Oxygenation

 

OXYGENATION

Oxygenation is the addition of oxygen to any system, including the human body. Oxygenation may also refer to the process of treating a patient with oxygen, or of combining a medication or other substance with oxygen.

REVIEW OF RESPIRATORY PHYSIOLOGY

Structure and function of the Respiratory System:

               The respiratory system is divided structurally into the upper respiratory system and the lower respiratory system. The mouth, nose, pharynx, and larynx compose the upper respiratory system. The lower respiratory system includes the trachea and lungs, with the bronchi, bronchioles, alveoli, pulmonary capillary network, and pleural membranes.

Air enters through the nose, where it is warmed, humidified, and filtered. Particles in the air are trapped by the hairs at the entrance of the nares. Inspired air passes from. the nose through the pharynx. The pharynx is a shared pathway for air and food. It includes both the nasopharynx and the oropharynx. The larynx is a cartilaginous structure that can be identified externally as the Adam's apple. During swallowing, the inlet to the larynx (the epiglottis) closes, routing food to the esophagus. The epiglottis is open during breathing, allowing air to move freely into the lower airways. Below the larynx, the trachea leads to the right and left main bronchi (primary bronchi) and the other conducting airways of the lungs. Within the lungs, the primary bronchi divide repeatedly into smaller and smaller bronchi, ending with the terminal bronchioles.

The respiratory zone of the lungs includes the respiratory bronchioles (which have scattered air sacs in their walls), the alveolar ducts, and the alveoli. The alveolar and capillary walls form the respiratory membrane, where gas exchange occurs the air on the alveolar side and the blood on the capillary side. The airways move air to and from the alveoli; the right ventricle and pulmonary vascular system transport blood to the capillary side of the membrane.

The function of the respiratory system is gas exchange. Inspiration is the act of drawing air into the lungs. The major muscle of inspiration is the diaphragm, the contraction of which creates a reduced pressure in the chest, causing the lungs to expand and air to flow inward. Expiration is the act of breathing forth or expelling air from the lungs. During exhalation, the process reverses. The diaphragm and intercostal muscles relax, causing thorax to return to its smaller resting size. Pressure in the chest increases allowing air to flow out of the lungs. Oxygen from inspired air diffuses from alveoli, in the lungs into the blood in pulmonary capillaries. Carbon dioxide produced during cell metabolism diffuses from the blood into the alveoli and is exhaled.

REVIEW OF CARDIOVASCULAR PHYSIOLOGY

Structure and function:

The right ventricle pumps deoxygenated blood through the pulmonary circulation.

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. Deoxygenated blood leaves the heart, goes to the lungs, and then re-enters the heart; deoxygenated blood leaves through the right ventricle through the pulmonary artery. The left ventricle pumps oxygenated blood through the systemic circulation.

Systemic circulation is the part of blood circulation that caries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart. The part of blood circulation that carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart is called as systemic circulation. As the blood passes through the circulatory system, there is an exchange of respiratory gases, nutrients and waste products between the blood and the tissues.

FACTORS AFFECTING OXYGENATION

The four factors which affect oxygenation are physiological, developmental, lifestyle and environmental.

Physiological factors: Any condition affecting cardiopulmonary functioning directly affects the ability of the body to meet oxygen demands. The respiratory disorders include hypoventilation, hypoxia, hyperventilation. The cardiac disorders include disturbances in conduction, impaired valvular function, myocardial hypoxia, peripheral tissue hypoxia and other physiological processes include alterations affecting oxygen carrying capacity of the blood, decreased inspired oxygen concentration, increased metabolic demand of the body and alterations affecting chest wall movement caused by musculoskeletal abnormalities or neuromuscular alterations.

·        Decreased oxygen carrying capacity: Hemoglobin carries the majority of oxygen to tissues. Anemia and inhalation of toxic substances such as CO decrease the oxygen carrying capacity of the blood by reducing the amount of available hemoglobin to transport oxygen.

·        Hypovolemia: Shock and severe dehydration cause extracellular fluid loss and hypovolemia. It results in hypoxia. With the significant fluid loss the body tries to adapt by peripheral vasoconstriction and increases the heart rate to increase the volume of blood returned to heart thus increasing the cardiac output.

