Pleural Pressure and Ventilation

Ventilation

• Ventilation is the process of moving gas (usually air) into and out of the lungs.
• It differs from respiration, which involves the physiological processes of oxygen use at the cellular level.
• The primary function of the lungs is to supply the body with oxygen and remove carbon dioxide.
• Ventilation is crucial for oxygenation.
• Without ventilation, there is no oxygenation.

Mechanics of Ventilation

• Ventilation is a cyclic process consisting of inspiration and expiration.
• Tidal volume (Vt) is the gas volume moved per phase during either inspiration or expiration.
• Respiratory muscles generate a pressure gradient enabling gas flow in and out of the lungs.
• Lung and thorax compliance and resistance affect ventilation.
• Healthy lungs have minimal inspiratory load and passive expiration.

Pressure Differences During Breathing

• Gases move due to pressure gradients.
• Thoracic expansion and contraction, along with the elastic properties of airways and alveoli in the chest wall, create these gradients.
• Transrespiratory pressure (PTR) is the difference between airway opening pressure and body surface pressure.
• Airway, lungs, and chest wall are components of PTR.
• This gradient causes gas flow in and out of the lungs.

Pressure Gradients (Pressure Differences)

• Gases or liquids move from areas of higher pressure to areas of lower pressure.
• This movement is driven by pressure gradients.
• In pulmonary physiology, pressure difference is called a pressure gradient.

Pressure Differences During Breathing (Cont.)

• Transairway pressure (PTAW) is the difference between airway opening pressure and alveolar pressure.
• Transalveolar pressure (PTA) is the difference between alveolar pressure and pleural pressure.
• Transchestwall pressure (PTCW) is the difference between pleural pressure and body surface pressure.
• Normal breathing maintains a negative transpulmonary pressure to keep the alveoli inflated.

Transpulmonary Pressure Difference (PTP)

• Transpulmonary pressure difference (PTP) maintains alveolar inflation.
• PTP is defined as the difference between alveolar pressure (PA) and pleural pressure (Ppl).

Transthoracic Pressure Difference (PTT)

• Transthoracic pressure difference (PTT) is the difference between alveolar pressure and body surface pressure.
• It drives gas flow into and out of alveoli during breathing.

Inspiration

• Inspiration begins when muscular effort expands the thorax.
• Thoracic expansion causes a decrease in pleural pressure (Ppl).
• Inspiration causes Ppl to decrease, which, in turn, increases PTP.
• The positive increase in PTP leads to the flow of air into the lungs.
• Alveolar filling slows as alveolar pressure approaches equilibrium with the atmosphere.

Expiration

• Inspiration is followed by expiration.
• During expiration, the muscles relax. The thorax and lungs recoil to their resting volume.
• This increased pleural pressure causes the alveolar pressure to increase, forcing air out of the lungs.
• Expiration is normally passive.

Balloon Model of Ventilation

• The balloon model demonstrates how changes in thoracic cavity dimensions affect lung pressure and gas flow during breathing.
• Inspiration is caused by the diaphragm's downward movement.
• Expiration is driven by the diaphragm's upward movement.

To Summarize Breathing

• Inspiration creates negative pleural pressure, pulling air into the lungs.
• Expiration causes positive pleural pressure and pushes air out as the lung recoils.

Work of Breathing

• Respiratory muscles perform work during both inhalation and forced exhalation.
• Pulmonary disease dramatically increases the work of breathing.
• Restrictive diseases increase work due to elastic tissue recoil.
• Obstructive diseases increase work due to increased airway resistance.

Pathology's Affect on WOB

• Different respiratory pathologies result in varying work of breathing patterns.

Metabolic Impact of Increased Work of Breathing

• Respiratory muscles' energy consumption reflects the workload.
• The rate of O2 consumption is used in diagnosing the effort of breathing.
• Increased work of breathing causes greater oxygen consumption.

Distribution of Ventilation

• In upright positions, ventilation and perfusion are best at the base, where air and blood flow match most effectively.
• Alveoli at the lung base expand more in an upright position than alveoli in the apexes (top).

Time Constants

• Time constants reflect how quickly lung regions inflate or deflate.
• Unequal time constants for differing lung regions have effects on the ventilator modes.

Efficiency of Ventilation

• Effective ventilation is necessary for oxygen uptake and carbon dioxide removal.
• Efficient ventilation minimizes gas waste.
• Anatomic and alveolar deadspace waste gas and reduce respiration efficiency.

Minute and Alveolar Ventilation

• Minute ventilation (VE) is a measure of total volume and rate of air movement in and out of the lungs.
• Normal minute ventilation is 5-10 liters per minute.
• Factors driving minute ventilation are metabolic rate and size.

Dead Space Ventilation

• Alveolar dead space refers to alveoli that get ventilated but are not perfused.
• Some alveoli cannot participate in gas exchange, due to blockage, diseases, or positioning.
• Total dead space is calculated by adding together anatomic, alveolar, and mechanical dead space.

Forces Opposing Lung Inflation

• Lungs' inward recoil and chest wall outward tendency oppose lung inflation.
• Elastic forces and frictional forces work against lung inflation.
• Elastic forces include the tissues and surface tension in the lungs.
• Frictional forces include resistance to gas flow.
• The force to stretch the lungs is directly related to the pressure and magnitude of the stretch.

Compliance

• Lung compliance is the measure of the lung's ability to stretch and expand.
• Lung pathologies, like emphysema, can increase compliance, so less pressure is needed for lung expansion.
• Lung pathologies like fibrosis may decrease compliance, so more pressure is needed for lung expansion.

Relationship Between Chest Wall and Lung

• Lungs and the chest wall pull in opposite directions during breathing.
• Opposing forces of chest and lung compliance determine normal resting lung volume.

Chest Wall

• Chest wall abnormalities such as severe kyphoscoliosis or ankylosing spondylitis can impact lung volume and breathing mechanics.

Frictional Resistance to Ventilation

• Frictional resistance to ventilation occurs when the system moves.
• Tissue viscosity generates resistance to the motion.
• Respiratory resistance occurs when tissues (like lungs, rib cage, diaphragm, abdominal organs) are displaced.
• Obesity or ascites can further increase resistance
• Airway resistance represents ~80% of the frictional resistance.
• Bronchospasm and airway size increase resistance.

Factors that affect resistance

• Laminar flow and turbulent flow affect inspiratory and expiratory pressure, the needed driving force.
• Resistance in nonventilated patients is measured during pulmonary function testing.

Airway Resistance

• Decreasing the radius of a tube requires a substantially increased pressure to maintain a constant gas flow.

Factors Affecting Ventilation Distribution

• Inspiration stretches surrounding lung tissue and widens transpulmonary pressure gradients.
• Increased lung volume decreases airway resistance.
• As lung volume decreases, airway diameters decrease, dramatically increasing airway resistance.
• Wheezing is mostly heard during expiration due to airway resistance.

Rule of Thumb

• Patients with emphysema can control pressure to decrease airway collapse.
• This occurs when patients exhale with pursed lips.
• Airway collapse can happen from poor support for lung structures.

This quiz covers the impact of positive pressure ventilation (PPV) on patient physiology, the mechanics of spontaneous breathing, and the role of pleural pressure in venous return. It also explores the physiological considerations when transitioning between PPV and spontaneous breathing, particularly in conditions like asthma.