RCP 100 Ch 11 Ventilation

• Lungs supply the body with oxygen and remove carbon dioxide
• Lungs must be adequately ventilated to perform their functions

Ventilation Basics

• Ventilation transfers gas in and out of the lungs
• Respiration is the use of oxygen at the cellular level
• Ventilation is generally regulated to suit the body's needs
• Impaired ventilation can increase the work needed to breathe

Mechanics of Ventilation

• Ventilation is cyclic, including inspiration and expiration
• Tidal volume is the amount of gas moved per phase and measured during inspiration or expiration
• Ventilation removes carbon dioxide and replenishes oxygen
• Respiratory muscles change pressure, allowing gas to flow in and out
• Respiratory muscles must overcome the load to produce ventilation
• Lung and thorax compliance and resistance impact ventilation
• Inspiratory loads are minimal with healthy lungs at rest, while expiration is passive

Pressure Differences

• Gas moves due to pressure gradients from thoracic expansion/contraction and elastic properties, due to airways, alveoli and the chest wall
• Transrespiratory pressure measures gradient between the airway opening and the body surface
• Transrespiratory pressure = Pressure measures at airway opening -pressure at body surface
• Transrespiratory pressure gradient causes gas flow in and out of the lungs
• Transairway pressure is gradients between the airway opening and alveoli of lungs
• Transalveolar pressure measures gradients between the alveoli and pleural space, and includes pressure measured in model alveolus and pressure in pleural space
• Transthoracic pressure difference PTT = PA-PBS, causes gas to flow into and out of alveoli during breathing
• Transchestwall pressure measures pressure between pleural space and body surface
• Transpulmonary pressure difference (airways and alveolar region) is needed to maintain alveolar inflation
• Pressure measurements derive mechanical properties of the pulmonary system

Inspiration

• Requires muscular effort to expand the thorax
• Thoracic expansion decreases pleural pressure, which induces airflow into lungs
• Inspiratory flow is proportional to the change in transairway pressure difference
• Higher change in transairway pressure means higher flow
• Pleural pressure decreases until the end of inspiration
• Alveolar filling slows as alveolar pressure equilibrates with the environment and inspiratory flow decreases

Expiration

• Thoracic recoil raises Pressure in the pleural space
• Transpulmonary pressure decreases, which is the opposite of inspiration
• Energy for expiratory flow from lung and chest wall elastances drives flow
• Pleural pressures remain subatmospheric during normal breathing
• Forceful inspiration can drop the pleural pressure as low as -50 cm H₂O

Inspiration and Expiration

• Both spontaneous breathing (SB) and positive pressure ventilation (PPV) result in inspiration
• Both SB and PPV increase the pressure that distends the lungs
• PPV can compress veins that carry blood to the heart, potentially impeding cardiac output
• Spontaneous breathing lowers pleural pressure further, increasing venous blood return to the heart

Inhalation Summary

• Expanding thoracic cavity decreases pleural pressure per Boyle's Law
• Reduced gas collisions from greater thoracic volume reduce pressure and causes air flow into lungs
• Transpulmonary pressure gradient maintains alveolar inflation, where Alveolar pressure falls until it equals the airway opening pressure

Balloon Model of Ventilation- Inspiration

• Atmospheric pressure is greater than pressure on inside of balloon
• Inspiration is caused by downward movements of a rubber sheet (diaphragm)

Balloon Model of Ventilation- End-Inspiration

• Equilibrium point, no gas flow, atmospheric pressure is equal to pressure inside the balloon

Balloon Model of Ventilation- Expiration

• Pressure inside balloon is greater than atmospheric pressure, caused by elastic recoil of diaphragm upward movement

Balloon Model of Ventilation- End-Expiration

• Atmospheric pressure is equal to pressure inside balloon, equilibrium point with no gas flow

Exhalation Summary

• Elasticity of diaphragm relaxes causing it to return to its dome shape, decreasing the vertical dimension of the thoracic cavity
• Lung recoils once inspiratory muscles relax to squeeze the alveolar gas volume
• This increases alveolar pressure to be higher than atmospheric pressure which will then cause air to flow from the higher pressure to the lower pressure
• Diaphragm relaxes, reducing the thoracic cavity's size and decreasing its volume

