RC 111 Q3 lecture 2025

Overview of Ventilation and Respiration

Ventilation

• Definition: The movement of gas (air) in and out of the lungs, essential for the exchange of oxygen and carbon dioxide.

• Mechanism: Involves inhalation (drawing air into the lungs) and exhalation (expelling air from the lungs).

• Importance: Maintaining adequate ventilation is crucial for homeostasis and metabolic function.

Respiration

• Definition: A physiological process at the cellular level where oxygen is utilized and carbon dioxide is produced as a byproduct.

• Types: Includes external respiration (gas exchange between the lungs and blood) and internal respiration (gas exchange between blood and tissues).

Key Differences Between Ventilation and Respiration

• Ventilation Issues: Directly affect CO2 levels, which can indicate lung function abnormalities.

  • High CO2 Levels: Suggest inadequate ventilation, potentially leading to respiratory acidosis.

  • Low CO2 Levels: Indicate hyperventilation (excessive elimination of CO2), resulting in respiratory alkalosis.

• Regulation of Ventilation: In healthy individuals, ventilation is regulated based on metabolic demands, increasing during physical activity or illness (e.g., fever).

Tidal Volume (Vt)

• Definition: The volume of air moved in and out of the lungs with each breath.

• Typical Value: Approximately 500 mL in adults, although it can vary based on body size, age, and physical condition.

• Role: Vital for effective gas exchange, helping to remove CO2 and replenish oxygen in the blood necessary for cellular functions.

Lung Mechanics and Compliance/Resistance

• Compliance: Refers to the lung's ability to expand; higher compliance indicates easier lung expansion.

  • Factors Affecting Compliance: Includes the elasticity of lung tissue, surface tension in alveoli, and presence of fluid or disease.

  • Compliance Equation: C = ΔV / ΔP

    • Where C = Compliance (mL/cm H₂O), ΔV = change in lung volume (mL), and ΔP = change in pressure (cm H₂O).

• Resistance: Refers to the opposition to airflow in the air passages; increased resistance can occur in conditions like asthma or bronchitis.

  • Clinical Relevance: Understanding compliance and resistance is essential for assessing and managing respiratory conditions.

• Airway Resistance: The resistance to airflow in the respiratory airways, affected by the diameter of the airways.

  • Equation: Raw = ΔP / Flow

    • Where Raw = Airway Resistance (cm H₂O/L/sec), ΔP = change in pressure (cm H₂O), and Flow = airflow (L/sec).

Mechanics of Breathing and Pressure Gradients

• Pressure Gradients: The difference in pressure between the atmosphere and the intrapleural pressure drives airflow during ventilation.

  • During inhalation, the diaphragm contracts, expanding the thoracic cavity, causing a decrease in intrathoracic pressure. This creates a negative pressure gradient, allowing air to flow into the lungs.

  • During exhalation, the diaphragm relaxes, and the elastic recoil of the lungs reduces the thoracic cavity volume, increasing intrathoracic pressure and pushing air out of the lungs.

• Pressure Changes:

  • Intrapleural Pressure: The pressure within the pleural cavity, which becomes more negative during inhalation to facilitate lung expansion.

  • Alveolar Pressure: The pressure within the alveoli that decreases during inhalation and increases during exhalation.

Role of Respiratory Muscles

• Function: The respiratory muscles (diaphragm, intercostal muscles) generate the pressure gradient necessary for air to flow into and out of the lungs.

• Weak Muscles: May necessitate mechanical ventilatory support (e.g., BiPAP or ventilators) in cases of respiratory failure or muscular diseases.

Measuring Pressure and Volume Changes

• Pressure Gradient: Air moves from areas of high pressure to areas of low pressure, facilitating airflow during breathing.

• Elastance: The ability of the lungs and thorax to return to their original shape after being distorted; crucial for effective breathing.

• Dynamic vs. Static Measurement: Dynamic refers to measurements taken with airflow, while static measurements occur without airflow, reflecting lung mechanics under rest conditions.

Components of Breathing

• Flow: Defined as the volume change per unit of time, indicating the speed of air movement (breathing rate).

• Dead Space: Refers to the portion of the tidal volume that does not participate in gas exchange, reducing effective ventilation.

  • Types of Dead Space:

    • Anatomical Dead Space: Air occupying the conducting airways (nose, trachea).

    • Alveolar Dead Space: Air in non-perfused regions of the lungs not participating in gas exchange.

Factors Influencing Ventilation Distribution

• Gravity: Affects blood perfusion in the lungs; in an upright position, more perfusion occurs at the lung bases compared to the apices.

• Time Constants: The time required for alveoli to fill or empty, which can be prolonged in diseased lungs, affecting overall ventilation.

ABG Interpretation and Acid-Base Balance

• Normal Ranges:

  • pH: 7.35-7.45 (Normal range indicates acid-base balance).

  • pCO2: 35-45 mmHg (Reflects respiratory function).

  • HCO3- (bicarbonate): 22-26 mEq/L (Reflects metabolic function).

