Lung Compliance II

• The primary focus of this session is to continue the discussion on compliance, specifically addressing how to overcome the elasticity and distensibility of the lung and how these factors become pathological in disease states.

• Learning Objectives:

  • Comprehensive understanding of lung compliance and how it alters with specific diseases.
    

    • Lung Compliance: This refers to how easily the lungs can expand when air enters. Healthy lungs are flexible, allowing them to stretch efficiently when breathing.

  • Understanding the inverse relationship between compliance and lung recoil forces.
    

    • Recoil Forces: When we inhale, the lung tissues stretch, but they naturally want to return to their original shape when we exhale. This returning force is called recoil.

  • Knowledge of the dynamic changes in pleural pressure ($P_{pl}$) and alveolar pressure ($P_{alv}$) during normal tidal volume breathing.
    

    • Pleural Pressure: This is the pressure within the pleural cavity (the space between the lungs and the chest wall). It plays a crucial role in keeping the lungs inflated.

    • Alveolar Pressure: This is the pressure inside the lungs, specifically the alveoli, which are tiny air sacs where gas exchange occurs.

• Tidal Volume Breathing Definition:

  • Normal breathing consists of moving approximately half a liter ($0.5 L$) of air in and out.
    

    • Tidal Volume: This is the amount of air we breathe in and out with each normal breath. It represents the basic volume needed for our regular activities.

  • Typical breathing frequency is approximately $12$ times per minute.
    

    • This means that in a minute, we take about a dozen breaths, which may vary with activity levels.

Defining and Calculating Lung Compliance

• Formal Definition: Compliance is defined as the change in lung volume for a given change in pressure.

  • To understand this concept, think about a balloon: how easily it expands when you blow air into it relates to the compliance of that balloon.

• Mathematical Representation:

  • $\text{Compliance} = \frac{\triangle V}{\triangle P}$
    

    • Here, $\triangle V$ is the change in volume of the lungs and $\triangle P$ is the change in pressure acting on the lungs.

• Normal Clinical Values:

  • On average, generating a pressure difference of $2.5 \text{ cm } H_2O$ results in an intake of $0.5 L$ of air.
    

    • This means that our lungs can effectively handle air movement at this pressure difference.

  • The typical compliance of a healthy human lung is approximately $0.2 L/\text{cm } H_2O$.
    

    • Higher values indicate better lung ability to inflate under pressure.

• Work of Breathing:

  • Moving the pleural space pressure from $-5 \text{ cm } H_2O$ down to $-7.5 \text{ cm } H_2O$ (a $2.5 \text{ cm } H_2O$ difference) represents the energy expended or work done to move that half-liter of air.
    

    • Think of it like using more effort to inflate a tire with higher resistance: more energy is needed when the pressure builds up.

The Compliance Curve and Transpulmonary Pressure

• Axes of the Compliance Graph:

  • Y-axis: Change in lung volume from the Residual Volume ($RV$), measured in liters (e.g., $1 L$, $2 L$, $3 L$, $4 L$, $5 L$).
    

    • This graph helps visualize how the lung volume changes as we go through different pressures.

  • X-axis: Transpulmonary pressure ($P_{tp}$), which is the pressure pushing the lung outward.
    

    • This pressure difference is what keeps the lungs inflated.

• Alternative View of the X-axis:

  • One can view the x-axis as pleural pressure ($P_{pl}$). The absolute numbers ($5$, $10$, $20$, $30$, $40$) would remain the same, but the signs would be inverted (negative).
    

    • It's important to understand that pressure readings can be negative, indicating suction or pulling, rather than pushing.

  • Negative pleural pressure pulls/distends the lung outward, while positive transpulmonary pressure pushes the lung outward.

• Curve Characteristics:

  • In a normal curve, compliance decreases (the line flattens out) as lung volume increases.
    

    • This means the lungs become harder to expand as they get fuller, illustrating the lungs' natural resistance.

  • Tidal volume breathing occurs at the lower, steeper portion of the curve.

  • Exercise occurs on the higher, flatter end of the curve, involving pleural pressures of $-20$, $-30$, or $-35 \text{ cm } H_2O$.
    

    • This means that during intense activities, our lungs need to pull much harder to get air in.

Low Lung Compliance: Pulmonary Fibrosis

• Definition: Characterized by a relatively small change in lung volume ($\triangle V$) for a given change in pressure ($\triangle P$).

  • This means the lungs don’t expand easily, requiring more effort to breathe.

• Disease Match: Pulmonary Fibrosis.

  • Clinical Distinction: Not to be confused with Cystic Fibrosis (which is a chloride channel defect).

  • Pathology: Loss of elastin fibers, increase in scar tissue and collagen.
    

    • This scarring makes it difficult for the lungs to expand and can significantly impact oxygen intake.

  • Descriptive Terms: Stiff lung, rigid lung, fibrotic lung, or "cement lung."
    

    • These terms illustrate how hard it is for people with this condition to breathe.

• Analogy: While a normal lung is like a single rubber band, a fibrotic lung is like trying to stretch four or five rubber bands bundled together.

  • It highlights how stiffness can complicate normal lung function.

• Impact on Work: Because the line on the graph is shallow, the patient must do significantly more work. To achieve a normal $0.5 L$ tidal volume, they might have to generate a transpulmonary pressure of $+10 \text{ cm } H_2O$ instead of the normal $+2.5 \text{ cm } H_2O$.