Forces Opposing Lung Inflation

• Lungs tend to recoil inward, the chest wall outwards
• Opposing forces maintain resting lung volumes (FRC, or functional residual capacity )
• Opposition to lung inflation includes elastic forces of tissues and surface tension in the alveoli
• Frictional forces relate to resistance from gas flow through airways, natural or artificial plus tissues during breathing
• Elastic opposition to ventilation and collagen fibers resist lung stretch
• Air pressure into lungs causes stretch
• Greater pressure causes greater stretch until the limit is reached
• Deflation is passive, requiring less force than inhalation

Hysteresis

• Hysteresis measures the difference between inflation and deflation curves
• Elastic opposition during deflation results in slightly higher lung volume at any given pressure than during inflation

Surface Tension

• Surface tension resists lung inflation, partly causing hysteresis
• Hysteresis can reveal issues not related to normal elastic tissue forces
• Lung recoil relies on the tissue elasticity and surface tension
• Pulmonary surfactant, produced in alveolar type II pneumocytes reduces lung surface tension and stabilizes alveoli against collapse
• Pulmonary surfactant lowers the surface tension if the surface area decreases

Hooke's Law

• Elastance is the ability of matter to respond to force and return to its original shape
• Elastance is defined as the measure of change in pressure per change in volume in pulmonary physiology
• Elastance = Change in Pressure / Change in Volume
• Volume varies directly with pressure until an elastic limit
• Hazards like pneumothorax can occur under increased mechanical ventilation pressure

Lung Compliance

• Compliance is how easily to stretch and expand the lung
• C₁ is the change in volume (ΔV) per unit of change in pressure (ΔΡ) difference
• Normal 0.2 L/cm H₂O
• Emphysema increases compliance due to loss of lung elastic tissue
• Fibrosis decreases lung compliance because of increased connective tissue making them stiff
• Lung compliance = ΔV L/ΔΡ cm H₂O

Lung Volume

• In restrictive lung disease, there are smaller changes in volume because there are stiffer lungs
• If compliance increases in emphysema, small pressure changes can lead to large changes in volume

Lungs and Chest Wall

• Airways recoil in opposite directions resulting in ~0.1 L/cm H₂O in the system compliance of ~0.1 L/cm H₂O
• The resting lung is the functional residual capacity (FRC) where the chest wall expands as much as the lungs collapse
• FRC occurs at ~40% TLC (total lung capacity)

Frictional Resistance

• Tissue displacement, obesity, fibrosis, and ascites, can increase impedance
• Tissue viscous resistance, and Impedance with lung rib cage/diaphragm make up approximately 20% of total resistance
• Airway resistance makes up ~80% of the frictional resistance
• Airway resistance gas trying to flow into airways but is being impeded
• Gas flow results in frictional resistance
• Airway radius exponential effect (r4) on resistance
• Artificial airway size or bronchospasm are components of airway resistance

Resistance

• Laminar flow requires less driving pressure than turbulent flow
• Driving pressure is the difference between inspiratory and expiratory pressure,
• Smaller endotracheal tubes can increase resistance
• Inspiration opens airways, thus more airflow, while volume decreases toward residual volume
• Wheezing occurs mainly during exhalation

Work of Breathing

• Respiratory muscles perform Inhalation and forced exhalation
• Pulmonary disease increases WOB dramatically via restrictive disease affecting recoil and obstructive due to greater resistance Raw

Oxygen Consumption

• Respiratory muscles consume O₂
• The rate of O₂ consumption V02 reflects energy needs, and correlates with WOB
• O2 cost of breathing (OCB) is indirect measurement of OWB
• In shock, intubation and ventilation can reduce O₂ consumption
• Intubation and ventilation preserves O₂ delivery for vital organs

Ventilation-Perfusion

• In an upright lung ventilation and perfusion are matched at the bases
• Ventilation is air movement in and out of lungs
• Perfusion is blood circulation through lung issue
• In healthy lungs, ventilation and perfusion are evenly distributed.
• Result uneven ventilation to perfusion ratio
• / ratio of 0.8
• Alveoli have different ventilation
• Apical alveoli have differential ventilation
• Apical alveoli have smaller perfusion
• In local disease, the "good lung" should be placed down for better /matching

Time Constants

• Long time constant (TC) means increased C₁ and Raw
• Short time constant (TC) means decreased C₁ and Raw
• Factors Varying Transpulmonary pressure gradients, Alveoli's size in Apexes
• Factors Alveoli at bases expand 4X as much than at Apexes
• 60% of total inflation