Respiratory Acidosis and Alkalosis Notes

Respiratory Acidosis

• Acidosis: Process lowering blood pH below 7.35. Respiratory refers to failure of respiratory system in pH balance.

• Normal Respiration:

  • Inhalation: Diaphragm and chest muscles contract, chest expands, air enters.

  • Exhalation: Muscles relax, lungs recoil due to elastin, air exits.

  • Lungs facilitate oxygen intake and carbon dioxide removal.

Carbon Dioxide and pH

• Carbon dioxide binds with blood water to form carbonic acid (H2CO3).

• Carbonic acid dissociates into hydrogen ions (H^+) and bicarbonate ions (HCO_3^-, a buffer).

• Partial pressure of carbon dioxide (pCO2) must be in a narrow range to prevent pH fluctuations.

• Lungs maintain a ventilation rate that matches carbon dioxide production by tissues.

• Chemoreceptors in carotid arteries and aortic arch detect rising pCO2 and falling pH.

• Respiratory centers in the brainstem increase respiratory rate and depth.

• Minute ventilation is increased, which is the volume of air moving in and out of the lungs per minute.

• Increased ventilation removes more carbon dioxide, reduces pCO2, and raises pH.

Respiratory Acidosis Explained

• Normal ventilation mechanism disrupted.

• Minute ventilation inadequate for pH balance.

• Potential Causes:

  • Brainstem Issues: Stroke or overdose (opioids, barbiturates) slowing respiratory centers.

  • Neuromuscular Disorders: Myasthenia gravis impairs nerve stimulation of muscles.

  • Muscle Impairment: Trauma or obesity hindering diaphragm and chest wall muscles.

  • Airway Obstruction: Foreign object blocking bronchus.

  • Impaired Gas Exchange: COPD, pneumonia (fluid in alveoli), pulmonary edema (fluid between alveoli/capillaries).

• Result: Inefficient carbon dioxide removal.

• Carbon dioxide accumulates, raising pCO2 above 45 mmHg.

• Blood pH decreases below 7.35.

Compensation Mechanisms

• If respiratory centers function, increased rate and depth of ventilation occur.

• Excess carbon dioxide diffuses into cells (especially red blood cells).

• Carbon dioxide reacts with water to form carbonic acid, then hydrogen ions and bicarbonate ions.

• Bicarbonate escapes to the circulation to counteract high pCO2.

• Hydrogen ions are buffered by intracellular proteins (amine groups like in hemoglobin).

• Issue: Limited intracellular protein concentration relative to carbon dioxide levels.

• Only small amounts of carbon dioxide enter cells to avoid disrupting intracellular pH.

• Limited bicarbonate generation (approximately 1 mEq/L increase per 10 mmHg increase in pCO2).

• Example: An acute rise of pCO2 from 40 to 60 mmHg (increase of 20 mmHg) only raises plasma bicarbonate from 24 to 26 mEq/L which has minimal impact of pH.

• Acute phase: pH remains low.

• Kidney Compensation (after 3-5 days):

  • Kidneys sense low pH and increase bicarbonate reabsorption in proximal convoluted tubules.

  • Kidneys increase bicarbonate concentration by roughly 4 mEq/L for each 10 mmHg increase in pCO2.

  • Example: If pCO2 increases from 20 to 40 mmHg (increase of 20 mmHg), plasma bicarbonate increases by 8 mEq/L (from 24 to 32 mEq/L).

  • This leads to a substantial pH increase.

Recap - Respiratory Acidosis

• Lungs fail to eliminate carbon dioxide.

• Carbon dioxide builds up, lowering blood pH below 7.35.

• Divided into acute (no renal compensation) and chronic phases (renal compensation present).

• Renal compensation raises bicarbonate concentration.

Respiratory Alkalosis

• Alkalosis: Process causing alkali accumulation or acid loss.

• Respiratory: Failure of the respiratory system in pH balance.

How Normal Respiration Works.

• Inhalation: Diaphragm and chest muscles contract, chest expands, air enters.

• Exhalation: Muscles relax, lungs recoil due to elastin, air exits.

• Lungs facilitate oxygen intake and carbon dioxide removal.

Carbon Dioxide and pH

• Carbon dioxide binds with blood water to form carbonic acid (H2CO3).

• Carbonic acid dissociates into hydrogen ions (H^+) and bicarbonate ions (HCO_3^-, a buffer).

• Partial pressure of carbon dioxide (pCO2) must be in a narrow range to prevent pH fluctuations.

• Lungs maintain a ventilation rate that matches carbon dioxide production by tissues.

• Chemoreceptors in carotid arteries and aortic arch detect falling pCO2 and rising pH.

• Respiratory centers in the brainstem decrease respiratory rate and depth.

• Minute ventilation is decreased, which is the volume of air moving in and out of the lungs per minute.

• Decreased ventilation means less carbon dioxide moves out, which increases pCO2 and lowers pH.

Respiratory Alkalosis Explained

• Normal ventilation mechanism disturbed.

• Minute ventilation exceeds what's needed to balance the pH.

• Respiratory centers fire more than usual for ventilation to increase.

• Causes:

  • Normal Response: Hypoxia (low oxygen) due to pneumonia, pulmonary embolism, or high altitude.

  • Abnormal Response: Anxiety, panic attacks, sepsis, salicylate overdoses.

  • Rare: Brainstem disorders irritating respiratory centers.

  • Iatrogenic: Over-ventilation (incorrect ventilator settings).

• Result: Lungs eliminate too much carbon dioxide.

• Carbon dioxide depleted, pCO2 falls below 35 mmHg.

• Blood pH increases above 7.45.

Compensation Mechanisms

• Acidic molecules (carboxyl groups from proteins like hemoglobin) in cells (especially red blood cells) release hydrogen ions.

• Hydrogen ions diffuse into blood and combine with bicarbonate to form carbonic acid, which breaks down into carbon dioxide and water.

• Plasma bicarbonate concentration decreases slightly.

• Limited intracellular protein concentration means limited hydrogen ions available to neutralize bicarbonate.

• Bicarbonate decrease is small (about 2 mEq/L for each 10 mmHg decrease in pCO2).

• Example: An acute drop of pCO2 from 40 to 20 mmHg (decrease of 20 mmHg) only decreases plasma bicarbonate from 24 to 20 mEq/L which has minimal impact of pH.

• Acute phase: pH remains high.

• Kidney Compensation (after 3-5 days):

  • Kidneys sense high pH and decrease bicarbonate reabsorption in proximal convoluted tubules.

  • Bicarbonate is excreted more in the urine.

  • Kidneys decrease bicarbonate concentration by roughly 4-5 mEq/L for each 10 mmHg decrease in pCO2.

  • Example: If pCO2 decreases from 40 to 20 mmHg (decrease of 20 mmHg), plasma bicarbonate decreases by 8 mEq/L (from 24 to 16 mEq/L).

  • This leads to a substantial pH decrease.

Recap - Respiratory Alkalosis

• Lungs blow off more carbon dioxide than needed.

• Blood pH increases above 7.45.

• Divided into acute (no renal compensation) and chronic phases (renal compensation present).

• Renal compensation decreases bicarbonate concentration.