Ventilation Mechanics: Pressure Differences in Lungs

Ventilation Mechanics: Pressure Differences Across the Lungs

• Pressure gradient is the pressure change responsible for moving air in and out of the lungs, with gas always flowing from high to low pressure.
• Gas flow ceases when the pressure gradient is zero.
• Includes transairway, transpulmonary, and transthoracic pressures.

Driving Pressure

• Driving pressure is the pressure difference between two points in a tube or vessel.
• Represents the force moving gas through the tube or vessel.

Transairway Pressure (Pta)

• Pta is the pressure difference between the mouth pressure (Pm) and the alveolar pressure (Palv).
• Pta = Pm – Palv
• Pta causes airflow in and out of the conducting airways.
• Example: If Pm = 760 mmHg and Palv = 757 mmHg, then Pta = +3 mmHg.
• Pta represents the driving pressure that forces gas in and out of the lungs.

Transpulmonary Pressure (Ptp)

• Ptp = Palv – Ppl, where Ppl is pleural pressure.
• Example: if Palv = 760 mmHg and Ppl = 755 mmHg, then Ptp = 5 mmHg.
• In the normal lung, Palv is always greater than Ppl, maintaining the lungs in an inflated state.

Transthoracic Pressure (Ptt)

• Ptt = Palv – Pbs, where Pbs is body surface pressure.
• Example: if Palv = 757 mmHg and Pbs = 760 mmHg, then Ptt = -3 mmHg during inspiration.
• Ptt is responsible for expanding the lungs and chest wall in tandem.

Role of the Diaphragm

• A pressure gradient (ΔP) is generated by the contraction and relaxation of the diaphragm.
• Inspiration: Contraction leads to low Ppl and Palv.
• End inspiration: Equilibrium is reached with no ΔP.
• Expiration: Upward movement leads to low thoracic volume and high Ppl and Palv.
• End expiration: Palv = Pm.
• Ppl is always negative in normal breathing.

Elastic Properties

• The chest wall has a natural tendency to move outward or expand due to the bones and muscles of the thorax.
• The lungs have a natural tendency to move inward or collapse due to their elastic properties and surface tension.
• Elastic forces of the lungs are routinely evaluated by measuring lung compliance.

Lung Compliance (CL)

• Lung compliance (CL) describes how readily the elastic force of the lungs accepts a volume of inspired air.
• CL = ΔV (L) / ΔP (cmH2O), measured in L/cm H2O
• Indicates how much air in liters the lungs will accommodate for each cm H2O pressure change.

Lung Compliance (CL) Example

• If an individual generates a negative intra-pleural pressure change of 5 cm H2O during inspiration, and the lungs accept a new volume of 0.75 L of gas, then CL = 0.75 L / 5 cm H2O = 0.15 L/cm H2O.

Lung Compliance (CL) cont.

• At rest, average CL for each breath is ~ 0.1 L/cm H2O, meaning approximately 100 mL of air is delivered into the lungs per 1 cm H2O pressure change.
• When CL is increased, the lungs accept a greater volume of gas per unit of pressure change.
• When CL is decreased, the lungs accept a smaller volume of gas per unit of pressure change.
• CL progressively decreases as the alveoli approach their total filling capacity.

Clinical Implications for CL

• Pulmonary disorders that decrease lung compliance (e.g., pneumonia, atelectasis, or acute respiratory distress syndrome) decrease the patient’s lung and chest wall expansion.
• Pulmonary disorders that cause the lungs to break away from the chest wall (e.g., pneumothorax) can result in an overexpansion of the chest wall on the affected side.

Hooke’s Law

• Elastance is the natural tendency to respond to force and return to its original resting position after the external force is removed.
• Elastance = ΔP / ΔV.
• Elastance is reciprocal (opposite) to Compliance (Lungs with high compliance have low elastance).
• Hooke’s law: if 1 unit of force (P) is applied to an elastic body, it will stretch 1 unit of length (V).
• After normal lung functional range, applying more force can cause rupture.

