(BQ) Part 2 book Evidence-based critical care presents the following contents: Arterial blood gas analysis, acute severe asthma, pleural effusions and atelectasis, hypertensive crises, acute decompensated cardiac failure, acute coronary syndromes, stress ulcer prophylaxis, acute and chronic liver disease,...
Chapter 22 Arterial Blood Gas Analysis Arterial blood gas (ABG) analysis plays a pivotal role in the management of critically ill patients Although no randomized controlled study has ever been performed evaluating the benefit of ABG analysis in the ICU, it is likely that this technology stands alone as that diagnostic test which has had the greatest impact on the management of critically ill patients; this has likely been translated into improved outcomes Prior to the 1960s clinicians were unable to detect hypoxemia until clinical cyanosis developed ABG analysis became available in the late 1950s when techniques developed by Clark, Stow and coworkers, and Severinghaus and Bradley permitted the measurement of the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2) [1–3] The ABG remains the definitive method to diagnose, categorize and quantitate respiratory failure In addition, ABG analysis is the only clinically applicable method of assessing a patient’s acid-base status ABGs are the most frequently ordered test in the ICU and have become essential to the management of critically ill patients [4] Indeed, a defining requirement of an ICU is that a clinical laboratory should be available on a 24-h basis to provide blood gas analysis [5] Indications for ABG Sampling ABGs are reported to be the most frequently performed test in the ICU [4] There are however no published guidelines and few clinical studies which provide guidance as to the indications for ABG sampling [6] It is likely that many ABGs are performed unnecessarily Muakkassa and coworkers studied the relationship between the presence of an arterial line and ABG sampling [7] These authors demonstrated that patients’ with an arterial line had more ABGs drawn than those who did not regardless of the value of the PaO2, PaCO2, APACHE II score or the use of a ventilator In this study, multivariate analysis demonstrated that the presence of an arterial line was the most powerful predictor of the number of ABGs drawn per patient independent of all other measures of the patient’s clinical © Springer International Publishing Switzerland 2015 P.E Marik, Evidence-Based Critical Care, DOI 10.1007/978-3-319-11020-2_22 329 22 330 Arterial Blood Gas Analysis status Roberts and Ostryznuik demonstrated that with use of a protocol they were able to reduce the number of ABGs by 44 % with no negative effects on patient outcomes [4] The ubiquitous use of pulse oximetry in the ICU has made the need for frequent ABG sampling to monitor arterial oxygenation unnecessary Furthermore (as discussed below), venous blood gas analysis can be used to estimate arterial pH and bicarbonate (HCO3−) but not arterial carbon dioxide tension (PaCO2) Previously, ABGs were drawn after every ventilator change and with each step of the weaning process; such an approach is no longer recommended The indications for ABG analysis should be guided by clinical circumstances However, as a “general rule” all patients should have an ABG performed on admission to the ICU and/or following (10–15 min) endotracheal intubation Patients’ with respiratory failure should have an ABG performed at least every 24–48 h Patients with type II respiratory failure will require more frequent ABG sampling than those with type I respiratory failure Furthermore, patients with complex acid-base disorders and patients undergoing permissive hypoventilation will require more frequent ABG sampling ABG Sampling ABG specimens may be obtained from an indwelling arterial catheter or by direct arterial puncture using a heparinized 1–5 mL syringe Indwelling arterial catheters should generally not be placed for the sole purpose of arterial blood gas sampling as they are associated with rare but serious complications Arterial puncture is usually performed at the radial site When a radial pulse is not palpable the brachial or femoral arteries are suitable alternatives Serious complications from arterial puncture are uncommon; the most common include pain and hematoma formation at the puncture site Laceration of the artery (with bleeding), thrombosis and aneurismal formation are rare but serious complications [8, 9] ABG analysis is typically performed on whole blood The partial pressure of oxygen (PaO2,), partial pressure of carbon dioxide (PaCO2), and pH are directly measured with standard electrodes and digital analyzers; oxygen saturation is calculated from standard O2 dissociation curves or may be directly measured with a cooximeter The bicarbonate (HCO−3) concentration is calculated using the Henderson-Hasselbalch equation: { } pH = pK A + log ⎡⎣ HCO3− ⎤⎦ / [ CO ] where pKA is the negative logarithm of the dissociation constant of carbonic acid The base excess is defined as the quantity of strong acid required to titrate blood to pH 7.40 with a PaCO2 of 40 mmHg at 37 °C In practice, acid is not titrated as suggested but calculated using a variety of established formulae or nomograms The base excess thus ‘removes’ the respiratory element of acid-base disturbance ABG Analysis 331 and identifies the metabolic contribution to interpret with pH and [H+] The standard bicarbonate is broadly similar and is the calculated [HCO3−] at a PaCO2 of 40 mmHg Although the base excess and standard bicarbonate allow for a metabolic acidosis to be diagnosed, it provides few clues as to the pathophysiology or underlying diagnosis As with any diagnostic test it is important that the specimen be collected and processed correctly and that quality assurance methods exist to ensure the accuracy of the measurements Aside from inter-laboratory variation, errors in calibration and electrode contamination with protein or other fluids may alter results Heparin is usually added to the blood to prevent coagulation and dilution with older liquid solutions previously caused spuriously low PaCO2 Sample preparation is important because air bubbles falsely elevate PaO2 The following points must be considered before obtaining sample to avoid errors in blood gas interpretation: s Steady State: Blood sampling must be done during steady state following the initiation or change in oxygen therapy or changes in ventilatory parameters in patients on mechanical ventilation In most ICU patients a steady state is reached between and 10 and in about 20–30 in patients with chronic airways obstruction [10] s Anticoagulants: Excess of heparin may affect the pH Only 0.05 mL is required to anticoagulate mL of blood s Delay in processing of the sample: Because blood is a living tissue, O2 is being consumed and CO2 is produced in the blood sample Red blood cell glycolysis may generate lactic acid and change pH Significant increases in PaCO2 and decreases in pH occur when samples are stored at room temperature for more than 20 In circumstances when a delay in excess of 20 is anticipated, the sample should be placed in ice; iced samples can be processed up to h without affecting the blood gas values s Hypothermia Blood gas values are temperature dependent, and if blood samples are warmed to 37 °C before analysis (as is common in most laboratories), PO2 and PCO2 will be overestimated and pH underestimated in hypothermic patients The following correction formulas can be used: – Subtract mmHg PO2 per °C that the patient’s temperature is 5 meq/L (i.e an anion gap >15 meq/L), the patient most likely has a metabolic acidosis Compare the fall in plasma HCO3− (25 − HCO3−) with the increase in the plasma anion gap (delta anion gap); these should be of similar magnitude If there is a gross discrepancy (>5 meq/L), then a mixed disturbance is present: s if increase AG >fall HCO3−; suggests that a component of the metabolic acidosis is due to HCO3− loss s if increase AG