·        Decreased inspired oxygen concentration: With the decline of the concentration of inspired oxygen, the oxygen carrying capacity of the blood decreases. It is caused by upper or lower airway obstruction, which limits delivery of inspired oxygen to alveoli; decreased environmental oxygen (at high altitudes) or hypoventilation (occurs in drug over dose).

·        Increased metabolic rate: This increases oxygen demand. The level of oxygenation declines when body systems are unable to meet this demand. Fever increases the need of tissues for oxygen; as a result carbon dioxide production increases the body's attempts to adapt to the increased C02 level as by increasing the rate and depth of respiration The patient's WOB increases and the patient eventually displays signs and symptoms of hypoxemia .

·        Conditions affecting chest wall movement: Any condition reducing chest wall movement results in decreased ventilation. If the diaphragm does not really fully descend with breathing, the volume of inspired air decreases, delivering less oxygen to the alveoli and tissues.

·        Pregnancy: As the fetus grows during pregnancy, the enlarging uterus pushes abdominal contents upward against the diaphragm in the last trimester of pregnancy, the inspiratory capacity declines, resulting in dyspnoea on exertion and increased fatigue.

·        Obesity: Patients who are morbidly obese have reduced lung volumes from the heavy lower thorax and abdomen, particularly in the recumbent and supine positions. Morbidly obese patients have a reduction in lung and chest wall compliance as a result of encroachment of the abdomen into the chest, increased WOB and decreased lung volumes.

·        Musculoskeletal abnormalities: Musculoskeletal impairments in the  thoracic region reduce oxygenation. Such impairments result from abnormal structural configurations, trauma, muscular diseases and diseases of central nervous system. Other examples include kyphosis, lordosis, or scoliosis etc.

·        Trauma: Flail chest is a condition in which multiple rib fractures cause instability in part of the chest wall. The unstable chest wall allows the lung underlying to contract on inspiration and bulge on expiration resulting in hypoxia. Patients with thoracic or upper abdominal surgical incisions use shallow respirations to avoid pain, which also decreases chest wall movement. Opioids used to treat pain depress the respiratory rate and chest wall expansion.

·        Neuromuscular diseases: Neuromuscular diseases affect tissue oxygenation by decreasing the patient's ability to expand and contract the chest wall. Ventilation is impaired, resulting in atelectasis, hypercapnia and hypoxemia. Examples include myasthenia gravis, Guillain-Barre syndrome, and poliomyelitis.

·        Central nervous system alterations: Diseases or trauma of the medulla oblongata and spinal cord result in impaired respiration. When the medulla oblongata is affected, neural regulation of respiration is impaired, and abnormal breathing patterns develop. Cervical trauma at C3 to C5 usually results in paralysis of the phrenic nerve. When phrenic nerve is damaged, the diaphragm does not descend properly, thus reducing the inspiratory lung volumes and causing hypoxemia.

·        Influences of chronic diseases:  Oxygenation decreases as a direct consequence of chronic lung disease. Changes in the antero posterior diameter of the chest wall (barrel chest) occur because of overuse of accessory muscles and air trapping in emphysema.

Developmental factors: The developmental stage of the client and normal aging process can affect oxygenation.

·        Infants and toddlers: They are at risk for upper respiratory tract infections.

·  School age children and adolescents: They are exposed to respiratory infections and respiratory risk factors such as cigarette smoking or second hand smoke.

·        Young and middle age adults: They are exposed to multiple cardiopulmonary risk factors: an unhealthy diet, lack of exercise, stress, illegal substances and smoking.

·        Older adults: The cardiac and respiratory systems undergo changes throughout the ageing process. The changes are associated with calcification of heart valves, SA node. The arterial system develops atherosclerotic plaques, the number of functional cilia is reduced, causing a decrease in the effectiveness of cough mechanisms thus the respiratory infections increase in the older adults.

Lifestyle: Lifestyle modifications are difficult for patients because they often have to change an enjoyable habit such as cigarette smoking or eating certain foods.

·        Nutrition: Severe obesity decreases lung expansion and increased body weight increases tissue oxygen demands. The malnourished patient experiences respiratory muscle wasting, resulting in decreased muscle strength and respiratory excursion.

·        Exercise: Increases metabolic activity and 02 demand of the body. The rate and depth of respiration increases, enabling the person to inhale more 02 and exhale excess C02.

