Pulmonary Mechanics and Gas Exchange

Pulmonary Mechanics

• Tidal volume refers to the air moving in or out during breathing.
• Vital capacity measures the amount of air exhaled after maximal inhalation.
• Lung compliance decreases as pressures rise and lung function worsens, sometimes requiring increased ventilator pressure per breath.

Alveolar-Capillary Gas Exchange

• Unoxygenated blood flows from the right heart to the lungs, then to the left atrium.
• Alveoli are represented by grey bulbs in diagrams of the alveolar-capillary interface.
• PaO2 indicates the partial pressure of oxygen in the venous side.
• PAO2 indicates the partial pressure of oxygen in the alveoli where gas exchange occurs.

Pulmonary Gas Diffusion Factors

• Lung compliance on ventilation depends on the ease of lung expansion.
• Lung compliance on ventilation is also affected by conditions impacting lung tissues.
• Surfactant helps alveoli remain open at the end of inspiration and prevents collapse at expiration.
• Surfactant deficiency increases the work of breathing and decreases tidal volume.
• Factors that affect surfactant include premature birth, aging, smoking, acute lung injury and adult respiratory distress disorder.
• Aging impacts ventilation by flattening the diaphragm, stiffening the chest wall, weakening respiratory muscles, and increasing the anterior-posterior diameter.
• Aging leads to decreased lung compliance, altered pulmonary mechanics, and air trapping which decreases gas exchange.

Affecting Gas Exchange

• The lung surface area is very large which is important for gas exchange.
• A greater alveolar-capillary membrane surface increases the area for gas exchange.
• Emphysema reduces surface area by destroying the alveolar capillary membrane.
• Thinner membranes facilitate more rapid diffusion.
• Conditions that thicken the membrane include fluid in alveoli and/or interstitial spaces (pulmonary edema from HF, pneumonia), inflammatory processes in alveoli and fibrosis of lungs.
• Diffusion of oxygen and carbon dioxide requires 0.25 seconds to reach equilibrium.
• During rest, blood flows through the alveolar-capillary system in 0.75 seconds.
• Increased carbon monoxide (CO) decreases exposure time, leading to hypoxemia.
• High CO states include sepsis.
• Aging decreases total lung surface area and increases alveolar-capillary membrane thickness.
• Aging can destroy alveoli, decrease diffusion, alter ventilation-to-perfusion relationships, and reduce the efficiency of gas exchange.

Pulmonary Perfusion

• Perfusion is the process of pumping blood to tissues and organs.
• Circulation is divided into the systemic and pulmonary systems.
• Oxygenation of all tissues depends on adequate perfusion in both systems.
• The systemic system is vast, moving from the aorta to the right side of the heart.
• The pulmonary system is smaller, moving from the pulmonary artery in the right ventricle, to the lungs, and then to the left ventricle.
• Adequate perfusion in the systemic system ensures good function of the pulmonary system.
• Cardiac output (CO) is calculated as heart rate (HR) multiplied by stroke volume (SV).
• Stroke volume is a function of ventricular preload, afterload, and contractility.
• Mean arterial pressure (MAP) should be between 60 and 100; less than 60 indicates MODS.
• MAP commonly reflects the adequacy of perfusion.
• Blood flow is gravity dependent, with the greatest gas exchange occurring in the bases.
• Gravity pulls blood toward the bases of the lungs.
• Normal airflow into and out of the alveoli is 4 L/min.
• Normal blood flow in and out of the alveoli is 5 L/min.
• The normal ventilation/perfusion (V/Q) ratio is 4:5 (0.8).
• The greatest amount of gas exchange occurs in the lung bases.
• Congestive heart failure (HF) slows perfusion.
• Respiratory failure impairs ventilation.
• Diseases altering the ventilation-perfusion ratio can cause unoxygenated blood to return to the left side of the heart, resulting in hypoxemia.
• The effects of a pulmonary shunt include the lungs receiving blood without gas exchange occurring (major cause of hypoxemia in high acuity patients).
• Anatomic shunts involves blood moving from the right heart to the left heart without coming into contact with the alveoli.
• Anatomic shunts can be normal (emptying of bronchial and other veins into the lung's venous system), or abnormal (heart/ lung problems, ventricular septal defect, trauma, or lung tumors).
• Capillary shunts involves normal blood flow past completely unventilated alveoli (consolidation, atelectasis and fluid in alveoli), and typically develop in high acuity patients.
• In shuntlike effects lung tissue is affected and refracts oxygen therapy because normal diffusion cannot take place if the alveoli are bypassed or nonfunctioning.
• A shuntlike effect is not a true shunt because the shunting is not complete
• With a shuntlike effect, alveolar ventilation is reduced, but not absent and the alveoli are still functioning. Causes are bronchospasm, hypoventilation, and pooling of secretions. Hypoxemia secondary to shuntlike effects is very responsive to oxygen therapy.
• Venous admixture is the effect that a pulmonary shunt has on the contents of the blood as it enters the left heart and systemic arterial blood.
• In venous admixture, oxygenated blood mixes with partially oxygenated blood, creating a new pressure balance.
• Normal > Venous admixture > shunt

