ATS Pulmonary Function Laboratory Manual

ATS Pulmonary Function Laboratory Management & Procedure Manual | 3rd Edition

Effective Date: 2014 Version #1

Chapter 16

Procedure Name: Blood Gas, pH, and Hemoximetry Analysis

Purpose or Principle This chapter addresses the analysis of blood for blood gases, pH, and hemoximetry (i.e., hemoglobin, hemoglobin fractions, O 2 content, and hemoglobin-O 2 saturation). It is limited to the measurement of these parameters in vitro using a single specimen. Arterial blood is the most common type of blood sample analyzed for these parameters, but it can also apply to capillary or venous blood samples. The analysis of blood for hydrogen ion concentration (pH), partial pressure of oxygen (Pa O 2 ) and partial pres- sure of carbon dioxide (Pa CO 2 ) are required to assess and revise therapeutic interventions to maintain acid-base and/or respiratory homeostasis. The evaluation of acid-base status is based on the direct measurement of pH and Pa CO 2 . The pH is measured using two chemical half-cells to measure the potential difference between solutions with different hydrogen ion [H + ] concentrations. The measuring half-cell is a Sanz electrode. The reference half- cell is composed of an electrode that supplies a constant reference voltage and is in a housing composed of potas- sium chloride solution (1). In traditional systems the Pa CO 2 is measured with a Severinghaus electrode. The Pa O 2 is measured with a Clark electrode. Base excess, standard bicarbonate, and total carbon dioxide may be calculated using the measured parameters. Hemoximetry, also known as whole blood CO-oximetry, is the spectrophotomet- ric analysis of the concentration of total hemoglobin (tHb) and percent saturations of the hemoglobin derivatives: oxyhemoglobin (O 2 Hb), reduced or deoxyhemoglobin (HHb), carboxyhemoglobin (COHb), and methemoglobin (MetHb). Hemoximetry of arterial blood is the only clinical laboratory test for the evaluation of anemic hypoxia, including carbon monoxide toxicity, methemoglobinemia, and sulfhemoglobinemia. The analyzer used for these measurements must be maintained and operated in a manner that provides accurate and precise measurements in an acceptable time frame. Blood gases, pH, and hemoximetry analyses must also meet current regulatory standards. Indications and Contraindications Indications for blood gas, pH, and hemoximetry analysis are provided in Table 16.1, and a list of relative contrain- dications is presented in Table 16.2.

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