Pulmonary Mechanics and Gas Exchange

Pulmonary Mechanics

• Tidal volume refers to the air moving in or out during breathing.
• Vital capacity relates to the amount of air exhaled after maximal inhalation.
• Lung compliance worsens as pressures rise, requiring the ventilator to apply more pressure per breath.

Alveolar-Capillary Interface

• This is where gas exchange occurs between alveoli and capillaries.
• Unoxygenated blood flows from the right heart to the lungs, then back to the left atrium.
• PaO2 measures the partial pressure of oxygen in venous blood.
• PAO2 measures the partial pressure of oxygen in the alveoli where gas exchange is occurring.

Pulmonary Gas Diffusion Factors

• Lung compliance on ventilation is determined by the ease of expansion.
• Lung compliance is sensitive to conditions affecting lung tissues.
• Decreased lung compliance can result from surfactant deficiency.
• Surfactant helps alveoli open during inspiration and prevents collapse during expiration.
• Decreased surfactant increases the work of breathing and causes decreased tidal volume.
• Surfactant can be affected by premature birth, aging, smoking, ARDS, and acute lung injury.
• Aging impacts ventilation by flattening the diaphragm, stiffening the chest wall, weakening respiratory muscles, and increasing the anterior-posterior diameter.
• These age-related changes decrease lung compliance, alter pulmonary mechanics, and cause air trapping.

Factors Affecting Gas Exchange

• Lung surface area significantly impacts gas exchange.
• A greater alveolar-capillary membrane surface area allows for better gas exchange.
• Emphysema reduces the alveolar capillary membrane, decreasing surface area.
• Thinner membranes facilitate faster diffusion.
• Conditions that thicken the membrane include fluid in alveoli, interstitial spaces like pulmonary edema from HF or pneumonia, inflammatory processes, and lung fibrosis.
• Diffusion of O2 and CO2 needs 0.25 seconds to reach equilibrium.
• Blood flows through the alveolar capillary system in 0.75 seconds at rest.
• Increased CO decreases exposure time, resulting in hypoxemia.
• Sepsis induces elevated CO states.
• Aging reduces total lung surface area and increases alveolar-capillary membrane thickness.
• Destroyed alveoli, decreased diffusion, altered ventilation-to-perfusion relationship, and less efficient gas exchange also results from aging.

Pulmonary Perfusion

• Perfusion is the process of pumping blood to tissues and organs
• It is split into the systemic and pulmonary systems
• Oxygenation of body tissues needs sufficient perfusion in both systems
• The systemic system originates from the aorta and extends to the right side of the heart.
• The pulmonary system involves the pulmonary artery, from the right ventricle to the lungs and then left ventricle, which relies on adequate perfusion in the systemic system
• CO (4-8 L/min) = HR x SV
• The SV relates to ventricular preload, afterload, and contractility.
• Adequate perfusion relies on MAP (60 -100,
• Greatest gas exchange occurs in the bases, where gravity pulls blood towards.
• Normal ventilation-to-perfusion ratio (V/Q) is 4:5 (0.8).
• Greatest amount of gas exchange occurs in bases.
• Heart failure slows perfusion.
• Respiratory failure impairs ventilation.
• Alterations in ventilation-perfusion ratio from diseases can cause unoxygenated blood to return to the left side of the heart, leading to hypoxemia.

Shunt Considerations

• Shunt is the percentage of cardiac output that flows from the right heart to the left heart without undergoing pulmonary gas exchange, a main cause of hypoxemia in high-acuity patients.
• True shunts can be anatomical or capillary.
• Anatomic shunts involve blood moving from the right heart to the left heart without contacting the alveoli and is a result of bronchial emptying, and abnormal blood flow from heart or lung problems
• Capillary shunts involve normal blood flow past completely unventilated alveoli because of consolidation, atelectasis or fluid.
• Shuntlike effects occurs if ventilation is reduced, but not absent with alveoli still functioning, results in alveoli can be affected by O2 therapy
• Shunting 15%+ CO equates to severe respiratory failure, and 30% or great usually constitutes ARDS.
• Hallmark of ARDS is Refractory hypoxemia.
• With venous admixture, oxygenated blood mixes with partially oxygenated blood, and this creates a new pressure balance.
• Resistance to blood flow in the pulmonary vascular system signifies right ventricular afterload.
• PVR is affected by vessel length, radius, and blood viscosity.

Arterial Blood Gases

• PaO2 measures partial pressure of oxygen in arterial blood.
• Normal PaO2 is 80-100 mm Hg.
• SaO2 indicates the percentage of O2 bound to hemoglobin.
• Normal values are above 95%.
• Normal hemoglobin levels are 12-15 g/dL for women and 13.5-16 g/dL for men.
• ABGs provide information on acid-base balance and oxygenation status.
• Normal ABG occurs at sea level with 21% oxygen, and normal blood temperature.
• At ages 30-80 there is a 25-30% decrease in PaO2.
• Mild hypoxemia: PaO2 60-75 mm Hg
• Moderate: PaO2 45-59 mm Hg
• Severe: PaO2 less than 45 mm Hg
• Normal pH range: 7.35-7.45
• Normal PaCO2 range: 35-45 mm Hg
• Normal HCO3 range: 24-28 mEq/L
• Normal BE range: +/- 2 mEq/L

Capnography

• Capnography, also known as end-tidal CO2 monitoring, is a noninvasive graphic display of CO2 concentration exhaled by the patient during breathing, which is displayed by a capnogram.
• Capnography can monitor ventilation, post-op recovery and PCA's, ED's or ICU's.
• ETCO2 monitoring assesses ventilation with normal range between 30-44.
• Abnormally low ETCO2 levels can relate to hyperventilation, hypothermia, PE, or decreased CO
• Elevated ETCO2 relates to increased CO2 production or hypoventilation.
• Inaccurate results can stem from morbid obesity, severe pulmonary edema, and ventilation-perfusion abnormalities or from a mis-positioned ET tube as capnography is not precise for ET placement.
• Colorimetric capnography uses pH-sensitive paper that changes color based on exhaled CO2 levels.
• Minute ventilation is the total volume of expired air in one minute and calculated: VE = TV x frequency. with a normal range of 5-10 L/min
• Respiration takes place on a cellular level, involving the exchange of Co2 and O2 across a semipermeable membrane.
• Ventilation provides air in and out of the lungs to maintain ABG's.