Arterial Blood Gas (ABG) Analysis PDF

Blood gases give us three different oxygen values: PaO2 (OXYGEN PRESSURE) SaO2 (OXYGEN SATURATION) CaO2 (OXYGEN CONTENT)  PaO2 = the partial pressure of oxygen in the plasma phase of arterial blood. In a mixture of gases, each gas has a partial pressure which is the hypothetica...

Blood gases give us three different oxygen values: PaO2 (OXYGEN PRESSURE) SaO2 (OXYGEN SATURATION) CaO2 (OXYGEN CONTENT)  PaO2 = the partial pressure of oxygen in the plasma phase of arterial blood. In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.  SaO2 = percentage of available haemoglobin that is saturated with oxygen. CaO2 = The arterial oxygen content is the amount of oxygen bound to haemoglobin plus the amount of oxygen dissolved in arterial blood.  Arterial Blood Gas (ABG) Analysis A sample of arterial blood is drawn (direct arterial puncture or from an arterial line) and analyzed to help to determine: – The quality and extent of pulmonary gas exchange – Acid-base status ABG test measures PaO2, SaO2, PaCO2, pH, and the bicarbonate (HCO3) level Arterial Blood Gas (ABG) Analysis – Normal Values Measurement Normal Value PaO2 80 – 100 mmHg SaO2 93% - 99% pH 7.35 – 7.45 PaCO2 35 – 45 mmHg HCO3 22 – 26 mEq/l Arterial Blood Gas (ABG) Analysis Measuring Oxygen in the blood – Oxygenation may be measured by evaluating the PaO2 and the SaO2 – Only 3% of oxygen is dissolved in the arterial blood; the remaining 97% is attached to haemoglobin – A normal PaO2 is 80 to 100 mmHg at sea level People living in higher altitudes, the normal PaO2 is lower – PaO2 tends to decrease with age (60 to 80 mmHg) Arterial Blood Gas (ABG) Analysis Measuring oxygen in the blood (contd.) – An abnormally low PaO2 is referred to as hypoxemia – Results from many conditions: Intrapulmonary (disturbances in the lung) Intracardiac (disturbance of flow to or from the heart) Perfusion deficits (inadequate perfusion of the lung tissues) – A normal SaO2 ranges between 93% to 97% – SaO2 is an important oxygenation value to assess Arterial Blood Gas (ABG) Analysis Measuring pH in the blood – pH is a measure of the hydrogen ion concentration in the blood – Provides information about the acidity or alkalinity of the blood – Normal pH is 7.35 to 7.45 – As hydrogen ions accumulate, the pH drops, resulting in acidemia Refers to a condition in which the blood is too acidic Acidosis refers to the process that caused the acidemia Arterial Blood Gas (ABG) Analysis Measuring pH in the blood (contd.) – A decrease in hydrogen ions results in an elevation of the pH and alkalemia Refers to a condition in which the blood is too alkaline Alkalosis refers to the process that causes the alkalemia – Acids A substance that can donate a hydrogen ion to a solution Volatile acids – Can move between the liquids and gaseous states – Once in the gaseous state, these acids can be removed by the lungs – Major acid in the blood serum is carbonic acid (H2CO3) – Broken down into carbon dioxide and water Arterial Blood Gas (ABG) Analysis Non volatile acids – Cannot change into a gaseous form – Cannot be excreted by the lungs – Can only be excreted by the kidneys (metabolic process) – Sample: lactic acid and keto acids – An acid-base disorder may be either respiratory or metabolic in origin – An excess of either kind of acid results in acidemia – If carbon dioxide accumulates, then respiratory acidosis exists – If non volatile acids accumulate, then metabolic acidosis exists Arterial Blood Gas (ABG) Analysis Measuring pH in the blood (contd.) – Bases A substance that can accept a hydrogen ion, thereby removing it from the circulating serum Main base found in the serum is bicarbonate (HCO3) Amount of bicarbonate available in the serum is regulated by the kidney (metabolic process) If there is too little bicarbonate in the serum, the result is metabolic acidosis If there is too much bicarbonate in the serum the result is metabolic alkalosis Arterial Blood Gas (ABG) Analysis Measuring pH in the blood (contd.) – Bases Conditions leading to acidemia or alkalemia are influenced by a multitude of physiological processes – Respiratory and renal function or dysfunction – Tissue oxygenation – Substance ingestion – Circulation – Lactic acid production – Electrolyte loss from the gastrointestinal tract Arterial Blood Gas (ABG) Analysis Measuring Carbon Dioxide in the Blood – PaCO2 refers to the pressure or tension exerted by dissolved carbon dioxide gas in arterial blood – Natural byproduct of cellular metabolism – Regulated primarily by the ventilatory function of the lung – Normal PaCO2 is 35 to 45 mmHg – PaCO2 is thought of as an ‘acid’ – Elimination of CO2 from the body is one of the main functions of the lungs – Important relationship exists between the amount of ventilation and the amount of CO2 in the blood Arterial Blood Gas (ABG) Analysis Measuring Carbon Dioxide in the Blood (contd.) – In hypoventilation, CO2 is accumulated and the PaCO2 value increased – Retention of CO2 results in respiratory acidosis – May occur even with normal lungs if the respiratory center is depressed and the respiratory rate or quality is insufficient to maintain normal CO2 concentrations – In hyperventilation, CO2 is eliminated from the body – PaCO2 value decreases – The loss of CO2 results in respiratory alkalosis Arterial Blood Gas (ABG) Analysis Measuring Bicarbonate in the Blood – Bicarbonate is the main base found in the serum – Helps the body regulate pH because of its ability to accept a hydrogen ion – Concentration is regulated by the kidneys (metabolic process of regulation) – Normal bicarbonate is 22 to 26 mEq/l – When bicarbonate level increases above 26 mEq/l, a metabolic alkalosis exists Results from a gain of base or a loss of metabolic acids – When bicarbonate level decreases below 22 mEq/l, a metabolic acidosis exists Arterial Blood Gas (ABG) Analysis Alterations in Acid-Base Balance – Disturbances in acid-base balance result from an abnormality of the metabolic or respiratory system – If the respiratory system is responsible, it is detected by the carbon dioxide in the serum – If the metabolic system is responsible, it is detected by the bicarbonate in the serum Arterial Blood Gas (ABG) Analysis Respiratory Acidosis – Defined as a PaCO2 greater than 45 mmHg and a pH of less than 7.35 – Characterized by inadequate elimination of carbon dioxide by the lungs – May be the result of inefficient pulmonary function or excessive production of carbon dioxide Respiratory Alkalosis – Defined as a PaCO2 less than 35 mmHg and a pH of greater than 7.45 mmHg – Characterized by excessive elimination of carbon dioxide Arterial Blood Gas (ABG) Analysis Metabolic Acidosis – Defined as a bicarbonate level of less than 22 mEq/l and a pH of less than 7.35 – Characterized by an excessive production of nonvolatile acids or an inadequate concentration of bicarbonate for the concentration of acid within the serum Metabolic Alkalosis – Defined as a bicarbonate level of greater than 26 mEq/l and a pH of greater than 7.45 – Characterized by excessive loss of nonvolatile acids or the excessive production of bicarbonate Arterial Blood Gas (ABG) Analysis Self-Test The normal pH is 7.35 – 7.45 Answer either Alkalosis or Acidosis pH 7.48 pH 7.28 pH 7.18 pH 7.30 pH 7.44 pH 7.50 Arterial Blood Gas (ABG) Analysis Self-Test The normal PCO2 is 35 to 45 mmHg Answer either Acidosis or Alkalosis. PCO2 33 PCO2 45 PCO2 28.6 PCO2 60 PCO2 55 PCO2 36 Arterial Blood Gas (ABG) Analysis Self-Test The normal HCO3 is 22 to 26 mmHg Answer either Acidosis or Alkalosis. HCO3 15 HCO3 28 HCO3 33.5 HCO3 45 HCO3 24 HCO3 20 Arterial Blood Gas (ABG) Analysis – Stepwise Approach In naming blood gas conditions, a consistent way of evaluating numbers must be followed. Step 1: Acidemic or Alkalemic – The pH of the ABG identifies the disorder as alkalemic or acidemic Step 2: Respiratory or Metabolic – PCO2 determines the respiratory component – HCO3 determines the metabolic component Arterial Blood Gas (ABG) Analysis – Stepwise Approach Step 2 (contd.) – Same-same principle – Whichever component is in the same state as the pH, that particular component is the “cause” of the pH state – Example: pH 7.32 (acidosis) PCO2 36 (normal) HCO3 18 (acidosis) Metabolic Acidosis Arterial Blood Gas (ABG) Analysis – Stepwise Approach Step 2 (contd.) – Your turn pH 7.55 PCO2 18 HCO3 20 pH 7.18 PCO2 54 HCO3 19