Respiratory Physiology and Maximum Expiratory Flow

Respiratory Insufficiency - Pathophysiology, Diagnosis, Oxygen Therapy

• Diagnosis and treatment of respiratory disorders relies heavily on understanding basic respiratory physiology and gas exchange.
• Some respiratory diseases stem from inadequate ventilation, others from issues with diffusion through the pulmonary membrane or blood transport of gases.
• Useful methods for studying respiratory abnormalities include measurements like vital capacity, tidal volume, functional residual capacity, dead space, physiological shunt, and physiological dead space. Additional methods are described for studying blood gases and blood pH.

Study of Blood Gases and Blood pH

• Fundamental tests for assessing pulmonary function include determining blood partial pressures of oxygen (P02), carbon dioxide (CO2), and pH; rapid measurements are frequently needed to guide treatment.
• Blood pH is measured using a miniaturized glass pH electrode, producing a voltage directly related to pH, often read from a voltmeter or charted.
• Blood CO2 is measured using a glass electrode surrounded by a thin membrane separating it from bicarbonate solution. Blood CO2 diffuses into the solution, and the resulting pH change is used to calculate CO2.
• Blood P02 is determined using polarography. Electric current between electrodes measures the proportional rate of O2 deposition, directly related to P02. Small electrodes with thin membranes surrounding them are used to minimize interference from other blood components.
• Modern equipment combines pH, CO2, and P02 measurements into a single device allowing bedside monitoring of blood gas levels and pH.

Measurement of Maximum Expiratory Flow

• Maximum expiratory flow is the maximal airflow rate during maximum exhalation, useful for assessing airflow resistance in diseases such as asthma.
• Maximum expiratory flow is greater at larger lung volumes compared to smaller lung volumes due to differences in bronchiolar and airway support.
• Abnormalities in the max. expiratory flow-volume curve indicate lung conditions like constricted lungs and airway obstruction.
• Constricted lung conditions (fibrotic diseases, chest cage constrictions) result in lower maximum expiratory flow rates overall.
• Airway obstruction makes exhalation more difficult, leading to a distinctly shallower maximum expiratory flow-volume curve shape.

Forced Expiratory Vital Capacity (FVC) and Forced Expiratory Volume (FEV1)

• Forced expiratory vital capacity (FVC) and forced expiratory volume in the first second (FEV1) help assess lung function.
• FVC measures the total capacity to exhale forcefully; FEV1 measures the volume exhaled in the first second of forced exhalation.
• Normal FEV1/FVC ratio is approximately 80%, in diseases with airway obstruction this ratio is reduced.

Pathophysiology of Specific Pulmonary Abnormalities

• Chronic pulmonary emphysema involves a complex destructive process in the lungs, characterized by chronic infection, excessive mucus, and inflammatory edema.
• The obstructive process results in air trapping, alveolar overstretching, and alveolar wall destruction.
• Key factors causing reduced lung function in emphysema include bronchiolar obstruction limiting airflow, and diminished diffusion capacity as alveolar walls are destroyed.

Pneumonia - Lung Inflammation and Fluid in Alveoli

• Pneumonia is an inflammatory lung condition where alveoli fill with fluid and cells.
• Bacterial pneumonia, frequently caused by pneumococci, involves infection and ensuing fluid buildup in alveoli.
• Pneumonia negatively impacts lung's gas exchange function by reducing respiratory membrane surface area and increasing membrane thickness.

Atelectasis - Collapse of Alveoli

• Atelectasis refers to the collapse of alveoli, potentially due to airway obstruction or surfactant deficiency in the alveolar fluid.
• Airway obstruction can result in atelectasis by trapping air in the blocked lung segment, leading to collapse.
• Insufficient surfactant in alveolar fluid reduces surface tension, creating a propensity for alveolar collapse (especially in newborns).

Hypoxia and Oxygen Therapy

• Hypoxia, or low blood oxygen, results from various causes including inadequate oxygenation of the blood, impaired blood transport, and tissue utilization.
• Various types of hypoxia exist; causes of hypoxia are classified for appropriate management.
• Oxygen therapy can be useful in treating hypoxia and hypercapnia, particularly when resulting from hypoventilation and certain types of lung disease.
• Cyanosis is a bluish discoloration of the skin caused by excessive deoxygenated hemoglobin in blood vessels.

Hypercapnia - Excess Carbon Dioxide

• Hypercapnia occurs when CO2 levels in the body fluids rise; this typically arises from hypoventilation or reduced respiratory function.
• Elevated CO2 levels can hinder respiration, causing a potential cycle of decreased respiration and elevated CO2.
• Hypercapnia is not associated with all types of hypoxia, unlike hypoventilation-related hypoxia.

Artificial Respiration

• Resuscitators provide intermittent positive pressure to assist breathing.
• Tank respirators employ a mechanism to deliver positive or negative pressure to aid inhalation and exhalation cycles.
• Excessively high positive or negative pressures in these methods can impede venous return to the heart.