Therapeutic Gases Oxygen

1
Therapeutic Gases - Oxygen

• Robert L. Copeland, Ph.D.
• rlcopeland_at_fac.howard.edu

2
Oxygen
Oxygen, water, and food are of fundamental
importance to the animal organism. Of these three
basic essentials for the maintenance of life, the
deprivation of oxygen leads to death most
rapidly. Therapy with oxygen is useful or
necessary for life in several diseases and
intoxications that interfere with normal
oxygenation of the blood or tissues.
3
Normal Oxygenation

• Oxygen moves down a stepwise series of partial
  pressure gradients from the inspired air to the
  body's cells and their mitochondria. Air normally
  contains 20.9 oxygen, equivalent (at normal
  barometric pressure) to a partial pressure of 159
  mm Hg

4
Normal Oxygenation

• Blood Oxygen Content
• Oxygen in blood is carried primarily in chemical
  combination with hemoglobin and to a small extent
  in physical solution in plasma.
• When fully saturated, each gram of hemoglobin
  binds about 1.3 ml of oxygen.
• Hemoglobin is about 98 saturated when air is
  breathed under normal circumstances

5
Oxygen Deprivation

• Hypoxia is the term used to denote insufficient
  oxygenation of the tissues.

6
Causes of Hypoxia

• Prepulmonary Hypoxia. Hypoxia can be caused by
  inadequate delivery of oxygen to the lung.
• results from inadequate ventilation brought about
  by airway obstruction (laryngospasm,
  bronchospasm), muscular weakness (disease or
  neuromuscular-blocking drugs), or impaired
  respiratory drive central nervous system (CNS)
  disease, opioids, anesthetics.

7
Causes of Hypoxia

• Pulmonary Hypoxia - abnormal pulmonary function
  can impair oxygenation of the blood.
• mismatch between ventilation and perfusion-
  (e.g., adult respiratory distress syndrome,
  asthma, emphysema).
• thickened barrier to diffusion and intrapulmonary
  shunting of venous blood (fibrosis, pulmonary
  edema).

8
Causes of Hypoxia

• Postpulmonary Hypoxia
• inadequate delivery of oxygen to tissues may be
  the result of low cardiac output (shock),
  maldistribution of cardiac output (sepsis,
  vascular occlusion)
• an inadequate concentration of oxygen in arterial
  blood (anemia, hemoglobinopathies, carbon
  monoxide poisoning).

9

• the tissues may be unable to extract or utilize
  sufficient oxygen. This may result from an
  unusually high metabolic demand (thyrotoxicosis,
  hyperpyrexia) or to malfunction of cellular
  enzyme systems (cyanide poisoning).

10
Effects of Hypoxia

• Respiration
• Cardiovascular System
• Central Nervous System
• Cellular and Metabolic Effects

11
Respiration

• Ventilatory rate and depth progressively increase
  during hypoxia as a result of stimulation of
  carotid and aortic chemoreceptors minute
  ventilation almost doubles when normal
  individuals inspire gas with a PO2 of 50 mm Hg

12
Cardiovascular System

• Cardiac output increases with hypoxia as a result
  of increased heart rate and decreased peripheral
  vascular resistance.
• Severe hypoxia, however, can produce bradycardia
  and, ultimately, circulatory failure.

13
CNS

• The CNS is least able to tolerate hypoxia.
  Hypoxia is accompanied initially by decreased
  intellectual capacity and impaired judgment and
  psychomotor ability this state progresses to
  confusion and restlessness and ultimately to
  stupor, coma, and death as the PaO2 decreases
  below 30 to 40 mm Hg.

14
Cellular and Metabolic Effects

• Delivery of oxygen to mitochondria slows as the
  partial pressure gradient from capillaries to
  tissues decreases.
• At a mitochondrial PO2 of less than about 1 mm Hg
  (130 Pa), aerobic metabolism stops, and the less
  efficient anaerobic pathways of glycolysis become
  responsible for the production of cellular
  energy.
• The cellular concentrations of Na, Ca2, and H
  increase, leading to cell death.

15
Adaptation to Hypoxia

• Long-term hypoxia results in adaptive
  physiological changes
• increased numbers of pulmonary alveoli,
  increased concentrations of hemoglobin in blood
  and myoglobin in muscle, and a decreased
  ventilatory response to hypoxia
• Short-term exposure to altitude produces similar
  adaptive changes

16

• Acute exposure - "mountain sickness
• a syndrome characterized initially by headache,
  nausea, dyspnea, and impaired judgment,
  progressing to pulmonary and cerebral edema
• Mountain sickness is treated by inhalation of
  oxygen, descent to lower altitude, or by an
  increase in ambient pressure. Treatment with
  diuretics (carbonic anhydrase inhibitors) and
  steroids also may be helpful.

17
Effects of Oxygen Inhalation

• The primary use for inhalation of oxygen is to
  reverse the effects of hypoxia other
  consequences usually are minor. However, when
  oxygen is breathed in excessive amounts, toxic
  effects can occur

18
Respiration

• Inhalation of oxygen at 1 atmosphere or above
  causes a small degree of respiratory depression
  in normal subjects, presumably as a result of
  loss of tonic chemoreceptor activity.
• Within a few minutes, ventilation increases
  because of a paradoxical increase in the tension
  of carbon dioxide in tissues.

19

• Carbon dioxide is carried by blood in the form of
  bicarbonate. This mechanism of carbon dioxide
  transfer operates more readily when a hydrogen
  ion acceptor, such as deoxyhemoglobin (a stronger
  base than oxyhemoglobin), is available.
• Oxygen at a high PO2, (e.g., during hyperbaric
  oxygenation), the amount of physically dissolved
  oxygen may be sufficient to satisfy the
  requirements of tissue.
• little or no oxygen is extracted from
  oxyhemoglobin, and deoxyhemoglobin is not formed.
  
• Carbon dioxide is then buffered less efficiently,
  and the PCO2 of the tissues rises by several mm
  Hg.

20
Oxygen Toxicity

• Oxygen toxicity probably results from an
  increased production of reactive species such as
  superoxide anion, singlet oxygen, hydroxyl
  radical, and hydrogen peroxide. The oxidative
  damage initiated by these substances is
  propagated by lipid peroxidation and ultimately
  involves all components of the cell. Cell injury
  and death are presumed to result from loss of
  membrane integrity.

21

• Central Nervous System. CNS oxygen toxicity does
  not occur when the partial pressure of inspired
  oxygen is less than 2 atmospheres its occurrence
  is thus limited to a small number of hyperbaric
  applications. CNS toxicity is observed before
  pulmonary toxicity when oxygen is administered at
  partial pressures above 2.5 atmospheres
• characterized by convulsions, which may be
  preceded by visual symptoms or muscular twitching

22
Therapeutic Uses

• Correction of Hypoxia
• Reduction of the Partial Pressure of an Inert Gas
• Oxygen as a Diluent
• Hyperbaric Oxygen Therapy