·        Smoking: Inhaled Nicotine causes vasoconstriction of peripheral vascular and coronary blood vessels, increases BP and decreases blood flow to peripheral vessels.

·        Substance abuse: The person who chronically abuses substances often has a poor nutritional intake. With the resultant decrease in intake of iron rich foods, Hb production declines. Excessive use of alcohol and certain other drugs depresses respiratory center and reduces rate and depth of respiration and amount of inhaled 02.

·        Stress: A continuous state of stress or severe anxiety increases the metabolic rate and oxygen demand of the body. The body responds to anxiety and other stresses with an increased rate and depth of respiration.

Environmental factors: The environment also influences oxygenation. The higher the altitude, lower the Pa02 an individual breathes. Person at high altitudes has increased respiratory and cardiac rates and increased respiratory depth. Pulmonary disease is higher in smoggy, urban areas than in rural areas. Occupational pollutants such as asbestos, talcum powder, dust, airborne fibers increase the risk of pulmonary diseases.

ALTERATIONS IN OXYGENATION

Alterations in oxygenation can be due to alterations in respiratory functioning and alterations in cardiac functioning.

Alterations in respiratory functioning:

Illnesses and conditions affecting ventilation or oxygen transport cause alterations in respiratory functioning. The three primary alterations are hypoventilation, hyperventilation and hypoxia. The goal of ventilation is to produce a normal arterial C02 tension (paC02) between 35 and 45 mmHg and a normal arterial oxygen tension (pa02) between 80 and 100mmHg.

·        Hypoventilation: It occurs when to alveolar ventilation is inadequate to meet the oxygen demand of the body or eliminate sufficient carbon dioxide. As alveolar function decreases, the body retains carbon dioxide. Signs and symptoms of hypoventilation include mental status changes, dysrhythmias and cardiac arrest (is a sudden stop in effective blood circulation due to failure of the heart to contract effectively). If untreated the patient's condition rapidly declines, leading to convulsions, unconsciousness and death.

·        Hyperventilation: often called alveolar hyperventilation, is a state of ventilation in which the lungs remove carbon dioxide faster than it is  produced by cellular metabolism (increased movement of air into and out of the lungs). During hyperventilation, the rate and depth of respirations increase. causes of hyperventilation are severe anxiety, infection, drugs, acid base balance, fever (increase metabolic rate) etc. The symptoms include rapid respirations, numbness or tingling  of hands/feet, light headedness an loss of consciousness. A particular type of hyperventilation is kussmaul’s breathing, by which the body attempts to compensate (give off excess body acids) by blowing off the carbon dioxide through deep and rapid breathing. Hyperventilation can also occur in response to stress or anxiety.

·        Hypoxia: Is inadequate tissue oxygenation at the cellular level. It results from deficiency of oxygen delivery or oxygen use at the cellular level. It is a life threatening situation. causes are decreased hemoglobin level and lowered oxygen carrying capacity of the blood, diminished concentration of inspired oxygen which occurs at high altitudes, the inability of the tissues to extract oxygen from the blood as with cyanide poisoning, decreased diffusion of oxygen from the alveoli to the blood as in pneumonia, poor tissue perfusion with oxygenated blood as with shock, impaired ventilation as with multiple rib fractures or chest trauma. Hypoventilation, due to diseases of the respiratory muscles, drugs, or anesthesia can also lead to hypoxia. Signs and symptoms include restlessness, inability to concentrate, decreased level of consciousness, dizziness behavioural changes, unable to lie flat and appears both fatigued and agitated, increased pulse rate and depth of respiration and cyanosis (late sign).

Cyanosis is caused by the desaturated Hb in capillaries. Central cyanosis, observed in the tongue, soft palate and conjunctiva of the eye where blood flow is high indicates hypoxemia. Peripheral cyanosis, seen in the extremities, nail beds and ear lobes is often a result of vasoconstriction and stagnant blood flow.

Altered Breathing Patterns:

Breathing patterns refer to the rate, volume, rhythm, and relative ease or effort of respiration. Normal respiration (eupnea) is quiet, rhythmic, and effortless.

Tachypnea (rapid rate) is seen with fevers, metabolic acidosis, pain and hypercapnia or hypoxemia.

Bradypnea is an abnormally slow respiratory rate, which may be seen in clients who have taken drugs such as morphine, who have increased intracranial pressure (e.g., from brain injuries).

