Pulmonary Mechanics and Gas Diffusion

Pulmonary Mechanics

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

Alveolar-Capillary Interface

• The alveolar-capillary interface is where gas exchange occurs.
• Unoxygenated blood flows from the right heart to the lungs, then to the left atrium.
• PaO2 is the partial pressure of oxygen in the venous side.
• PAO2 is the partial pressure of oxygen in the alveoli where gas exchange is occurring.

Factors Affecting Pulmonary Gas Diffusion

• Lung compliance on ventilation is related to ease of expansion and sensitivity to conditions affecting lung tissues.
• Decreased compliance with surfactant deficiency occurs when surfactant helps alveoli open during inspiration and can cause collapse during expiration.
  • Decreased compliance increases the work of breathing
  • Decreased compliance causes decreased tidal volume.
• Factors affecting surfactant include premature birth, aging, smoking, acute lung injury, and adult respiratory distress disorder.
• Aging on ventilation includes diaphragm flattening, chest wall rigidity, respiratory muscle weakening, and increased anterior-posterior diameter, which leads to:
  • Decreased lung compliance
  • Altered pulmonary mechanics
  • Air trapping which decreases gas exchange

Affecting Gas Exchange

• A large lung surface area increases gas exchange.
• Emphysema destroys alveolar capillary membranes, surface area is reduced.
• Thinner membranes lead to more rapid diffusion.
• Conditions that thicken the membrane include fluid in alveoli and/or interstitial spaces.
  • Pulmonary edema from heart failure and pneumonia
  • Inflammatory processes involving alveoli
  • Fibrosis of lungs
• Diffusion of O2 and CO2 takes 0.25 seconds to reach equilibrium.
• Blood flows in 0.75 seconds through the alveolar capillary system at rest.
• Increased CO concentrations decrease exposure time, leading to hypoxemia.
  • Indicates high CO states like sepsis
• Aging decreases total lung surface area
• Aging increases alveolar-capillary membrane thickness.
• Alveoli destroyed can also decrease diffusion or alter ventilation-perfusion relationships to make gas exchange less efficient

Pulmonary Perfusion

• Perfusion is the pumping of blood to tissues and organs.

• The two circulatory systems include systemic and pulmonary systems.

• Systemic circulation is a vast system from the aorta to the right side of the heart.

• Pulmonary circulation is a smaller system moving blood from the pulmonary artery in the right ventricle to the lungs and then to the left ventricle.

• Adequate perfusion depends on systemic circulation.

• CO, which is between 4-8 L/min, is calculated as HR x SV.

• SV is the function of ventricular preload, afterload, and contractility. -A MAP between 60-100, and less than 60 indicates MODS

• Blood distribution relies on gravity, with the greatest gas exchange occurring in the bases.

• Ventilation is 4 L/min of air flow in and out of alveoli

• Perfusion is 5 L/min of blood flow in and out of alveoli.

• The normal ventilation/perfusion ratio (V/Q) is 4:5 (0.8).

• Heart failure slows perfusion and respiratory failure impairs ventilation.

• Altered ventilation-perfusion ratios can cause unoxygenated blood to return to the left side of the heart, resulting in hypoxemia.

Shunt

• A shunt is a percentage of CO that flows from the right heart to the left heart without undergoing pulmonary gas exchange.
• This is a major cause of hypoxemia primarily in acutely ill patients.
• There are 2 True shunts which include Anatomic shunt and Capillary shunt
• Anatomic shunts involve blood moving from the right heart to the left heart without coming into contact with the alveoli. -Normal result when emptying bronchial or venous system.
  • Abnormal when there are heart or lung problems, ventricular septal defects, traumatic injuries, or lung tumors
• Capillary shunts are normal blood flow past completely unventilated alveoli. -Conditions include consolidation, atelectasis, and fluid in alveoli Other Shunts include Absolute which are combined Anatomic + capillary shunts
• Lung tissue affected, which is refracted by O2 therapy
• No matter how much O2 is administered, diffusion cannot take place if alveoli are bypassed or non-functioning.