  • This requires more energy from the person, making breathing harder.

High Lung Compliance: Emphysema

• Definition: Characterized by a relatively large change in lung volume ($\triangle V$) for a given change in pressure ($\triangle P$).

  • This means the lungs can expand easily but may not function properly.

• Disease Match: Emphysema.

• Pathology: Chronic smoking (e.g., two packs a day for 30 years) destroys the elastic properties of the lung parenchyma.

• Analogy: A high-compliant lung is like an old sock in the back of a drawer that has lost all its elasticity. It is "flabby" or completely loose.

  • This means while inhaling is easy, exhaling requires more energy.

• Curve Characteristics: The slope of the line is very steep. A pressure change of $2.5 \text{ cm } H_2O$ might result in a volume change of nearly $1.0 L$ instead of $0.5 L$.

• Clinical Values: Compliance values might reach $0.3$, $0.4$, or $0.5 L/\text{cm } H_2O$.

• The Downside: While it is easier to inflate the lung (more "bang for the buck" during inspiration), the lack of elasticity means there is no passive recoil.

  • This leads to "air trapping," where it becomes increasingly difficult to deflate the lung over time.

Inverse Relationship Between Compliance and Recoil

• Compliance is inversely proportional to lung recoil (\text{Compliance} \text{
  } \frac{1}{\text{Recoil}}).
  

  • Low Compliance / High Recoil (Fibrosis):
    

    • Very stiff; hard to pull air in.

    • Once distended, the "rubber bands" have a massive force pulling back, resulting in high recoil.

  • High Compliance / Low Recoil (Emphysema):
    

    • Very loose; easy to distend.

    • Because the tissue is "flabby," there is no force to snap the lung back, resulting in dangerously low recoil.

Pressure and Volume Dynamics: Inspiration

• Initiation: The diaphragm (the most important pressure generator) contracts and moves downward.

  • Diaphragm: This muscle separates the chest cavity from the abdominal cavity and is crucial for breathing.

• Pleural Pressure ($P_{pl}$):

  • As the diaphragm moves down, the volume of the pleural space increases, causing the pressure to become more negative.
    

    • Negative pressure aids in sucking air into the lungs, similar to a syringe pulling liquid.

  • It starts at $-5 \text{ cm } H_2O$ and bottoms out at $-7.5 \text{ cm } H_2O$.

• Alveolar Pressure ($P_{alv}$):

  • Before inspiration (the pivot point), $P_{alv} = 0 \text{ cm } H_2O$ (equilibrium with atmosphere).

  • As $-P_{pl}$ pulls the lung out, the alveoli and airways are physically stretched.

  • This increase in alveolar volume causes $P_{alv}$ to drop (subatmospheric).

  • Alveolar pressure reaches its lowest point at mid-inspiration, approximately $-1 \text{ cm } H_2O$.

• Airflow: Air moves from the atmosphere ($0 \text{ cm } H_2O$) into the lower pressure alveoli ($-1 \text{ cm } H_2O$).

• End of Inspiration: As air fills the lungs, $P_{alv}$ rises back to $0 \text{ cm } H_2O$. At this point, no air is moving, but lung volume has increased by approximately $0.4$ to $0.5 L$.

Pressure and Volume Dynamics: Expiration

• Initiation: The diaphragm relaxes and moves upward toward the thoracic cavity.

  • This is a passive process, meaning it mostly happens naturally rather than needing muscle effort.

• Pleural Pressure ($P_{pl}$):

  • The volume of the pleural space decreases, making the pressure less negative.

  • $-P_{pl}$ moves from $-7.5 \text{ cm } H_2O$ back toward $-5 \text{ cm } H_2O$.

• Alveolar Pressure ($P_{alv}$):

  • As the distending negative pressure is lost, the lung's natural recoil squeezes the alveoli.

  • This compression decreases alveolar volume, causing $P_{alv}$ to become positive (supra-atmospheric).

  • Mid-expiration, $P_{alv}$ peaks at approximately $+1 \text{ cm } H_2O$.

• Airflow: Air moves from the high-pressure alveoli ($+1 \text{ cm } H_2O$) to the atmosphere ($0 \text{ cm } H_2O$).

• Hysteresis: The symmetry of the pressure-volume lines for inspiration and expiration is not identical; the difference in the shape of these curves is known as hysteresis.

  • This means that it's easier to breathe in than out due to the distinct paths the lung pressures take.

Pressure Changes During Strenuous Exercise

• In aerobic or strenuous exercise, the body moves beyond tidal volume limits:

  • Inspiratory limits: Pleural pressure can drop as low as $-35 \text{ cm } H_2O$ to $-40 \text{ cm } H_2O$ during maximal inspiration.
    

    • During heavy exercise, we take in even more air by significantly lowering the pressure to draw in more air.

  • Expiratory limits: To blow air out quickly, the body recruits expiratory muscles to forcefully compress the lung. This can raise pleural pressure to a positive value of approximately $+30 \text{ cm } H_2O$.
    

    • This increase in pressure allows for a fast release of air, which is crucial during intense activities.

• This positive pressure actively squeezes the lung to force air out of the alveoli much faster than passive recoil alone would allow.

  • Understanding these dynamics helps appreciate how our body adapts to increased demands during exercise, influencing our breathing and overall respiratory function.