Surface Tension (ST)

• Liquid molecules surrounded by liquid molecules are mutually attracted to each other, moving freely in all directions.
• In a liquid-gas interface, the liquid molecules at the surface are attracted to the liquid molecules within the liquid mass.
• This cohesive force is called surface tension, which maintains the shape of a water droplet.

Laplace’s Law

• The distending pressure of a liquid bubble is:
  • Directly proportional to surface tension.
  • Inversely proportional to the size of the bubble.
• P = 4ST/r, where P is pressure, ST is surface tension, and r is the radius.
• It takes more pressure to hold a bubble open if ST increases or r decreases.
• When two different size bubbles with the same ST are in direct communication, the greater pressure in the smaller bubble will cause it to empty into the larger bubble.

Critical Opening Pressure

• Laplace’s law does not apply until critical opening pressure is reached.
• At first, a High P with little V change is observed, similar to the initial pressure required to blow up a new balloon.
• Once critical opening pressure is reached, distending pressure progressively decreases as the bubble increases its radius.

Pulmonary Surfactant

• High Ptp must be generated to keep the small alveoli open, which is offset by the pulmonary surfactant.
• Produced by type II cells.
• Decreases surface tension in proportion to the ratio of surfactant to alveolar surface area.
  • Large alveolus = high ST
  • Small alveolus = low ST

Dynamic Characteristics of the Lung

• Study of forces in action.
• Focuses on the movement of gas in and out of the lungs and pressure changes required to move the gas.
• Explained by Poiseuille’s law and the airway resistance equation.

Poiseuille’s Law

• During inspiration:
  • Ppl decreases.
  • Bronchial airways lengthen and increase in diameter (passive dilation).
• During exhalation: Opposite phenomenon occurs.
• In abnormal situations, bronchial gas flow and Ppl may change, affecting gas flow.

Poiseuille’s Law for Flow

• °V= ΔPr⁴π /8lη, where:
  • °V = gas flowing through the tube,
  • η (eta) = viscosity of gas or fluid,
  • ΔP = change in pressure,
  • l = length of the tube. π/8 = constants
• Flow will increase if pressure and radius increase and decrease if viscosity and length increase.
• If pressure remains constant and r is decreased by half, flow decreases to 1/16th of initial flow because halving the radius of an airway would cause resistance to increase 16-fold.

Poiseuille’s Law for Pressure

• P = °V 8lη /r⁴π
• Pressure is directly proportional to flow, length, and viscosity of the gas.
• Pressure decreases in response to increased radius.
• Combined: P = °V/r⁴ and °V=Pr⁴

Airway Resistance (Raw)

• Raw = Pressure difference between the mouth and the alveoli (Pta) divided by flow rate.
• Represents the resistance to gas flow created by the airways.
• Raw = P (cmH2O) / °V (L/sec)
• Includes laminar flow and turbulent flow.

Types of Flow

• Laminar Flow:
  • Streamlined.
  • Gas molecules move parallel to the sides of the tube.
  • Occurs at low flow rates and low-pressure gradients.
• Turbulent Flow:
  • Random movement of gas.
  • Gas flow encounters resistance from both sides of the tube and collides with itself.
  • Occurs at high flow rates and high-pressure gradients.
• Obstructions in the airways (e.g., in asthma or COPD) produce turbulence.
• High flow speeds also cause turbulence.
• Turbulence leads to a need for a much greater difference in pressure to move the air.
• In physiological terms, Pta (pressure difference between the outside air and within the lungs) would need to be increased, so the intercostal muscles and diaphragm would need to work harder to expand the lungs.

Airway Resistance (Raw) cont.

