the presence of metabolic alkalosis as the acid would have to be added to return the blood ph to normal. BE allows the calculation of the buffer quantity that needs to be infused. Measurement Temperature The Proxima Sensor is heated to 37ºC during the Sample measurement. PCO 2 is a temperature-dependent quantity and is measured at 37ºC. Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2

5 System Performance These are example performance data given in the tables below. Precision data in controls: Two different levels of quality control material were estimated using a minimum of 6 Proxima Systems over a minimum of 27 days. Precision data (mmhg) N Mean SD* %CV Level % Level % Precision data (kpa) N Mean SD* %CV Level % Level % *Based on total SD Method comparison data The study was conducted in a laboratory setting with transfusion blood samples. Arterial blood samples were collected and compared with the Siemens 1200 reference device. Method comparison N Slope Intercept RMSE* R 2 Min Max pco 2 (mmhg) pco 2 (kpa) *RMSE: Root mean square error Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2

6 Factors affecting the results Adequate sample should be drawn to ensure a representative arterial blood sample is measured. If the sample is drawn too quickly, this may de-gas the sample leading to lower concentration to be reported. As Proxima operates in a closed system the arterial blood sample couldn t be exposed to air if the product is used following the Proxima User Manual. pco 2 on the sample is measured on the Proxima Sensor within less than 1 min ensuring the prompt analysis of the sample. Potential interference by exogenous and endogenous substances studies (based on CLSI guideline EP7 A2 Interference Testing in Clinical Chemistry; Approved Guideline-Second Edition ) have assessed the specificity of the pco 2 measurement on the Proxima Sensor. Sodium pentothal has been shown to have a potential interference on the measurement of pco 2 on the Proxima Sensor at the following concentration > 20.6 μmol/l. References 1. Severinghaus J., Bradley F. Electrodes for po 2 and pco 2 determination. Journal of Applied Physiology, 13 (3):515 (1958) 2. Scott M, Heusel J, LeGrys V, Siggaard-Andersen O; Electrolytes and Blood Gases. Tietz Textbook of Clinical Chemistry, Third Edition, ed. C.A. Burtis and E.R. Ashwood. Saunders Rev. 29/08/ pco 2 Test Information Sheet PL:028 v2