Transcript
Chapter 10: Determinants and Assessment of Pulmonary Gas Exchange
Chapter Summary
The respiratory tract can be divided into two sets of airways, the conducting and respiratory airways. The conducting airways are a conduit for airflow, providing protective functions such as humidification, filtration, and warming of air. The trachea branches into the right and left mainstem bronchi, which further divide into bronchioles. Bronchioles regulate airflow distribution by constricting and dilating. The respiratory airways consist of the terminal units, the alveoli, and are responsible for gas exchange.
The respiratory process has three components: ventilation, diffusion, and perfusion. Ventilation is the mechanical movement of airflow to and from the atmosphere and the alveoli. During inhalation, air moves from an area of higher pressure (atmosphere) to an area of lower pressure (lungs) until the pressure in the lungs becomes slightly higher than atmospheric pressure. At this point, air passively flows back out of the lungs (expiration) until pressures are equalized again.
Lung tissue and alveoli have to constantly overcome the tendency to collapse. Collapse is prevented by surfactant, which reduces surface tension to keep alveoli open. Lung compliance is decreased by any condition that affects the lung tissues. Decreased compliance represents stiffness of the lungs and requires more force to increase lung volumes, resulting in increased work of breathing and decreased tidal volume.
Diffusion, the second component of the respiratory process, is the movement of gases across the alveolar–capillary membrane from an area of high pressure to an area of low pressure. External respiration is the movement of gases across the alveolar–capillary membrane by the process of diffusion. Diffusion is affected by the partial pressures of oxygen and carbon dioxide and the oxyhemoglobin dissociation curve. The effectiveness of diffusion is influenced by gradient, surface area, thickness, and length of exposure.
Perfusion, the third component of the respiratory process, refers to the blood flow into and from the lungs, beginning with the pulmonary artery and ending at the junction of the pulmonary veins and left atrium. Pulmonary perfusion is dependent on three factors: cardiac output, gravity, and peripheral vascular resistance (PVR).
An overall balance of the relationship of ventilation to perfusion (V/Q) must be maintained to optimize proper diffusion of gases. If the V/Q ratio becomes significantly mismatched, hypoxia results.
Pulmonary shunt refers to blood that does not take part in pulmonary gas exchange. There are two types of pulmonary shunt, true/physiologic and shuntlike effect. Capillary shunt represents complete nonfunctioning alveolar units, which cannot respond (are refractory) to oxygen therapy since no diffusion is taking place. Shuntlike effect is partially responsive to oxygen therapy because the alveoli function to some extent. The P/F ratio can be calculated to estimate the amount of intrapulmonary shunt.
PVR is a measure of the resistance to blood flow in the pulmonary vascular system. PVR is determined by the length and radius of the vessels and viscosity of the blood. Prolonged increased PVR results in dilation and hypertrophy of the right heart and is known as cor pulmonale.
To differentiate between primary metabolic acidosis and alkalosis, the HCO3 and pH are evaluated. Compensation of a metabolic acid–base disturbance requires evaluation of PaCO2.
To differentiate between primary respiratory acidosis or alkalosis, the PaCO2 and pH are evaluated. Compensation of respiratory acid–base disturbance requires evaluation of HCO3. Compensatory mechanisms for acid–base disturbances include buffering, excretion or retention of CO2 (respiratory), and excretion or retention of HCO3 (metabolic). Treatment of acid–base disturbances requires correction of the underlying problem.
The most common respiratory complaints and subjective symptoms are dyspnea, cough, chest pain, sputum, and hemoptysis. A focused respiratory assessment includes vital signs, pulse oximetry, inspection of skin color, chest movement/pattern of breathing, and auscultation for adventitious breath sounds such as crackles, wheeze, rhonchi, or pleural rub.
Tidal volume, vital capacity, and total lung capacity are all components of pulmonary function tests that help to measure the effects of disease processes on ventilation.
Arterial blood gas analysis is a step-by-step approach evaluating each component individually and in relationship to other components.
Pulse oximetry and end tidal CO2 monitoring are noninvasive tools used to assist in monitoring oxygenation and ventilation. Arterial catheters are an invasive means to monitoring oxygenation and ventilation.
