Acid-Base Balance and ABG Analysis

Acid-Base Balance and Arterial Blood Gases

• Acid-base balance is crucial for homeostasis, maintaining a steady balance between acids and bases.
• Imbalances can result from various health issues like diabetes, vomiting, diarrhea, and respiratory conditions (COPD, asthma, bronchitis, flu, COVID-19).

Purpose

• The purpose is to maintain homeostasis, a steady balance between acids and bases.
• Health problems can disrupt this balance.

How and Why... (Arterial Blood Gas (ABG))

• Arterial blood gas (ABG) analysis includes:
  • pH: Measures the concentration of H+ ions. Higher H+ = more acidic; lower H+ = more alkaline.
  • PaCO2: Reflects lung ventilation, specifically the exhalation of CO2. High PaCO2 indicates poor lung function.
  • HCO3-: Reflects kidney function, specifically the reabsorption or excretion of bicarbonate. High HCO3- often indicates compensation for a problem.

pH

• pH measures the concentration of H+ ions.
• Blood is slightly alkaline, with a pH range of 7.35-7.45.
• A pH below 7.40 indicates acidosis; above 7.40 indicates alkalosis.

Figure 16-16

• A diagram showing the relationship between carbonic acid and base bicarbonate, and how the body attempts to regulate pH.
• A balance of these two is important to maintaining a healthy pH level in blood.

Compensation

• The body attempts to correct blood pH changes through various mechanisms, primarily the lungs and kidneys.
• Respiratory acidosis is often compensated for by metabolic alkalosis (and vice versa). This is a compensatory mechanism for respiratory and metabolic imbalances.

Compensation Mechanisms

• Chemical buffers are the first line of defense, acting within seconds to counteract minor acid-base imbalances in tissues.
• Lungs compensate for imbalances within minutes by adjusting the rate and depth of respiration to excrete or retain CO2, altering the acid-base balance.
• Kidneys are the more permanent solution, regulating bicarbonate (HCO3-) and excreting acids within hours to days.

Acid-Base Control Actions & Mechanisms: Respiratory

• Hyperventilation: An increase in the respiratory rate to remove more CO2 from the body if blood acid levels increase,
• Hypoventilation: A decrease in the respiratory rate if blood becomes too alkaline.
• Lungs compensate for metabolic imbalances.

Regulators of Acid/Base: Respiratory System

• Respiratory system eliminates CO2, and the Respiratory Center in the medulla controls breathing.
• The body responds rapidly (within minutes/hours) to changes in acid/base balance by adjusting the CO2 levels in the blood.
• Increased respiratory rate/depth (↑ RR) leads to increased CO2 elimination that results in decreased CO2 (and thus decreased H+) in the blood.
• Decreased respiratory rate/depth (↓ RR) leads to increased CO2 retention that results in increased CO2 (and thus increased H+) in the blood.

Regulators of Acid/Base: Renal System

• Renal system eliminates H+ and reabsorbs HCO3-.
• This system regulates electrolytes (e.g., Na+, Cl-) which is important to this system's function.
• The kidneys' response time to acid/base imbalances takes hours to days.

Alterations in Acid-Base Balance

• Imbalances arise when compensatory mechanisms fail.
• Common causes of imbalance include respiratory issues (changes in CO2 levels), and metabolic issues (changes in bicarbonate levels).

Respiratory Acidosis

• Caused by hypoventilation (shallow/infrequent breathing) that leads to CO2 accumulation in the blood.
• Hallmarks include increased PaCO2 and decreased PaO2.
• Acute acidosis shows pH <7.35; while chronic acidosis may have a normal pH due to kidney compensation. The kidneys retain bicarbonate and excrete hydrogen ions in the urine (more so in chronic respiratory acidosis).
• Elevated PaCO2 remains high, while the compensatory mechanism of ↑ HCO3 is apparent in chronic cases.

Respiratory Alkalosis

• Resulting from hyperventilation (excessive breathing) causing CO2 loss.
• Causes include anxiety, pain, and some medical conditions/trauma.
• Compensation is often less apparent or negligible when considering the acute phase of the condition.
• Compensatory mechanisms might lead to a decreased HCO3-.

Metabolic Acidosis

• Characterized by a decrease in pH and a decrease in bicarbonate levels (HCO3-).
• Several possible causes - including diabetic ketoacidosis, lactic acidosis, starvation, severe diarrhea, renal tubular acidosis, kidney failure, and gastrointestinal fistulas.
• Compensatory mechanisms include increased CO2 excretion—leading to 'Kussmaul respirations', which are rapid and deep, to counter the excess acid. Kidneys excrete acid to further compensate.

Metabolic Alkalosis

• Results from excess base (bicarbonate).
• Causes include prolonged vomiting, excessive gastric suctioning, diuretic therapy, hypokalemia, excessive mineralocorticoids use, and excessive NaHCO3 intake.
• The body responds by decreasing breathing rate (↓ RR) to retain CO2 and decreasing kidney reabsorption of HCO3-. This is a compensatory mechanism in metabolic alkalosis.

Interpretation of ABGs

• Interpretation steps include evaluating pH, PaCO2, and HCO3- values.
• Determining if the patient is compensating is critical for proper diagnosis.
• Normal pH ranges from 7.35 to 7.45.
• Normal PaCO2 ranges from 35 mmHg to 45 mmHg.
• Normal HCO3- ranges from 22 to 26 mEq/L.
• Assess PaO2 (partial pressure of oxygen) and O2 saturation (how much oxygen is binding to hemoglobin) for a full picture of the patient's respiratory status.

ROME Method for Interpretation

• Provides a mnemonic for acid-base interpretation.
• Respiratory and Opposite, Metabolic and Equal

Tic Tac Toe

• A visual aid for quickly identifying acid-base disorders based on the pH and CO2 & HCO3 levels. The order is important!

Partial vs Full Compensation

• Full compensation means pH is now within normal limits.
• Partial compensation means pH is still outside of normal range and compensating.

This quiz covers the importance of acid-base balance and the role of arterial blood gas (ABG) analysis in maintaining homeostasis. It addresses how health issues can disrupt this balance and explains key components like pH, PaCO2, and HCO3-. Test your understanding of these critical concepts in physiology.