• Steps for ABG Interpretation:

  1. Determine if the pH indicates acidosis or alkalosis.

  2. Check if CO2 or HCO3 explains the pH change (respiratory vs metabolic).

  3. Assess compensation status: fully compensated, partially compensated, or non-compensated conditions.

Combined Conditions in ABG

• Combined Acidosis: Recognition of both metabolic and respiratory systems contributing to acidemia indicates a more complex clinical scenario that often requires comprehensive management.

• Fully Compensated: When pH is normal, but both CO2 and HCO3 are abnormal, indicating that the body has compensated for the underlying issue.

Specific Conditions Affecting Patient Oxygenation

• Anemia: A condition characterized by lower than normal hemoglobin levels, reducing the oxygen-carrying capacity of the blood, leading to hypoxia even with normal ventilation.

• Ventilation-Perfusion Mismatch: The most common cause of hypoxemia; occurs when air reaches the alveoli (ventilation) but blood flow (perfusion) is inadequate, leading to inefficient oxygen exchange.

• Acute vs. Chronic Conditions: Chronic respiratory issues often involve the body’s compensatory strategies, which can obscure the severity of the condition if standard values appear stable despite underlying abnormalities.

Calculations and Equations in Ventilation and Respiration

1. Tidal Volume (Vt):

   • Equation: Vt = Tidal Volume (mL) = Total Air Volume / Total Breaths

   • Example: If a person takes 12 breaths and the total air volume is 6000 mL, then Vt = 6000 mL / 12 = 500 mL per breath.

2. Minute Ventilation (Ve):

   • Equation: Ve = Tidal Volume (Vt) × Respiratory Rate (RR)

   • Example: With a tidal volume of 500 mL and a respiratory rate of 12 breaths per minute, Ve = 500 mL × 12 = 6000 mL/min (or 6 L/min).

3. Alveolar Ventilation (Va):

   • Equation: Va = (Tidal Volume (Vt) - Dead Space Volume (Vd)) × Respiratory Rate (RR)

   • Example: If Vt = 500 mL, Vd = 150 mL, and RR = 12 breaths/min, then Va = (500 mL - 150 mL) × 12 = 4200 mL/min (or 4.2 L/min).

4. Ventilation-Perfusion Ratio (V/Q):

   • Equation: V/Q = Ventilation (L/min) / Perfusion (L/min)

   • Example: If ventilation is 4 L/min and perfusion is 5 L/min, then V/Q = 4/5 = 0.8, indicating potential issues in gas exchange depending on the physiological context.

5. Dead Space Calculation:

   • Equation: Vd/Vt = (PaCO2 - PeCO2) / PaCO2

   • Where PaCO2 = arterial CO2 pressure and PeCO2 = end-tidal CO2 pressure. This calculates the proportion of tidal volume that does not participate in gas exchange, aiding in assessing lung function.

6. Airway Resistance Calculation:

   • Equation: Raw = ΔP / Flow

     • Where Raw = Airway Resistance (cm H₂O/L/sec), ΔP = change in pressure (cm H₂O), and Flow = airflow (L/sec).

7. Compliance Equation:

   • Equation: C = ΔV / ΔP

     • Where C = Compliance (mL/cm H₂O), ΔV = change in lung volume (mL), and ΔP = change in pressure (cm H₂O).

Minute Volume (Ve)

• Definition: The total volume of air inhaled or exhaled from the lungs in one minute.

• Equation: Ve = Tidal Volume (Vt) × Respiratory Rate (RR)

• Typical Value: Normal minute ventilation for an adult at rest is approximately 6-10 L/min.

• Importance: Minute volume is critical for assessing respiratory function and ensuring that adequate oxygen meets the body's metabolic demands.

Calculations in Respiratory Physiology

CaO2 (Oxygen Content in Arterial Blood)

• Equation: CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.003)

  • Where:

    • Hb = Hemoglobin concentration (g/dL)

    • SaO2 = Arterial oxygen saturation (

  • Unit: mL O2/dL blood

PaO2 (Partial Pressure of Oxygen in Arterial Blood)

• Normal Range: 75-100 mmHg (depending on altitude and individual factors)

• Knowledge: Reflects the amount of dissolved oxygen in blood, critical for assessing lung function.

PO2 (Partial Pressure of Oxygen)

• Calculation (General): PO2 = Total Barometric Pressure × Fraction of Inspired Oxygen (FiO2)

• Example: At sea level (760 mmHg) with FiO2 of 0.21 (21% O2), PO2 = 760 mmHg × 0.21 = 159 mmHg.

Dead Space (Vd)

• Equation: Vd = (PaCO2 - PeCO2) / PaCO2 × Vt

  • Where:

    • PaCO2 = arterial CO2 pressure

    • PeCO2 = end-tidal CO2 pressure

    • Vt = Tidal Volume (mL)

Ideal Body Weight (IBW)

• Calculation for Males: IBW (kg) = 50 + 2.3 × (height in cm - 60)

• Calculation for Females: IBW (kg) = 45.5 + 2.3 × (height in cm - 60)

• Purpose: Useful for determining dosages of medications and evaluating respiratory mechanics in clinical settings.