Lecture 18 - Blood Gas Analysis PDF

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EPM608 Physiological Measurement Lecture 18 - Blood Gas Analysis Introduction All large hospitals have Blood Gas Analysers (BGAs). BGAs enable clinical decisions to be made regarding the care of critically ill patients with consequences to their outcome. Modern instruments are situated in ‘near-patient’ areas. Originally analysers were based in the biochemistry laboratory. The measurement process is fully automated i.e. the sample is automatically aspirated from the syringe, measured and the sample path flushed with detergent. Calibrations also occur automatically at regular intervals. What can BGAs measure? All BGAs measure pCO2, pO2 and pH. pCO2 and pO2 are abbreviations for partial pressures of carbon dioxide and oxygen respectively. pH is a logarithmic measure of acidity (pH = -log10[H+], where [H+] is hydrogen ion concentration). Modern BGAs can also make many other measurements. Why measure Blood Gases? Blood gas analysers provide indication of the function of the respiratory and metabolic systems. One of the main functions of blood is to transport O2 from the lungs to the tissue and to transport CO2 back to the lungs where it is expired. Blood Gas Analysis gives a useful indication of the effectiveness of this process and highlights respiratory problems. The body maintains its overall acid-base balance to ensure proper cell function and to prevent irreparable cell damage. The respiratory and metabolic systems are complementary. Blood gas analysis helps with the diagnosis of metabolic insufficiency. Indications for BGA (i.e. when do we measure blood gases?) Indications for blood gas analysis include:  Respiratory Disorders (e.g. Asthma, Respiratory Depression, Emphysema, Cystic Fibrosis)  Intensive Care Patients  Anaesthetised Patients  Metabolic Disorders (e.g. Kidney Failure)  Circulatory Disorders (e.g. Heart Disease) ...etc. pO2 The normal range of pO2 in adults for arterial blood is 12.0 – 18.0 kPa. Typically 13.3 kPa in healthy adults. Below 12 kPa, the patient is said to be hypoxaemic. pCO2 The normal range for pCO2 in adults for arterial blood is 4.7 - 6.0 kPa Above 6.0 kPa, patient is said to be hypercapnic, or suffering from respiratory acidosis. Below 4.7 kPa, patient is said to be hypocapnic, or suffering from respiratory alkylosis.

Acid-Base Chemistry

We produce large amounts of water (H2O), acid (H+) and carbon dioxide (CO2) each day. These are waste products of the food we metabolize to produce energy. This process occurs at a cellular level and if it remains unchecked, these products build up and cause irreversible cell damage. The respiratory system is the primary mechanism for CO 2 removal, but can also remove H + as a backup. Likewise, the metabolic system removes H+ but can also support the respiratory system in the removal of CO 2. The two systems are complementary and work together, i.e. not separate mechanisms. Hydrolysis reaction When CO2 dissolves into water (or blood – as happens in the systemic capillaries): CO2 + H2O  CH2O3 

H+ + HCO3–

Carbonic acid (CH2O3) dissociates, as it is unstable. By “law of mass action” or Le Chatelier’s Principle: CO2 from tissues dissolves in blood plasma and reaction shifts to the right. When the blood reaches the lungs, the low concentration of CO 2 in the alveoli cause the equilibrium to shift back to the left and CO2 is eliminated (exhaled from the lungs) Respiratory Acidosis and metabolic compensation If there is an increase in arterial pCO2, for example if ventilation is compromised, we have a respiratory acidosis. The reaction shifts to the left, H+ increases (and pH decreases). 2 things can happen. pH continues to decrease (if...