Shuntlike Effect

• Shuntlike effect is not a true shunt because the shunting is not complete.
• Alveolar ventilation is reduced, but, although some alveoli are still functioning, hypoxemia secondary to this effect is responsive to O2 therapy.
• Common causes include bronchospasm, hypoventilation, and pooling of secretions.
• Venous admixture is the effect a pulmonary shunt has on the contents of the blood as it enters the left heart and system as arterial blood.
  • Mixture of oxygenated and partially oxygenated blood
  • Normal > Venous admixture > shunt

Pulmonary Vascular Resistance (PVR)

• PVR is the resistance to blood flow in the pulmonary vascular system
• Represents the right ventricular afterload.
• Factors affecting PVR include the length, radius and viscosity of the vessel.
  • PVR is much lower SVR is shorter

Arterial Blood Gases

• Arterial blood gases interpreted with compensatory status
• PaO2 is the partial pressure of oxygen dissolved in arterial blood, not what is available.
  • Normal range: 80-100 mm Hg.

SaO2 and SpO2

• SO2 is the percentage of O2 bound to hemoglobin in blood.
• SaO2 is arterial blood oxygen saturation.
• SpO2 is oxygen saturation with pulse oximetry.
• Normal range: 95%+.
• Hemoglobin (Hgb/Hb) is a major component of RBCs.
  • Normal for women: 12-15 g/dL
  • Normal for men: 13.5-16 g/dL

Arterial Blood Gas Normals

• ABGs provide information on acid-base and oxygenation status.
• Normal conditions include sea level (760 mm Hg partial pressures), room air (21% oxygen), and blood temperature of 37°C/98.6°F.
• Age can affect ABG readings.
  • 30-80 year olds may have a 25-30% decrease in PaO2.

Interpretation

• Interpret ABGs as trends, not single points.
• Interpretation includes determination of acid-base state, level of compensation, and oxygenation status.
  • Alveolar ventilation
  • Amount of oxygen available in arterial blood for possible tissue use.
  • Oxygen-carrying capacity ​- Oxygen transport

Hypoxemia

• Mild hypoxemia has a PaO2 of 60-75 mm Hg.
• Moderate hypoxemia has a PaO2 of 45-59 mm Hg.
• Severe hypoxemia has a PaO2 of less than 45 mm Hg.
• Normal pH ranges from 7.35-7.45.
• Normal PaCO2 ranges from 35-45 mm Hg.
• Normal HCO3 ranges from 24-28 mEq/L.
• Normal BE is +/- 2 mEq/L.
• Normal PaO2 ranges from 80-100 mm Hg.
• Normal SaO2 is 95%+.
• Hemoglobin measurements range from 13.5-18 g/dL for men and 12-15 g/dL for women.

Capnography

• Capnometry involves a numeric measurement of CO2.
• Capnography, also known as end-tidal CO2 monitoring, uses a non-invasive graphic display of CO2 concentration exhaled by the patient captured by a capnogram.
• Clinical uses include surgical and procedural anesthesia, post-op recovery, post-op PCA recipients, critical care units, and EDs.
• Capnography confirms endotracheal and enteric feeding tube placement.
  • Also helps in the detection of mechanical ventilator problems
• ETCO2 monitoring assesses ventilation status and it is an early warning sign for ventilation changes with acceptable range between 30-44 mm Hg:
  • Abnormally low ETCO2 (Less than 30 mm Hg) due to Hyperventilation , Hypothermia, PE, or Decreased CO;
  • Increased ETCO2 (Greater than 44 mm Hg) with Increased CO2 production from Fever or increased CO or Hypoventilation from Respiratory center depression or neuromuscular disease
• Capnography might not be accurate in 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.
  • Not precise for ET tube but more for ET tube placement. Process
• The CO2 detector is attached to an ET tube.
• The patient breathes six times.
• The device is ready to full expiration.
• It then responds with three color ranges
• Purple indicates ET tube absence in the trachea -Brown or Yellow indicate the ET tube placement in the Esophagus or low pulmonary blood flow or properly in the Trachea, respectively.