Lung Compliance I

• This session is the first of a two-part series on lung compliance, specifically pertaining to the content found in Chapter 21 of the Boron and Boulpaep medical physiology textbook.

• The primary objectives of this study guide include:

  • Understanding the concept and calculation of transpulmonary pressure: This involves knowing how the pressures inside the lungs and pleural space work together to affect breathing.

  • Understanding the pathology and mechanisms of a pneumothorax: A pneumothorax is an important medical condition where air enters the pleural space, causing the lung to collapse.

  • Defining lung compliance and identifying normal clinical values: Lung compliance refers to how easily the lungs can stretch and fill with air, and knowing normal values helps in diagnosing respiratory conditions.

  • Understanding why compliance changes in the context of respiratory disease: Diseases can alter how easily lungs stretch, which can be crucial for assessing a person's lung health.

Fundamental Forces in Pulmonary Physiology

• To facilitate breathing and the movement of air, two primary forces must be overcome:

  • Lung Compliance: Think of this as the lungs’ ability to stretch. Just like a rubber band, the lung tissue must be stretched out to allow breathing. If it's stiff, like an old rubber band, it requires more effort to breathe.

  • Airway Resistance: This is the resistance that air faces while moving through the tubes that lead to the lungs (trachea and bronchi). If there is too much resistance, it can be harder to breathe.

• Functional Analogy: The lungs act like rubber bands. In a healthy state, they have a certain level of distensibility, or flexibility. In disease states, lungs may become very stiff, like a rigid, thick rubber band, requiring significantly more work and effort to stretch and fill with air, leading to difficulty in breathing.

Transpulmonary Pressure ($P_{tp}$)

• Measurement Convention: Pressures in respiratory physiology are measured in centimeters of water ($cm H_2O$). This standard allows for easy comparison and calculation.

• Atmospheric Pressure ($P_{atm}$): By convention, the atmospheric pressure is defined as $0 cm H_2O$. - Note: This doesn't mean there’s no air pressure. For instance, in locations like Gainesville, the actual atmospheric pressure is about $1\text{ atmosphere}$ or $760 mmHg$, but for our calculations, we start from zero.

• Alveolar Pressure ($P_{alv}$): This is the pressure inside the individual air sacs (alveoli) in our lungs, crucial for the process of gas exchange.

• Pleural Space/Pleural Pressure ($P_{pl}$): The pleural space is the area surrounding the lungs. It acts like a sealed container. The space is airtight and separates the two lungs, which is important for their independent operation.

• Definition of Transpulmonary Pressure: Transpulmonary pressure is the pressure difference across the lung walls, calculated using the difference between alveolar pressure and pleural pressure. - Formula: $P_{tp} = P_{alv} - P_{pl}$ which tells us how much strength is required to keep the lungs expanded.

Pressures at the Start of a Normal Breath

• At the "pivot point" between the end of expiration (breathing out) and the beginning of inspiration (breathing in), air movement ceases momentarily when they are equal:

  • Alveolar Pressure ($P_{alv}$) = $0 cm H_2O$.

• Alveolar pressure equals zero at this moment because there's no difference pushing air in or out, so no flow occurs.

• However, the Pleural Pressure ($P_{pl}$) is subatmospheric (negative pressure), around $-5 cm H_2O$.

• Calculation of $P_{tp}$ at rest: - $P_{tp} = 0 cm H_2O - (-5 cm H_2O) = +5 cm H_2O$. This means there’s a positive pressure pushing the lungs away from collapsing.

• This resulting $+5 cm H_2O$ is the force pushing the lungs outward, counteracting the natural elastic tendency of the lungs to collapse.

The Vacuum Concept and the Role of the Diaphragm

• Vacuum Analogy: - When you blow into a straw in a glass of water, you create a positive pressure, making bubbles.

• When you suck on a straw, you create negative pressure (a vacuum), which pulls liquid upward. Breathing works similarly:

• Breathing requires this negative pressure to expand the lungs.