Apnea is the cessation of breathing. Abnormal respirat01Y rhythms create an irregular breathing pattern. Two abnormal respiratory rhythms are Cheyne-Stokes respirations and Biot^ts (cluster) respirations. Cheyne-stokes respirations are very deep to very shallow breathing and temporary apnea; common causes include congestive heart failure, increased intracranial pressure, and overdose of certain drugs.

Biot's (cluster) respirations are shallow breaths interrupted by apnea; may be seen in clients with central nervous system disorders.

Orthopnea is the inability to breathe except in an upright or standing position. Difficult or uncomfortable breathing is called dyspnea. The dyspnoeic person often appears anxious and may experience shortness of breath (SOB), a feeling of being  unable to get enough air (breathlessness). Often the nostrils are flared because of the increased effort of inspiration. The skin may appear dusky; heart rate is increased. Dyspnea may have many causes, most of which stem from cardiac or respiratory disorders. The term hypoxemia refers to reduced oxygen in the blood. Carbon dioxide accumulation in the blood, a condition called hypercarbia (hypercapnia).

Alterations in cardiac functioning:

Illness and conditions affecting cardiac rhythm, strength of contraction, blood flow through the heart muscle, and decreased peripheral circulation cause alterations in cardiac functioning.

•   Disturbances in Conduction: Electrical impulses that do not originate from the SA node cause conduction disturbances. The rhythm disturbances are called dysrhythmias, meaning a deviation from the normal sinus heart rhythm. Dysrhythmias occur as primary conduction disturbances such as in response to ischemia; valvular abnormality; anxiety; drug toxicity; caffeine, alcohol, or tobacco use; or as a complication of acid-base or electrolyte imbalance.
•    Altered Cardiac Output: Failure of the myocardium to eject sufficient volume to the systemic and pulmonary circulations occurs in heart failure. Primary coronary artery diseases, valvular disorders, and pulmonary disease lead to myocardial pump failure.
•      Left-sided heart failure: It is an abnormal condition characterized by decreased functioning of the left ventricle. If left ventricular failure is significant, the amount of blood ejected from the left ventricle drops greatly, resulting in decreased cardiac output. Signs and symptoms include fatigue, breathlessness, dizziness and confusions as a result of tissue hypoxia form the diminished cardiac output. As the left ventricle continues to fail, blood begins to pool in the pulmonary circulation, causing pulmonary congestions. Clinical findings include crackles in the base of the lungs on auscultation, hypoxia, shortness of breath on exertion, cough, and paroxysmal nocturnal dyspnea.
•       Right —sided heart failure: It results from impaired functioning of the right ventricle. It most commonly results from pulmonary disease or as a result of long term left sided failure. The primary pathological factor in right-sided heart failure is elevated pulmonary vascular resistance (PVR). As the PVR continuous to rise, the right ventricle works harder and the oxygen demand of the heart increases. As the failure continues, the amount of blood ejected from the right ventricle declines and the blood begins to back up in the systemic circulation. Clinically the patient will have weight gain, distended neck veins, hepatomegaly, splenomegaly and peripheral edema.
•    Impaired valvular function: It is an acquired or congenital disorder of a cardiac valve that causes either hardening (stenosis) or impaired closure (regurgitation) of the valves. When the stenosis occurs the flow of blood through the valves is obstructed. When the stenosis occurs in the semilunar valves, the adjacent ventricles have to work harder to move the ventricular blood volume beyond the stenotic valve. When regurgitation occurs, there is a back flow of blood into an adjacent chamber. In mitral regurgitation, the mitral leaflets do not close completely. When the ventricles contract, blood escapes back into the atria, causing a murmur or whooshing sound.
•    Myocardial ischemia: Occurs when the supply of blood to the myocardium from the coronary arteries is insufficient to meet myocardial oxygen demands. Two common outcomes of this ischemia are angina pectoris and MI

ü Angina pectoris: It is a transient imbalance between myocardial oxygen supply and demand. The condition results in chest pain that is aching, sharp, tingling or burning that feels like pressure.

ü Myocardial infarction (MI): Results from sudden decrease in myocardial blood flow or an increase in myocardial oxygen demand without adequate coronary perfusion. Infarction occurs because ischemia is not reversed.