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Acid-Base: Basics of Respiratory Disturbance
- 1. Acid & Base –II Respiratory Acidosis & Alkalosis
- 2. Acid-Base Disorders BASE ACID 7.35 – 7.45 pH Acidemia <7.35 Decreased HCO3 - Metabolic Acidosis Increased PCO2 Respiratory Acidosis Alkalemia >7.45 Increased HCO3 - Metabolic Alkalosis Decreased PCO2 Respiratory Alkalosis pH = 7.35 – 7.45 PaCO2 = 38 - 42 HCO3 - = 22 - 26
- 3. Arterial Blood Gas Analysis pH <7.35 = acidemia >7.45 = alkalemia PaCO2 Elevated Normal Lowered HCO3 - Elevated Normal Lowered
- 4. Respiratory Acidosis (Hypoventilation) Respiratory Acidosis CO2 Acute (intracellular buffering) HCO3 - : Increase 1mEq/L per each 10mmHg CO2 increase pH: 0.08 change per 10 mmHg change in PCO2 Chronic (renal compensation) HCO3 - : Increase 4mEq/L per each 10mmHg CO2 increase pH: 0.03 change per 10 mmHg change in PCO2 Step 1: pH < 7.35 Step 2: PaCO2 >40 Step 3: HCO3 - ?? Step 4: pH
- 5. Respiratory Alkalosis (Hyperventilation) Respiratory Alkalosis CO2 Acute HCO3 - : Decrease of 2mEq/L per each 10mmHg CO2 Decrease pH: 0.08 change per 10 mmHg change in PCO2 Chronic HCO3 - : Decrease of 5mEq/L per each 10mmHg CO2 decrease pH: 0.08 change per 10 mmHg change in PCO2 Step 1: pH > 7.45 Step 2: PaCO2 <40 Step 3: HCO3 - Step 4: pH
- 6. Online resources are available
Editor's Notes
- Arterial blood gas analysis can be used to determine pH, the partial pressure of carbon dioxide and the concentration of serum bicarbonate. These measurements help providers determine the cause of acid-base derangements so that appropriate clinical interventions can be initiated.
- Normal physiologic pH ranges from 7.35 to 7.45. Normal partial pressure of carbon dioxide ranges from 38-42mmHg. Normal serum Bicarbonate range is from 22-26 mEq/L. Acidemia can occur from too little bicarbonate or too much carbon dioxide. Alkalemia can occur from too little carbon dioxide or too much bicarbonate. When the process is driven by carbon dioxide it is termed – “respiratory”. When the process is driven by bicarbonate, it is termed – “metabolic”.
- The first step in interpreting the arterial blood gas analysis, or ABG, is determining the value of the pH. This will determine if the patient has an acidemia or alkalemia. The second step is to determine the value of the partial pressure of carbon dioxide. Too much carbon dioxide leads to acidosis while decreased levels of carbon dioxide lead to an alkalosis. Third, providers should determine the value of the bicarbonate. Bicarbonate is a buffer and will be used to compensate for an acute acidotic process. Over time, in more chronic states, the kidneys will begin to retain more bicarbonate in an attempt to compensate for an underlying acidosis.
- Acidosis occurs from too much carbon dioxide or too little bicarbonate. When the process is driven by carbon dioxide it is termed – “respiratory”. Respiratory acidosis, then, is caused by the retention of carbon dioxide. Following a stepwise approach, if the pH is less than 7.35 there is an acidosis and if the PaCO2 is elevated, then there is a respiratory acidosis. In the acute phase, bicarbonate can shift out of cells to compensate for an acidosis, however this compensation is limited. Renal compensation, through the retention of bicarbonate, starts within 12 hours. It has its maximal effect at 3-4 days. Determining whether the process is acute or chronic can be done by looking at the changes in pH and bicarbonate as compared to the carbon dioxide. For an acute respiratory acidosis, the bicarbonate will increase 1 mEq/L for every 10mmHg change in carbon dioxide and the pH will decrease by 0.08. Examples of acute respiratory acidoses include oversedation, airway obstruction or apnea. For a chronic respiratory acidosis, the bicarbonate will increase by 4 mEq/L for every 10mmHg rise in carbon dioxide and the pH will decrease by 0.03. COPD is an example of a chronic respiratory acidosis.
- If the first step reveals a pH > 7.45, an alkalemia is present. Alkalemia can occur from too little carbon dioxide or too much bicarbonate. If the second step reveals a PaCO2 < 40, a respiratory alkalosis is present. Respiratory alkalosis occurs from hyperventilation. Determining whether the process is acute or chronic can be done by evaluating the pH and bicarbonate. If the process is acute, the bicarbonate will decrease by 2 mEq/L for each 10mmHg decrease in CO2. The pH will increase by 0.08. Examples of acute respiratory alkalosis include states that result in tachypnea such as early asthma exacerbations or anxiety. If a respiratory alkalosis is chronic, the bicarbonate will decrease by 5 mEq/L for every 10mmHg decrease in carbon dioxide and the pH will increase by 0.08. CNS injury, pulmonary embolus and certain medications or toxins can cause chronic hyperventilation and respiratory alkalosis. It is also possible to cause a respiratory alkalosis by over-ventilating patients who are intubated and mechanically ventilated.
- Learners should know that there are many websites and smartphone applications that can assist providers with calculations and interpretations of arterial blood gases.