Pulmonary Vascular Resistance (PVR)

• PVR is a representation of right ventricular afterload.
• Factors affecting PVR are the length of the vessel (shorter length equals lower PVR), radius of the vessel, and viscosity of blood.

ABG Interpretation

• PaO2 is the partial pressure of oxygen dissolved in arterial blood, not the total amount of oxygen available.
• Normal PaO2 is 80-100 mm Hg.
• SaO2 represents the percentage of oxygen bound to hemoglobin in blood.
• SaO2 is arterial blood oxygen saturation.
• SpO2 is oxygen saturation measured with pulse oximetry.
• Normal SaO2/SpO2 is 95%+.
• Hemoglobin (Hgb/Hb) is a major component of red blood cells.
• Normal hemoglobin levels are: Women: 12-15 g/dL, Men: 13.5-16 g/dL.
• ABGs provide information on acid-base balance and oxygenation status.
• Normal ABG conditions include sea level partial pressures (760 mm Hg), room air (21% oxygen), and a body temperature of 37°C/98.6°F.
• Age can affect ABGs; those 30-80 years old may have a 25-30% decrease in PaO2.
• ABG interpretation includes determination of acid-base state, level of compensation, oxygenation status, alveolar ventilation, available oxygen in arterial blood for use, oxygen-carrying capacity, and oxygen transport.
• Mild hypoxemia: PaO2 60-75 mm Hg.
• Moderate hypoxemia: PaO2 45-59 mm Hg.
• Severe hypoxemia: PaO2 less than 45 mm Hg.
• Normal pH is 7.35-7.45.
• Normal PaCO2 is 35-45 mm Hg.
• Normal HCO3 is 24-28 mEq/L.
• Normal Base Excess (BE) is +/- 2 mEq/L.
• Hemoglobin normal ranges: Men 13.5-18 g/dL, Women 12-15 g/dL.
• Capnometry is the numeric measurement of carbon dioxide.

Capnography

• Capnography, also known as end-tidal CO2 monitoring, is a noninvasive graphic display of CO2 concentration exhaled by the patient during breathing.
• Capnography monitors adequacy of ventilation in surgical and procedural anesthesia, postop recovery, post op patients receiving PCA, in critical care units and in EDs.
• Capnography confirms endotracheal and enteric feeding tube placement and detects mechanical ventilator problems.
• ETCO2 monitoring assesses ventilation status and serves as an early warning sign for ventilation changes.
• Normal ETCO2 ranges from 30-44 mm Hg.
• Abnormally low ETCO2 (less than 30 mm Hg) can indicate hyperventilation, hypothermia, PE, or decreased CO.
• Increased ETCO2 (greater than 44 mm Hg) can indicate increased CO2 production (fever, increased CO) or hypoventilation (respiratory center depression, neuromuscular disease).
• Capnography is not accurate in patients with morbid obesity, severe pulmonary edema, or ventilation-perfusion abnormalities.
• Colorimetric capnography uses pH-sensitive paper that changes color based on exhaled CO2 to represent a range of ETCO2.
• Colorimetric capnography is not precise for ET tube placement.
• The process for colorimetric capnography is that a CO2 detector is attached to the ET tube, the patient breathes x6, and the device is read at full expiration which responds with 3 color ranges.
• A purple color indicates 0.03%-0.5% ETCO2 and that the ET tube is not in the trachea.
• A brown color indicates 0.5%-2% ETCO2, indicating that the ET tube is in the esophagus, or the patient has hypocarbia or low pulmonary blood flow.
• A yellow color indicates 2-5% ETCO2 and that the ET tube is properly placed in the trachea.