Respiratory Basics: Understanding Oxygenation, Ventilation, and Respiratory Complications
By Cheryl Palmer, RN - CVICU NOTES
What's the Difference: Oxygenation VS Ventilation
Oxygenation is the addition of oxygen to a system, in this case, the human body. On the other hand, Ventilation is the exchange of air between the lungs and the atmosphere so oxygen is absorbed, and carbon dioxide is eliminated.
Understanding the Normal Respiratory Process
The process of breathing starts with the contraction of the diaphragm, a key component of our respiratory anatomy. This contraction creates a vacuum in the lungs, drawing in air and filling the lungs with oxygen. The oxygen is then absorbed into the bloodstream and transported to cells throughout the body, where it is used to produce energy.
What Does Adequate Oxygenation Mean?
Adequate oxygenation refers to the sufficient delivery of oxygen to meet the cellular demands of the body. Some factors affecting adequate oxygenation include sufficient ventilation, effective oxygen (O2) transfer across the alveolar/capillary membrane, the amount of hemoglobin (Hgb) available to transport the oxygen, cardiac output, the release of O2 from Hgb to the tissue bed, and the cellular ability to utilize O2. For the body to be adequately oxygenated, the O2 Saturation should be more than 92%, and the PaO2 should be between 80-100 mmHg.
How Does Adequate Ventilation Occur?
Several factors stimulate adequate ventilation. These include sufficient movement of air to facilitate appropriate oxygen and carbon dioxide (CO2) concentrations, a Tidal Volume (Vt) of 6-8mL/kg, a respiratory rate of 12-20 breaths per minute, an End Tidal CO2 (ETCO2/PaCO2) of 35-45 mmHg, and the presence of adequate pH and PaCO2 levels. The diaphragm, the primary muscle of ventilation, is crucial in this process. Additionally, the concept of Dead Space, where no gas exchange occurs (anatomic vs pathologic), is also important in understanding ventilation. Ventilation can be calculated as MV=TVxRR (L/min).
What is Adequate Perfusion?
Adequate perfusion refers to sufficient blood movement through the capillary bed, past the alveoli. This is necessary for effective gas exchange. However, an imbalance between the amount of air getting into the alveoli and the amount of blood flow to the capillaries surrounding those alveoli can lead to reduced oxygen exchange in the lungs. This is known as V/Q Mismatch.
Understanding V/Q Mismatch
When discussing V/Q Mismatch, it is crucial to understand Ventilation (V) and Perfusion (Q). Ventilation refers to the process of air getting into the alveoli, with inadequate ventilation indicating that not enough air is reaching the alveoli. This can result from conditions like airway blockages or reduced airflow. Perfusion refers to the blood flow to the capillaries surrounding the alveoli. Insufficient perfusion means that not enough blood is flowing through the pulmonary capillaries, which can occur due to conditions affecting the heart or blood vessels.
More Definitions in Respiratory Care
PEEP or positive end-expiratory pressure decreases surface tension and prevents atelectasis, extending the time for gas transfer. Lung Compliance refers to the elasticity of tissues while Plateau Pressure is a measurement of the elastic properties of the alveoli. The Peak Inspiratory Pressure is a measurement of the elastic properties of the airway.
Respiratory Complications
Various respiratory complications can arise, including COPD, Emphysema, Asthma, and Bronchitis. These conditions can lead to dynamic hyperinflation, air trapping, low expiratory flow rates, and chronic CO2 retention. Symptoms of these conditions include worsening dyspnea, increased sputum, hypercapnia, and hypoxemia. Treatments can include titrating FiO2 to PaO2 > 60 mmHg or SaO2 > 90%, using bronchodilators, corticosteroids, antibiotics, and mechanical ventilation.
Pulmonary Embolism
A pulmonary embolism is a blockage in the pulmonary artery, the blood vessel that carries blood from the heart to the lungs. This blockage, usually a blood clot, can be life-threatening. The blockage may be partial or complete and can lead to decreased perfusion, increased PA pressures, RV failure, decreased cardiac output, and hypotension. Symptoms include dyspnea, tachypnea, tachycardia, chest pain, cough, hemoptysis, respiratory alkalosis, hypoxemia, hypotension, and PEA. Treatments include maintaining the airway, oxygenation, ventilation, fluids, and anticoagulation.
VTE Prevention
Preventing venous thromboembolism (VTE) involves early mobilization, sequential compression devices (SCDs), and anticoagulation therapy with drugs like Heparin or Lovenox.
Pneumonia
Pneumonia is an acute inflammation caused by infection resulting in alveolar consolidation. It can be community-acquired or hospital-acquired. The diagnosis involves CXR showing consolidation and patchy infiltrates, sputum culture showing leukocytosis, and the presence of hypoxemia. Treatments include positioning the “Good Lung Down”, antibiotics, pulmonary hygiene, and systemic support.
Prevention of Hospital Acquired and Ventilator-Acquired Pneumonia
Prevention strategies include keeping the head of the bed elevated >30 degrees, maintaining oral care, practicing hand hygiene, and managing tube feeding. Aspiration is a common cause and can be due to an altered level of consciousness, depressed cough/gag reflex, the presence of feeding tubes, patient positions, ileus, gastric distention, and secretions.
Understanding ARDS vs ALI
ARDS (Acute Respiratory Distress Syndrome) and ALI (Acute Lung Injury) are conditions that cause fluid to leak into the lungs, making breathing difficult or impossible. ARDS is considered more severe, while ALI represents a milder form of respiratory failure. Both conditions have similar pathophysiology, involving an inflammatory response triggered by an underlying injury or condition. This leads to increased capillary bed permeability causing leakage of fluid and proteins from the bloodstream into the lung tissue and airspaces. There is increased transudate due to the release of inflammatory mediators and the presence of infiltrating immune cells, leading to massive atelectasis from alveolar cell damage.
Identifying ARDS vs ALI
Early signs of ARDS and ALI include tachycardia, restlessness, mild dyspnea, respiratory alkalosis, crackles, CXR showing a “ground glass” appearance, and PaO2 on room air 60. Later signs include agitation, extreme dyspnea leading to respiratory/metabolic acidosis, crackles and wheezes, CXR showing “white out”, and PaO2 on room air 30.
Treatment for ARDS and ALI
Treatment for ARDS and ALI focuses on supportive measures for end-organ stabilization. These include mechanical ventilation, maintaining PEEP at 10-15 cmH2O, keeping plateau pressure <30, limiting tidal volume to 4-6 mL/kg, allowing permissive hypercapnia, proning, using ECMO, providing fluids and vasopressors, administering analgesia and sedation, ensuring nutrition, and providing emotional support. The mortality rate for these conditions is around 30%, often due to multisystem organ failure.
Phases of ARDS
ARDS progresses through three phases - the Exudative phase where the direct or indirect insult to the alveoli or pulmonary capillary endothelium results in surface destruction and alveolar flooding, the Proliferative phase involving macrophage “clean-up” and fibrin ‘scaffolding', and the Fibrotic phase characterized by scar tissue fibrosis and remodeling. The severity of the initial damage determines the extent of the fibrosis and the length of ventilator days.
Resources
For more information on respiratory care, check out the
AACN Essentials of Critical Care Nursing, Fifth Edition.
You can also visit cherylpalmer.com or follow Cheryl Palmer on Instagram @thecherylpalmer