Acid - Base Balance

Acid–Base Balance: Main Ideas

How the Body Regulates pH

• Normal arterial blood pH is 7.35–7.45.
  • A pH below 7.35 indicates acidosis.
  • A pH above 7.45 indicates alkalosis.
• pH depends on hydrogen-ion concentration:
  • More hydrogen ions make the blood more acidic and lower the pH.
  • Fewer hydrogen ions make it more alkaline and raise the pH.
• The body maintains pH through buffering systems and the coordinated work of the lungs and kidneys:
  • Kidneys regulate bicarbonate and hydrogen ions. In acidosis, they excrete more hydrogen ions and retain bicarbonate; in alkalosis, they excrete bicarbonate and retain hydrogen ions. Renal compensation takes longer—typically days.
  • Lungs regulate carbon dioxide (CO₂), which affects carbonic acid in the blood. Faster breathing removes CO₂ and tends to raise pH; slower breathing retains CO₂ and tends to lower pH. Respiratory adjustments can occur within hours.

Arterial Blood Gas (ABG) Reference Values

The lecture gives these typical ranges:

• pH: 7.35–7.45
• PaO₂: 80–100 mmHg; lower values suggest hypoxemia.
• PaCO₂: 35–45 mmHg; CO₂ acts as an acid.
• HCO₃⁻ (bicarbonate): about 22–26; bicarbonate acts as a base.
• O₂ saturation: 95–100% in a healthy adult; lower values suggest inadequate oxygenation.

The lecturer states that a pH below 6.8 or above 7.8 is generally incompatible with life.

The Four Primary Acid–Base Imbalances

Metabolic Acidosis

What it is: Excess acid or loss of bicarbonate lowers blood pH. The ABG pattern is low pH and low bicarbonate.

Causes covered:

• Lactic-acid buildup, including in sepsis, shock, or cardiac arrest.
• Salicylate (aspirin) toxicity.
• Renal failure, when the kidneys cannot adequately excrete hydrogen ions or retain bicarbonate.
• Toxic alcohol ingestion, such as ethylene or propylene glycol.
• Diabetic ketoacidosis (DKA) and starvation-related ketosis.

Possible signs and compensation:

• Headache, confusion, and drowsiness.
• Faster, deeper breathing—described in the lecture as Kussmaul respirations—to remove CO₂.
• Decreased blood pressure and cardiac output may occur.
• The kidneys may also compensate by excreting hydrogen ions and retaining bicarbonate, but this is slower.

Management discussed:

• Correct the underlying cause.
• Depending on the case, treatment may include bicarbonate, IV fluids, and monitoring electrolytes—especially potassium.
• Severe or persistent acidosis, particularly with renal failure or toxicity, may require dialysis.

Metabolic Alkalosis

What it is: Loss of acid or an increase in bicarbonate raises blood pH. The ABG pattern is high pH and high bicarbonate.

Causes covered:

• Vomiting or gastric suction, which removes stomach acid.
• Long-term use of loop or thiazide diuretics, which can increase losses of hydrogen and potassium.
• Hyperaldosteronism and Cushing syndrome, which can affect electrolyte balance.
• Hypokalemia (low potassium), which contributes to the imbalance.

Possible signs and compensation:

• Tingling, dizziness, and, in more severe cases, tetany or muscle spasms.
• Tachycardia or rhythm changes, particularly when potassium is low.
• Respiratory depression as the body retains more CO₂.
• Reduced gastrointestinal motility, which may cause bloating or constipation.

Management discussed:

• Treat the underlying cause—for example, address vomiting or gastric suction and consider anti-nausea medication.
• IV fluids may be needed.
• Monitor intake and output, electrolytes, and cardiac rhythm.
• Hyperaldosteronism may require medication and, in some cases, surgery.

Respiratory Acidosis

What it is: The lungs do not remove enough CO₂, so it accumulates and lowers pH. The primary ABG pattern is low pH and high PaCO₂.

Causes covered:

• Pulmonary edema, atelectasis, pneumothorax, or pneumonia.
• Respiratory depression from narcotics or sedatives.
• Severe obesity that restricts lung expansion.
• COPD.
• Neuromuscular disorders, such as muscular dystrophy, multiple sclerosis, or myasthenia gravis, that impair breathing.

Possible signs and compensation:

• Acute cases may involve increased heart and respiratory rates and elevated blood pressure.
• Increased CO₂ can cause confusion, drowsiness, and reduced consciousness.
• Chronic respiratory conditions may allow the kidneys to retain more bicarbonate, so symptoms can be mild or less apparent.

Management discussed:

• Improve ventilation and treat the underlying problem.
• Monitor oxygenation; treatment may include oxygen, bronchodilators, antibiotics for infection, or other cause-specific care.
• The lecture also mentions pulmonary physiotherapy and hydration to help clear secretions.
• Some patients may need mechanical ventilation.

Respiratory Alkalosis

What it is: Rapid or deep breathing removes too much CO₂, raising blood pH. The primary ABG pattern is high pH and low PaCO₂.

Causes covered:

• Anxiety or panic.
• Hypoxemia.
• Salicylate toxicity.
• Sepsis, which can increase respiratory rate; severe progression to septic shock may lead to acidosis instead.
• Overventilation by a mechanical ventilator.

Possible signs:

• Lightheadedness and difficulty concentrating.
• Numbness or tingling.
• Tachycardia or arrhythmias.

Management discussed:

• Treat the underlying cause.
• For anxiety-related hyperventilation, the lecture mentions anxiety medication and breathing techniques.
• Give oxygen when hypoxemia is the cause.
• If ventilator settings are contributing, they may need adjustment by the clinical team.

The lecturer mentions breathing into a paper bag as an “old-school” technique. This should not be treated as a general recommendation: it can be unsafe if the cause is hypoxemia or another serious condition.

A Method for Interpreting ABGs

The lecture’s examples use the following approach:

1. Check the pH first.

   • Below 7.35: the blood is acidotic.
   • Above 7.45: the blood is alkalotic.
   • Within range: determine whether it is nearer the acidic or alkaline end.

2. Check PaCO₂ and bicarbonate.

   • High PaCO₂ points toward a respiratory acidifying effect; low PaCO₂ points toward a respiratory alkalizing effect.
   • Low bicarbonate points toward metabolic acidosis; high bicarbonate points toward metabolic alkalosis.

3. Identify which value matches the pH direction.

   • If bicarbonate matches the pH, the primary problem is metabolic.
   • If PaCO₂ matches the pH’s acidifying or alkalizing direction, the primary problem is respiratory.

4. Determine the degree of compensation.

   • Uncompensated: The pH is abnormal and the other system has not brought it back into range.
   • Partially compensated: The pH remains abnormal, while the other system is also responding.
   • Fully compensated: The pH is back within range, but PaCO₂ and/or bicarbonate remain abnormal. The pH’s position within the normal range helps identify the direction of the original imbalance.

The lecture’s examples include a normal ABG, metabolic alkalosis, metabolic acidosis, partially compensated metabolic alkalosis and acidosis, and fully compensated examples. One fully compensated example appears to have a captioning error: with pH 7.35, elevated PaCO₂, and elevated bicarbonate, it is described as leaning toward respiratory acidosis.

Speakers and Sources Featured

• One instructor/lecturer presents the lesson as spoken narration. The subtitles do not name the speaker; the video is from the Eileen Hawkins channel.
• ABG ranges and worked examples are presented within the lecture. No other speakers or external sources are identified in the subtitles.