Blood Gas Analysis Methods PDF

\] Calculations Detail of measurement chamber of a modern blood gas analyzer showing the measurement electrodes. (Cobas b 121 - Roche Diagnostics) The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bi...

\] Calculations Detail of measurement chamber of a modern blood gas analyzer showing the measurement electrodes. (Cobas b 121 - Roche Diagnostics) The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.\[citation needed\] Two methods have been used in medicine in the management of blood gases of patients in hypothermia: pH-stat method and alpha-stat method. Recent studies suggest that the α-stat method is superior.\[citation needed\] pH-stat: The pH and other ABG results are measured at the patient\'s actual temperature. The goal is to maintain a pH of 7.40 and the arterial carbon dioxide tension (paCO2) at 5.3 kPa (40 mmHg) at the actual patient temperature. It is necessary to add CO2 to the oxygenator to accomplish this goal. α-stat (alpha-stat): The pH and other ABG results are measured at 37 °C, despite the patient\'s actual temperature. The goal is to maintain the arterial carbon dioxide tension at 5.3 kPa (40mmHg) and the pH at 7.40 when measured at +37 °C. Both the pH-stat and alpha-stat strategies have theoretical disadvantages. α-stat method is the method of choice for optimal myocardial function. The pH-stat method may result in loss of autoregulation in the brain (coupling of the cerebral blood flow with the metabolic rate in the brain). By increasing the cerebral blood flow beyond the metabolic requirements, the pH-stat method may lead to cerebral microembolisation and intracranial hypertension.\[10\] Guidelines A 1 mmHg change in PaCO2 above or below 40 mmHg results in 0.008 unit change in pH in the opposite direction.\[11\] The PaCO2 will decrease by about 1 mmHg for every 1 mEq/L reduction in \[HCO− 3\] below 24 mEq/L A change in \[HCO− 3\] of 10 mEq/L will result in a change in pH of approximately 0.15 pH units in the same direction. Assess relation of pCO2 with pH: If pCO2 & pH are moving in opposite directions i.e., pCO2 ↑ when pH is \ 7.4, it is a primary respiratory disorder. If pCO2 & pH are moving in same direction i.e., pCO2 ↑when pH is \>7.4 or pCO2 ↓ when pH \< 7.4, it is a primary metabolic disorder.\[12\] Parameters and reference ranges See also: Reference ranges for blood tests § Acid--base and blood gases These are typical reference ranges, although various analysers and laboratories may employ different ranges. Analyte Range Interpretation pH 7.34--7.44\[13\] The pH or H+ indicates if a person is acidemic (pH \< 7.35; H+ \>45) or alkalemic (pH \> 7.45; H+ \< 35). H+ 35--45 nmol/L (nM) Arterial oxygen partial pressure (PaO2) 10--13 kPa 75--100 mmHg\[13\] A low PaO2 indicates abnormal oxygenation of blood and a person is known as having hypoxemia. (Note that a low PaO2 is not required for the person to have hypoxia as in cases of Ischemia, a lack of oxygen in tissues or organs as opposed to arterial blood.) At a PaO2 of less than 60 mm Hg, supplemental oxygen should be administered. Arterial carbon dioxide partial pressure (PaCO2) 4.7--6.0 kPa 35--45 mmHg\[13\] The carbon dioxide partial pressure (PaCO2) is an indicator of CO2 production and elimination: for a constant metabolic rate, the PaCO2 is determined entirely by its elimination through ventilation.\[14\] A high PaCO2 (respiratory acidosis, alternatively hypercapnia) indicates underventilation (or, more rarely, a hypermetabolic disorder), a low PaCO2 (respiratory alkalosis, alternatively hypocapnia) hyper- or overventilation. HCO − 3 22--26 mEq/L The HCO − 3 ion indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO − 3 indicates metabolic acidosis, a high HCO − 3 indicates metabolic alkalosis. As this value when given with blood gas results is often calculated by the analyzer, correlation should be checked with total CO2 levels as directly measured (see below). SBCe 21 to 27 mmol/L the bicarbonate concentration in the blood at a CO2 of 5.33 kPa, full oxygen saturation and 37 Celsius.\[15\] Base excess −2 to +2 mmol/L The base excess is used for the assessment of the metabolic component of acid-base disorders, and indicates whether the person has metabolic acidosis or metabolic alkalosis. Contrasted with the bicarbonate levels, the base excess is a calculated value intended to completely isolate the non-respiratory portion of the pH change.\[16\] There are two calculations for base excess (extra cellular fluid - BE(ecf); blood - BE(b)). The calculation used for the BE(ecf) = \[HCO − 3 \]− 24.8 + 16.2 × (pH − 7.4). The calculation used for BE(b) = (1 − 0.014 × Hgb) × (\[HCO − 3 \]− 24.8 + (1.43 × Hgb + 7.7) × (pH − 7.4). total CO2 (tCO2 (P)c) 23--30 mmol/L\[17\] 100--132 mg/dL\[18\] This is the total amount of CO2, and is the sum of HCO − 3 and PCO2 by the formula: tCO2 = \[HCO − 3 \] + α×PCO2, where α=0.226 mM/kPa, HCO − 3 is expressed in millimolar concentration (mM) (mmol/L) and PCO2 is expressed in kPa O2 Content (CaO2, CvO2, CcO2) 94-100%\[19\] (mL O2/dL blood) This is the sum of oxygen dissolved in plasma and chemically bound to hemoglobin as determined by the calculation: CaO2 = (PaO2 × 0.003) + (SaO2 × 1.34 × Hgb) where hemoglobin concentration is expressed in g/dL.\[20\]