Understanding Pulmonary Mechanics and Ventilation

Posted on Nov 4, 2024 in Physics

Key Terms

• Ventilation
• Respiration
• Pressure Gradient
• Tidal Volume
• Pulmonary Mechanics
• Ventilation/Perfusion
• Compliance
• Resistance
• Flow

Assessment of Ventilation

• Respiratory Pattern (Rate and Depth)
• Oxygen and carbon dioxide influence rate and depth of respiration. CO₂ is the primary stimulus.
• Accessory Muscle Use
• Prolonged Expiration
• Shortness of Breath
• Cyanosis
• Minute Ventilation
• ABG’s (Primarily CO₂)
• Pulse Oximetry
• Capnography
• Transcutaneous Monitoring

Ventilation

• Negative Pressure
  • Spontaneous Breathing
  • Negative Pressure Ventilators
  • Iron Lung
  • Cuirass
• Positive Pressure
  • Volume Ventilators
  • Pressure Ventilators
• High-Frequency Ventilation

Pulmonary Mechanics

• Movement of chest wall (thorax) and respiratory muscles
• Opposing forces keep lungs inflated (thorax moving out, lungs moving in)
• Diaphragm contracts/relaxes
• Inspiration
• Expiration

Review

Ventilation occurs due to a pressure gradient between the lungs and mouth. Contraction of the respiratory muscles results in a pressure-volume change in the lungs. As pressure decreases, air moves into the lungs during inspiration, and as the pressure increases, gas moves out of the lungs during expiration. The compliance characteristics of the pulmonary system influence the amount of pressure required to affect a volume change. The airway resistance characteristics also influence the effort needed to create a volume change.

Forces Opposing Inflation of the Lungs

Elastic Forces/Compliance

Compliance

• Distensibility of the lungs
• C_L = V (Liters) / P (cm H₂O)
• The higher the compliance, the easier to ventilate the lungs (lungs require less pressure to ventilate)
• The lower the compliance, the stiffer the lung is and the harder it is to ventilate (lungs require more pressure to ventilate)
• Normal total lung compliance (sum of the compliance of the lung tissue and thoracic cage) is 0.1 L/cm H₂O (100 mL/cm H₂O)

Compliance: Dynamic Compliance

• Is measured as air is flowing through the circuit and airways, therefore is actually a measurement of airway resistance (R_AW)
• Dynamic compliance changes with changes in R_AW caused by:
  1. Water in the ventilator tubing
  2. Bronchospasm
  3. Secretions
  4. Mucosal edema
• Is not an accurate measurement of how compliant the lungs are

Compliance: Static Compliance

• Is more accurate measurement of lung compliance
• It is measured with no air flowing through the circuit and airways
• Air flow may be stopped with the volume remaining in the lungs by adjusting a 1-2 sec. inspiratory hold or by pinching off the expiratory drive line after inspiration has begun.
• Once the flow has stopped, a plateau pressure occurs after peak pressure has been reached. The drop from peak to plateau pressure represents the pressure change from when gas flow is occurring to when it stops.

Compliance Calculations

• Dynamic C_L = Vt / (PIP – PEEP)
• Static C_L = Vt / (Plateau Pressure – PEEP)

Frictional (Nonelastic) Opposition to Ventilation

• Frictional forces also oppose ventilation.
• It occurs only when the system is in motion.
• Frictional opposition has the following two components: 1- Tissue Viscous Resistance 2- Airway Resistance.
• Tissue Viscous Resistance
• Tissue viscous resistance is the impedance of motion caused by displacement of tissues during ventilation.
• The tissues displaced include the lungs, rib cage, diaphragm, and abdominal organs.
• Tissue resistance accounts for only approximately 20% of the total resistance to lung inflation.

Frictional (Nonelastic) Opposition to Ventilation: Airway Resistance

• Airway Resistance
• Gas flow through the airways also causes frictional resistance.
• Airway resistance (R_AW) accounts for approximately 80% of the frictional resistance to ventilation.
• Airway resistance is the ratio of driving pressure (pressure difference between the alveoli and the airway opening) responsible for gas movement to the flow of the gas.

Factors Affecting Airway Resistance

• Two patterns characterize the flow of gas through the respiratory tract: laminar flow, turbulent flow- and tracheobronchial flow.
• Poiseuille’s law defines laminar flow through a smooth, unbranched tube of fixed dimension.
• ΔP = the pressure required to cause a specific flow of gas through a tube.
• To maintain ventilation in the presence of narrowing airways, large increases in driving pressure may be needed.
• Reducing the airway radius by half will decrease the flow sixteenfold at a constant pressure.

Rule of Thumb

A change in the caliber of an airway by a factor of 2 causes a sixteenfold change in resistance. This applies to human airways, as well as artificial airways (i.e. endotracheal and tracheostomy tubes). If the size of a patient’s airway is reduced from 2 mm to 1mm, airway resistance increases by a factor of 16. Similarly, if a 4.5 mm E.T. tube is replaced with a 9 mm tube, the pressure required to cause a flow of 1 L/sec through the tube will decrease sixteenfold. This rule has many practical consequences. It is the basis for bronchodilator therapy and for using the largest practical size of artificial airway.

Distribution of Airway Resistance

• Approximately 80% of the resistance to gas flow occurs in the nose, mouth, and the large airways.
• Branching of the tracheobronchial tree increases the cross-sectional area with each airway generation. As gas moves from the mouth to the alveoli, the combined cross-sectional area of the airways increases exponentially.
• According to the laws of fluid dynamics, this increase in cross-sectional area causes a decrease in gas velocity. The decrease in gas velocity promotes a laminar flow pattern, particularly in the smaller airways.

