acid- base disturbances unit 6

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Unit 6: Acid-Base Balance

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Most biochemical reactions occur in aqueous solutions, where compounds may dissociate to form acids or bases. Deviations in hydrogen concentration from the body’s normal range will cause alterations in the rates of chemical reactions within the cell and affect many metabolic processes of the body. Consequently, acid-base balance is primarily concerned with the regulation of these hydrogen ions within the body fluids. Introduction to Acid-Base Balance

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The concentration of hydrogen ions in the body fluid is regulated primarily by 2 buffer systems: The chemical buffer system that acts rapidly. The physiological buffer systems that may require several minutes to several days to begin resisting a change in pH. Acid-Base Regulation

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Chemical Buffer System – The body’s first line of defense against changes in hydrogen concentration is the acid-base buffer system. This type of system is usually composed of sets of two or more chemicals that take up hydrogen ions when body fluids are becoming more acidic, and give up hydrogen ions when the fluids are becoming more basic. Specifically, one of the most important acid-base buffer systems in body fluids is the bicarbonate buffer system. Acid-Base Regulation

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Bicarbonate Buffer System Components Bicarbonate (HCO 3 -) & Carbonic acid (H 2 CO 3 ) Within the body’s red blood cells, carbonic acid may be produced from carbon dioxide as it reacts with the water in the presence of the enzyme, carbonic anhydrase. As carbonic acid dissociates, bicarbonate is formed to act as a buffer to avoid excess hydrogen ion release. Acid-Base Regulation CO 2 + H 2 0  H 2 CO 3  H + + HCO 3 - (carbonic acid)

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Henderson-Hasselbalch Equation - The Henderson-Hasselbalch equation is used for calculating the pH in the blood by using a dissociation constant (pK a ) and the ratio of base (HCO 3 -) to acid (H 2 CO 3 ). In other words, this equation relates pH of blood plasma to equilibrium of bicarbonate salt and carbonic acid. Acid-Base Regulation pH = pK a + log [Base] [Acid]

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Physiological Buffer System: The body’s second line of defense against pH shifts is the physiological buffer system where the respiratory mechanism may act within minutes, but the renal mechanism may take up to several days. Acid-Base Regulation

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Respiratory Mechanism – The respiratory system may regulate hydrogen ion concentrations in the body fluids by controlling the rate and depth of breathing. Respiratory Response for  H+ In response, the respiratory system increases the depth and rate of breathing, so that the lungs excrete more carbon dioxide. Respiratory Response for  H+ As a result, breathing rate and depth fall to prevent further loss of CO 2 . Acid-Base Regulation

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Renal Mechanism – When the hydrogen ion concentration deviates from normal, the kidneys also respond by selectively excreting or reabsorbing hydrogen, bicarbonate ions, or other electrolytes to restore the equilibrium between the production and removal of hydrogen ions. Kidney Response for  H+ The kidneys selectively excrete hydrogen ions whenever there is excess. They also act to retain bicarbonate ions. Kidney Response for  H+ Whenever there is a loss of hydrogen ions, the kidneys increase the rate of their reabsorption. They also act to selectively excrete bicarbonate ions. Acid-Base Regulation

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Ordinarily, chemical and physiological buffer systems maintain the hydrogen ion concentration of body fluids within very narrow pH ranges. Abnormal conditions, however, may disturb this acid-base balance. Because the body’s cellular and metabolic activities are so pH-dependent, the body tries to return the pH toward normal whenever an imbalance occurs. This action by the body is termed compensation. Acid-Base Disturbances

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Classifications Acid-base disturbances are classified by pH and based on the primary origin of the disorder. The normal pH of arterial blood is 7.35 - 7.45 A pH below this reference range is referred to as acidosis. A pH above this reference range is referred to as alkalosis. Acid-Base Disturbances

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Acidosis - Acidosis results from an accumulation of acids or loss of bases, both of which cause abnormal increases in the hydrogen ion concentration of body fluids. The two major types of acidosis are: Respiratory acidosis Metabolic acidosis Acid-Base Disturbances pH

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Alkalosis - Conversely, alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in hydrogen ion concentrations. The two major types of alkalosis are: Respiratory alkalosis Metabolic alkalosis Acid-Base Disturbances pH