• Diaphragm Mechanics: - The diaphragm is a muscle that helps our lungs fill with air. When the diaphragm contracts, it moves downwards, increasing the size of the space around the lungs.

• Since this space is airtight, when the volume increases, the pressure decreases, allowing air to flow in (this is due to Boyle's Law, which states that volume and pressure are inversely related).

• Pleural pressure may drop from $-5 cm H_2O$ to $-10$, $-20$, $-30$, or even $-40 cm H_2O$ during deep or heavy inspiration.

• A more negative pleural pressure creates a stronger vacuum force, allowing the lungs to stretch all the way to Total Lung Capacity (TLC).

• Frame of Reference: The size of transpulmonary pressure and pleural pressure are often comparable (like $5$ and $5$), but with opposite signs. For example, positive transpulmonary pressure ($+5$) indicates the lung is pushing outward, while negative pleural pressure ($-5$) indicates the pleural space is pulling inward.

Pneumothorax

• Definition: A pneumothorax is the medical term for a collapsed lung, occurring when the pleural cavity is breached, such as from an accident.

• Pathophysiology:- When the pleural cavity is compromised, air rushes in from high pressure (the atmosphere) to low pressure (the pleural space), causing the lung to collapse.

• The pleural pressure will keep rising until it matches atmospheric pressure, leading to a loss of the transpulmonary pressure. - $P_{tp} = P_{alv} (0) - P_{pl} (0) = 0 cm H_2O$, meaning there’s no force keeping the lung inflated anymore.

• Without this positive distending pressure or negative vacuum force from the pleural space, the lung collapses due to its natural elasticity immediately.

• Unilateral Pneumothorax: This is a condition where only one lung collapses. Despite causing significant breathing difficulty, it’s usually not fatal because the other lung can still function.

• Clinical Treatment: Often, a chest tube is inserted at the injury site. This tube connects to a vacuum pump that extracts air from the pleural space, restoring the needed negative pressure and allowing the lung to reinflate.

Defining Lung Compliance ($C_L$)

• Definition: Compliance is a term used to describe how easily the lungs can stretch, or distend. Think of it as a measure of flexibility.

• Mathematical Formula: Compliance is calculated as the change in lung volume (V$) divided by the change in pressure ($$P$). - $C_L = \frac{\Delta V}{\Delta P}$. If the lungs stretch easily, compliance is higher.

• Normal Physiological Values:- A normal breath takes in about $0.5 liters$ (or $500 mL$) of air when someone inhales above the Functional Residual Capacity (FRC).

• The average drop in pleural pressure required for this inhalation is about $2.5 cm H_2O$ (like moving from $-5 cm H_2O$ to $-7.5 cm H_2O$).

• Normal Compliance Calculation: - $C_L = \frac{0.5 liters}{2.5 cm H_2O} = 0.2 L/cm H_2O$. Normal compliance values allow doctors to assess lung function effectively.

• Clinical Significance: - Doctors may measure lung compliance for patients, assessing how well lungs can expand. - Decreased Compliance: If compliance is low, it means the lungs are stiffer, and less air enters for a given pressure change, indicating possible lung disease (like pulmonary fibrosis). - Increased Compliance: Higher compliance means much more air enters with the same pressure change, often seen in conditions like emphysema, where lung tissue is damaged. Both very high and low compliance values can suggest lung diseases that may need further examination.

Summary of Key Points

• Air movement depends on pressure differences established between the atmosphere and the alveoli which are essential for effective breathing.

• The diaphragm plays a vital role in creating a negative pleural pressure: the foundation of inhalation and air movement into the lungs.

• Pneumothorax is a serious condition causing the loss of negative pleural pressure leading to lung collapse, necessitating quick medical Response.

• Compliance serves as an important clinical measurement ($0.2 L/cm H_2O$), reflecting how effectively the lungs can stretch, and changes in compliance can indicate specific respiratory disorders.