Review of Medical Physiology - William F. Ganong 2002

Respiration
Adaptive respiratory changes in health and disease
Hypoxia

Hypoxia is defined as a deficiency of O₂ in Tissues. Although the term anoxia is technically more precise, complete depletion of O₂ from tissue rarely occurs.

Traditionally, hypoxia is classified into four types. While numerous other Classification systems exist, this framework remains the most valuable and practical due to its clear terminology. The four types are: Hypoxic hypoxia (anoxic anoxia), characterized by a reduced arterial P_O2; anemic hypoxia, where arterial P_O2 is normal, but The amount of Hemoglobin available for O₂ transport is reduced; stagnant, or ischemic, hypoxia, in which tissue perfusion is so low that an adequate amount of O₂ fails to reach the tissues despite normal P_O2 and hemoglobin concentrations; and histotoxic (tissue) hypoxia, in which the delivery of O₂ to tissues is adequate, but cellular utilization of O₂ is impaired due to the action of a toxic agent.

Effects of Hypoxia

The effects of stagnant hypoxia depend on the specific tissue affected. In contrast, hypoxic hypoxia and Other forms of generalized hypoxia impact the Brain first. A sudden decrease of 20 mmHg in the P_O2 of inspired air—such as that experienced during a rapid cabin depressurization at an altitude of 16,000 m—results in a loss of consciousness within 10–20 s (Fig. 37-5) and death within 4–5 min. Less severe hypoxia induces various neurological impairments, somewhat resembling alcohol intoxication: impaired judgment, somnolence, blunted pain perception, excitation (euphoria), disorientation, loss of time perception, and headache. Other symptoms include anorexia, nausea, vomiting, tachycardia, and, in severe cases, Hypertension. Ventilation rate increases in direct proportion to the severity of hypoxia, mediated by carotid chemoreceptor Cells.

Respiratory Stimulation

Dyspnea refers to difficult or labored breathing accompanied by an increased respiratory rate. Hypercapnia is a general term denoting an increase in respiratory frequency or depth, regardless of the patient's subjective sensations. Tachypnea refers to rapid, shallow breathing. In healthy individuals, breathing is largely an automatic process until ventilation doubles, at which point Respiration becomes noticeably labored if ventilation increases three- to fourfold. When the required level of ventilation is not met, breathing becomes labored and is further influenced by various other factors. Hypercapnia and hypoxia play a somewhat lesser role in triggering dyspnea. Current hypotheses suggest that dyspnea arises from a mismatch between incoming afferent signals and outgoing motor commands from the brain to the Respiratory Muscles, though this remains difficult to prove. An additional contributing factor is the mechanical work required to move air into and out of the Lungs (the work of breathing). For instance, in asthma accompanied by bronchoconstriction—particularly during inspiration—overcoming the increased airway resistance significantly elevates the work of breathing.

Class="center">

Fig. 37-5. Duration of consciousness following sudden exposure to ambient pressure at various altitudes. Circulation time from the lungs to the brain is 10 s.

Cyanosis

A decrease in the level of deoxygenated hemoglobin alters the color of the tissues. This bluish-purple discoloration is termed cyanosis and typically becomes clinically apparent when the concentration of capillary deoxygenated hemoglobin falls to 5 g/dL. The appearance of cyanosis depends on the total amount of hemoglobin in the Blood, the degree of hemoglobin desaturation, and the state of capillary blood flow.

Cyanosis is most readily observed in the nail beds, mucous membranes, and areas with thin Skin, such as the earlobes, Lips, and fingers. Cyanosis is absent in anemic hypoxia because the total hemoglobin content is low; in Carbon monoxide poisoning, because hemoglobin acquires a dark cherry-red color due to carboxyhemoglobin (see below); and in histotoxic hypoxia, because blood gas composition remains normal. A skin and mucous membrane discoloration similar to cyanosis can also occur in the presence of elevated circulating levels of methemoglobin (see Chapter 27).