Mini-Clinic

• Determine Cs and Raw
• An intubated, 36-year-old woman is being ventilated with a volume of 0.5 L (500 ml). The PIP is 24 cm H₂O, P_plateau is 19 cm H₂O, and baseline pressure is 0. The inspiratory gas flow is constant at 60 L/min (1 L/sec).
• What are the static compliance and airway resistance?
• Are these normal values?

Mechanics of Exhalation

• Airway caliber is determined by several factors, which include anatomical support (cartilage in the wall of the airway and traction provided by surrounding tissues) and pressure differences (transpulmonary pressure gradient) across their walls.
• The difference between the pleural pressure and the pressure inside the airway is called the transmural pressure gradient.
• The transmural pressure gradient during normal quiet breathing is negative, even during exhalation. This negative transmural pressure gradient helps maintain the caliber of the small airways.
• During a forced exhalation, contraction of expiratory muscles can increase pleural pressure above atmospheric pressure. This reverses the transmural pressure gradient, making it positive.

Mechanics of Exhalation

• If positive transmural pressure gradient exceeds the supporting force provided by the lung parenchyma, the small airways may collapse.
• In healthy airways, this occurs only with forced exhalation and at low lung volumes. In diseased airways, it may occur with normal breathing.
• In pulmonary emphysema, the elastic tissue responsible for supporting the small airways is destroyed. Destruction of elastic tissue has multiple outcomes. It increases the compliance of the lung. Emphysema also obliterates the anatomical structures responsible for small airway support. Expiratory flow is limited by airway collapse during exhalation.

Work of Breathing

• Work of breathing is done by respiratory muscles
• Requires energy to overcome the elastic and frictional forces opposing inflation
• Mechanical Work of Breathing
  • Work done on the object is the product of the force exerted on the object times the distance it is moved.
• Metabolic Work of Breathing
• To perform work the respiratory muscles consume oxygen

Long Time Constants (Obstructive)

Short Time Constants (Restrictive)

Frequency Dependence of Compliance

• At increased breathing rates, units with long time constants fill less and empty more slowly than do units with normal compliance and resistance. Increasingly more inspired gas goes to lung units with relatively normal time constants. When more inspired volume goes to a smaller number of lung units, higher transpulmonary pressures must be generated to maintain alveolar ventilation. Compliance of the lung appears to decrease as breathing frequency increases.
• This phenomenon is called frequency dependence of compliance. Compliance measured during breathing is not static. It includes pressure changes created by resistance to airflow.

Efficiency of Ventilation

Even in healthy lungs, ventilation is not efficient. A substantial volume of inspired gas is wasted with each breath.

VE = Minute Ventilation

VE = V_T x RR

VA = Alveolar Ventilation

VA = (V_T – V_DS) x RR

V_DS = Dead Space (Anatomical V_DS + Alveolar V_DS) = Physiological V_DS

V_DS / V_T = Dead Space / Tidal Volume

• Represents the efficiency of ventilation
• Normal range 0.2 to 0.4 or 20% to 40%

Ventilatory Pattern, Dead Space, and VA

• The rate and depth of ventilation affect VA and the VD/VT ratio. VE is not a reliable indicator of VA.
• In A, B, and C V_DANAT and VE are identical. (VE is 8000 ml/min and V_DANAT is 150 ml in each instance). Figure B represents a normal VT and respiratory rate.
• Figure B, VD equals 16 multiplied by 150, which equals 2400 ml/min. The VA of 5600 ml/min is equal to VE minus VD (8000-2400=5600). For the following discussion, it is assumed that this VA maintains a normal PaCO₂ of 40 mmHg, representing neither hyperventilation nor hypoventilation.

Ventilatory Pattern, Dead Space, and VA

• Figure A illustrates the inefficiency of rapid (tachypnea), shallow (hypopnea) breathing. The lung still achieves a VE of 8000 ml/min. However, VD necessarily increases (VD = 32 x 150 = 4800 ml/min, compared with a VD of 2400 ml/min in Figure B). This leaves only 3200 ml/min for VA, compared with 5600 ml/min in B. The VD/VT increases also; in B it is 150/500, which equals 30%, and in A it is 150/250 = 60%.
• Thus 70% of the VE is involved in gas exchange in Figure B, whereas only 40% is similarly involved in Figure A. Rapid shallow breathing is a common signal of respiratory distress and possible ventilatory failure.

Ventilatory Pattern, Dead Space, and VA

• Figure C illustrates slow (bradypnea), deep (hyperpnea) breathing, which also achieves a VE of 8000 ml/min. Because V_DANAT is constant, all the additional VT enters alveoli, increasing VA. VD is only 8 x 150, which equals 1200 ml/min, leaving 6800 ml/min for VA.
• The VD/VT is 150/1000 or equal to 15%, meaning 85% of the VE participates in gas exchange. Slow, deep breathing is thus the most efficient ventilatory pattern in terms of the fraction of VE received by alveoli.

Effectiveness of Ventilation

• Ventilation is effective when it removes CO₂ at a rate that maintains a normal pH.
• The balance between CO₂ and VA determines the PaCO₂ in the lungs and arterial blood.
• If alveolar ventilation increases, the CO₂ will be less than its rate of removal. PaCO₂ will fall below its normal value of 40 mmHg and pH will rise. (Hyperventilation)
• If VA falls, CO₂ will exceed its rate of removal. The PaCO₂ will rise above its normal value of 40 mmHg, and the arterial pH level will fall. (Hypoventilation)