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In order to interpret acid-base disturbances, the following five factors are considered: pH HCO 3 - pCO 2 anion gap assessment for compensation A step-by-step mechanism may be use when interpreting acid-base disturbances and determining which classification the patient is exhibiting. Interpreting Acid-Base Disturbances

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Step-by-Step Mechanism 1st Step - Based on pH, determine if the disturbance is caused by acidosis or alkalosis. 2nd Step - Determine the cause of the pH imbalance, based on HCO 3 -, pCO 2 , and the anion gap. 3rd Step - Assessment of compensation. Interpreting Acid-Base Disturbances

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Measurement of ABGs provides valuable information in assessing and managing a patient’s respiratory and metabolic acid/base and electrolyte homeostasis. It is also used to assess adequacy of oxygenation, especially those patients on ventilators or to monitor critically ill non-ventilator patients. Arterial Blood Gases (ABGs)

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Collection & Handling of ABGs: The specimen for blood gases and pH should be arterial blood collected in heparinized plastic containers, i.e. syringes. All air bubbles should be removed, and the end of the container fitted with a tight cover. The specimen must be placed in ice water until analysis, unless it is processed immediately at the patient’s bedside using a point-of-care instrument. Arterial Blood Gases (ABGs)

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Collection & Handling of ABGs: Pre-analytical errors - Variations in the anticoagulant Exposure to warm temperatures during transport of the specimen Air exposure Arterial Blood Gases (ABGs)

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ABG Analysis: A blood gas analyzer measures multiple parameters using a series of electrodes and spectrophotometric measurements. Arterial Blood Gases (ABGs) diagnostics.siemens.com/

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ABG Analysis: Parameters measured using selective electrodes: Blood gas analyzers use electrodes as sensing devices to selectively measure pH, pCO 2 , and pO 2 . pH – is a measure of alkalinity and acidity. pCO 2 – is a measure of the partial pressure of CO 2 dissolved in the blood. It is referred to as the respiratory component in acid-base determination because this value is controlled primarily by the lungs. pO 2 - is a measure of the partial pressure of oxygen dissolved in the plasma. Arterial Blood Gases (ABGs)

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ABG Analysis: Calculated parameters: HCO 3 - (Bicarbonate) – is a measure of the metabolic component of the acid-base equilibrium since it is regulated by the kidneys. When given the dissociation constant (pK a ), pH, and pCO 2 , the formula to solve for bicarbonate is as follows: Arterial Blood Gases (ABGs) pH = 6.1 + log [HCO 3 -] (pCO 2 x 0.0301) Where: H 2 CO 3 (Carbonic acid) = pCO 2 x 0.0301

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ABG Analysis: Calculated parameters: Base excess – is a parameter calculated by the blood gas analyzer to represent the amount of buffering anions in the blood. When interpreting base excess values: A negative base excess (deficit) indicates a metabolic acidosis. A positive base excess indicates metabolic alkalosis. Arterial Blood Gases (ABGs)

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ABG Analysis: Parameters measured spectrophotometrically - O 2 saturation – is an indication of the percentage of hemoglobin saturated with oxygen (O 2 ). Oxygen saturation is measured as oxyhemoglobin versus total hemoglobin by a spectrophotometer taking multiple absorbance readings at various wavelengths. The blood gas analyzer calculates the O 2 saturation by using the following formula: Arterial Blood Gases (ABGs) % of O 2 Saturation = Oxyhemoglobin x 100 Total Hemoglobin

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Pulse Oximetry Transcutaneous monitoring of oxygen saturation, also known as pulse oximetry, has been used for many years in pediatric and critical care patients since monitoring through the skin can be continuous and noninvasive. Principle: It operates on the principle that oxyhemoglobin found in capillary blood of warmed skin can be measured spectrophotometrically. Additional Measures of Oxygen Saturation

Reading Assignment:

Reading Assignment Please read Chapter 9 Assessment of Respiratory Disorders in your textbook Clinical Chemistry: A Laboratory Perspective. You may also find it helpful to review Chapter 6 pp. 215-216 that discusses renal control of acid-base balance.

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