• Opposition to the flow of gases through the airways.
• For laminar flow, resistance is quite low.
• Factors affecting Raw:
  • Radius (diameter)
  • Airway length
  • Flow rate

Factors Affecting Airway Diameter

• Mechanical obstruction or compression:
  • Extrinsic, e.g., by tumor
  • Dynamic compression, e.g., due to gas trapping
  • Artificial airways complications, e.g., kinked endotracheal tube
• Decreased internal cross-section:
  • Edema
  • Mucosal or smooth muscle hypertrophy
  • Encrusted secretions
• Increased smooth muscle tone:
  • Bronchospasm
  • Irritants, e.g., histamine

Time Constants

• Time in seconds necessary to inflate a lung region to 60% of its potential filling capacity.
• Lung regions with high Raw and high CL require more time to inflate (Long Time Constant).
• Equals the product of Raw and CL
• TC (sec) = Raw x CL

Dynamic Compliance

• Defined as the change in the volume of the lungs divided by the change in trans-pulmonary pressure (Ptp).
• Static lung compliance (CL stat) is determined during a period of no gas flow, whereas dynamic compliance (CL dyn) is measured during a period of gas flow.
• In the healthy lung, the CL dyn is about equal to CL stat.
• In the obstructed airways, CL dyn / CL stat falls at high respiratory rates (frequency-dependent alveoli). - This is because alveoli distal to the obstruction do not have enough time to fill to their potential filling capacity as the breathing frequency increases.

Alveolar Ventilation vs. Dead-Space Ventilation

• Alveolar ventilation: Inspired air that reaches the alveoli for gas exchange.
• Dead-space ventilation: Inspired gas that does not reach the alveoli.
• Dead-space is divided into:
  • Physiologic dead-space
  • Alveolar dead-space
  • Anatomic dead-space

Dead-Space Ventilation

• Alveolar Dead-space:
  • Alveolus is ventilated but not perfused with pulmonary blood.

Anatomic Dead-Space

• Volume of gas in the conducting airways.
• Normal: ~150 mL.
• VA = (VT - VD) x breaths/min, where VA is alveolar ventilation, VT is tidal volume, and VD is dead space.
• Example: If VT = 450 mL, VD = 150 mL, RR = 12 bpm, then VA = (450 – 150) x 12 = 3600 mL (3.6 L).
• Breathing pattern (rate & depth) can alter total alveolar ventilation.

Effects of Raw and CL on Ventilatory Patterns

• Normal: RR 15 bpm / VT 500mL.
• Decreased Compliance (↓ CL):
  • Decreased Tidal Volume (↓ VT)
  • Increased Respiratory Rate (↑ RR)
• Increased Airway Resistance (↑ Raw):
  • Increased Tidal Volume (↑ VT)
  • Decreased Respiratory Rate (↓ RR)
• The change in ventilatory pattern is thought to be based on minimum work requirements rather than ventilatory efficiency.

Normal Ventilatory Pattern

• Consists of:
  • Tidal Volume (VT)
  • Ventilatory Rate (RR)
  • Inspiratory to Expiratory Ratio (I:E)
• VT is the volume of air that moves in and out of the lungs in one quiet breath.
  • VT = 7-9 mL/kg.
• RR = ~ 15 bpm (12-20).
• I:E = 1:2. (1:ins) + (1:exp, 1:pause).

Specific Ventilatory Patterns

• Apnea: Complete absence of spontaneous ventilation
• Eupnea: Normal, spontaneous breathing.
• Biot’s breathing: Rapid, deep inspirations followed by 10-30 seconds of apnea (observed in meningitis).
• Hyperpnea: Increased depth of breathing with or without increased RR.
• Hyperventilation: Increased VA (↑ RR or ↑ VT).
• Hypoventilation: Decreased VA (↓ RR or ↓ VT).
• Tachypnea: Increased RR.
• Cheyne-Stokes breathing: Gradual increase in volume and rate followed by a gradual decrease in volume and rate until a period of apnea of 10-30 seconds (e.g., pulmonary edema and cerebral disorders).
• Kussmaul breathing: Increased depth and rate of breathing (observed in diabetic ketoacidosis).
• Orthopnea: The individual is able to breathe most comfortably in an upright position.
• Dyspnea: Difficulty in breathing.