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Unformatted text preview: Acid-Base Balance
NURS 216 Spring 2010 Sabra Smith, MS, RN Objectives Understand concepts of pH and buffer systems Understand bicarbonate buffer system Understand and apply concepts of respiratory acidosis/alkalosis and metabolic acidosis/alkalosis Interpret ABGs pH pH is a measure of the H+ concentration pH and H+ concentration are inversely related pH = log[H+] pH 7 = neutral, <7 = acidic, >7 = basic Ex: water is neutral b/c there is a balance between positive and negative ions high pH=lower concentration of H+ low pH = higher concentration of H+ Acid-Base Balance Normal ECF/blood pH is 7.4 Body functions and cell survival are VERY dependent on a blood pH of 7.357.45 An acid molecule is one that can release a H+ ion (has a positive charge) Stronger acids dump more of their H+ A Base is a molecule that can accept a H+ ion (has a negative charge) Acidic by-products Metabolism produces volatile and nonvolatile acids Volatile carbonic acid (H2CO3) is in equilibrium with CO2 Nonvolatile acids less important when measuring blood pH difficult to measure in the blood stream so CO2 is measured in its place CO2 Byproduct of cellular respiration
+ CO2 + H20 = H2C03 = H + HC03 Because C02 combines with H20 to make an acid, it can be measured as a substitute for acid CO2 is excreted by the lungs Base HCO3 (bicarbonate, bicarb) Kidneys control excretion of HCO3 NaHCO3 is a weak base because it partially dissolves in solution and therefore reacts weakly with acids Buffers A buffer is an weak acidbase mixture that can absorb acids or bases This serves to minimize changes in pH keep the ECF pH within range (7.357.45) Bicarbonate – carbonic acid buffer system NaHCO3 and H2CO3 controls the ECF pH Hemoglobinoxyhemoglobin buffer system in RBCs (works with bicarbonate system) Bicarbonate-Carbonic Acid Bicarbonate-Carbonic Buffer System Buffer
+ CO2 + H20 H2C03 H + HC03
Respiratory component metabolic/kidney component Acidemia occurs when the number of H+ ions increases Regulatory Mechanisms Buffer systems (mainly bicarbonate) immediate but temporary fix Respiratory control of CO2 levels responds within minutes Kidney control of HCO3 levels slower, takes a few days Primary Cause The primary cause is the initiating event that causes an acidbase imbalance in the blood Can occur in either the lungs (respiratory) or kidneys (metabolic) Respiratory problems cause changes in C02 levels Metabolic problems cause changes in HC03 levels Compensation The other system will attempt to counteract the primary problem metabolic system will compensate for a respiratory problem respiratory system will compensate for a metabolic problem The compensating system attempts to push the acidbase imbalance back toward 7.4!!! A word about compensation A mechanism that attempts to balance out the state created by the primary cause An interim measure that adjusts pH but does not correct the cause Permits survival while the body attempts to fix the problem Changes in pH Characterized by: initiating system (primary cause) respiratory metabolic pH level acidosis alkalosis Respiratory acidosis If the blood becomes too acidic due to decreased CO2 expiration by the lungs… Then the kidneys will produce more HCO3 to balance out the acid pH will be < 7.35, PaCO2 > 45 If kidney (metabolic) compensation works, the pH will move back toward 7.4 Respiratory Acidosis Causes: hypoventilation (narcotic/sedative overdose, sleep apnea, muscular disorders, COPD, pulmonary edema) S/sx: depend upon rapidity of onset, mainly due to hypoxemia CNS changes, lethargy, weakness, eventual coma Tmt: restore effective ventilation, give O2 Respiratory alkalosis If the blood becomes too alkaline due to increased C02 expiration by the lungs… Then the kidneys will excrete more HCO3 in the urine to balance out the alkaline C02 level pH will be >7.45, PaCO2 < 35 If kidney (metabolic) compensation works, the pH will move back toward 7.4 Respiratory Alkalosis Causes: hyperventilation (increased rate or tidal volume), brain injury/tumor, lung disease (pneumonia, pulmonary edema) S/sx: dizziness, numbness and tingling in extremities, confusion, seizures Tmt: fix underlying cause breathe into a paper bag Metabolic Acidosis If HCO3 is lost or acid is retained by the kidneys… The lungs will blow off more CO2 (hyperventilate) to balance the acidity pH will be < 7.35, HCO3- < 22 If respiratory compensation works, the pH will move back toward 7.4 Metabolic Acidosis Causes: bicarbonate loss (diarrhea), increased acid production (lactic acidosis, diabetic ketoacidosis, ingestion of poisions, acute or chronic renal failure) S/sx: GI disturbances, CNS changes, bradycardia and dysrhythmias Tmt: slow increase of pH, possibly IV NaHCO3, replace fluid and other electrolytes Metabolic alkalosis If H+ is lost, the kidneys retain too much HCO3, or if other alkaline metabolic byproduct build up d/t a dysfunctional metabolic process… Then the lungs will blow off less CO2 (hypoventilate) to balance out the alkalinity pH will be > 7.35, HCO3- > 28 If respiratory compensation works, the pH will move back toward 7.4 Metabolic Alkalosis Causes: vomiting, certain diuretics, hypokalemia, excess ingestion or administration of bicarbonate or alkaline substances S/sx: vague, problems usually d/t hypokalemia or fluid volume deficit. If severe, confusion and hyperactive reflexes, hypoventilation Tmt: replace necessary electrolytes, fluid Arterial Blood Gases (ABG) Drawn from an artery Measures Usually by respiratory therapy pH PaO2 PaCO2 how well O2 can move from lungs to blood how well CO2 can move from blood to lungs HCO3 PaO2 Arterial Blood Gases (ABG) Step 1 - assess O2
Purpose: make sure patient is oxygenating well Should be 80100% for arterial blood If low, consider:
was this arterial blood? is patient oxygenating well? Arterial Blood Gases (ABG) Step 2 - assess pH Normal pH 7.357.45 If < 7.35 = acidosis If > 7.45 = alkalosis Arterial Blood Gases (ABG) Step 3 - assess acids/bases CO2 3545 mmHg HCO3 2228mEq/L if high: respiratory acidosis if low: respiratory alkalosis if high: metabolic alkalosis if low: metabolic acidosis pH 7.35 7.45 CO2 45 35 HCO3- 22 28 Patient pH 7.31 PaCO2 50mmHg HCO3 23mEq/L Example 1 Normal 7.357.45 3545 mmHg 2228mEq/L Example 2 Patient pH 7.31 PaCO2 44mmHg HCO3 19mEq/L Normal 7.357.45 3545 mmHg 2228mEq/L Example 3 Patient pH 7.48 PaCO2 33mmHg HCO3 24mEq/L Normal 7.357.45 3545 mmHg 2228mEq/L Example 4 Patient pH 7.48 PaCO2 43mmHg HCO3 33mEq/L Normal 7.357.45 3545 mmHg 2228mEq/L ...
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- Spring '10