15 The Effects of Pregnancy, Altitude, and Diving on the Respiratory System Flashcards Preview

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Flashcards in 15 The Effects of Pregnancy, Altitude, and Diving on the Respiratory System Deck (24)
1

Pregnancy

  • The respiratory system during pregnancy is affected by...
  • As gestation progresses,...
  • Functional residual capacity (FRC)
  • Total lung capacity (TLC)
  • Vital capacity
  • The gravid uterus
  • Tidal volume
  • Respiratory rate
  • Dead space/tidal volume ratio
  • Minute ventilation

  • The respiratory system during pregnancy is affected by...
    • Both anatomic changes and alterations in metabolism
  • As gestation progresses,...
    • The diaphragmatic position elevates 4 cm
    • The diameter of the lower rib cage increases by as much as 5 cm
    • Gives a more barrel-chested appearance to the thorax
  • Functional residual capacity (FRC)
    • These alterations result in a 10 - 25% reduction in FRC, predominantly due to a lower residual volume
  • Total lung capacity (TLC)
    • Although FRC is reduced, TLC decreases only marginally
  • Vital capacity
    • Not affected
  • The gravid uterus
    • Does not impair diaphragmatic excursion
  • Tidal volume
    • Increased
  • Respiratory rate
    • Does not change
  • Dead space/tidal volume ratio
    • Does not change
  • Minute ventilation
    • Increased minute ventilation results in an increase in alveolar ventilation

2

Pregnancy

  • Most pronounced alterations in the respiratory physiology of pregnancy
  • Progesterone
  • Part of the increased respiratory drive results from...
  • The greatly augmented minute ventilation over-compensates for...
  • ABG measurements normally show...
  • Alveolar air equation

  • Most pronounced alterations in the respiratory physiology of pregnancy
    • Increased respiratory drive
    • Increased minute ventilation
  • Progesterone
    • Increases throughout gestation
    • Stimulates respiratory drive directly
    • Increases the sensitivity of the respiratory center to PCO2
  • Part of the increased respiratory drive results from...
    • Increased metabolic rate
    • Associated carbon dioxide (CO2) production (an increase of as much as 30% by the third trimester)
  • The greatly augmented minute ventilation over-compensates for...
    • This increase in CO2 production
    • Results in a primary respiratory alkalosis with renal compensation
  • ABG measurements normally show...
    • A pH ranging between 7.40 and 7.47
    • PCO2 ranging from 28 to 32 mmHg
  • Alveolar air equation
    • Increased alveolar ventilation also increases PaO2

3

Dyspnea during pregnancy

  • Frequency of dyspnea during pregnancy
  • Dyspnea during first, second, and third trimesters
  • The likely mechanisms of dyspnea during normal pregnancy
  • Indications of a respiratory illness rather than normal physiologic dyspnea of pregnancy

  • Frequency of dyspnea during pregnancy
    • Up to 75% of pregnant women experience dyspnea at rest or with mild exertion by the 30th week of pregnancy
  • Dyspnea during first, second, and third trimesters
    • This symptom commonly starts during the first or second trimester, before it can be explained by an increase in abdominal girth
    • The frequency of dyspnea increases during the second trimester and is reasonably stable during the third trimester
  • The likely mechanisms of dyspnea during normal pregnancy
    • Progesterone-induced hyperventilation is at least partially responsible, since dyspnea has been shown to correlate with a low PaCO2
    • Increased mechanical load imposed by the enlarging uterus that increases the work of breathing, nasal congestion, increased pulmonary blood volume and anemia
  • Indications of a respiratory illness rather than normal physiologic dyspnea of pregnancy
    • Abrupt onset of symptoms
    • The presence of a cough, sputum or tachypnea
    • Abnormal findings on physical exam

4

Diving

  • The physiologic alterations that occur during diving result primarily from...
  • Boyle's law
  • For every 33 ft of seawater, ambient pressure...
  • Lung volume vs. depth
  • At a depth of 33 ft,...
    • External pressure
    • Lung volume

  • The physiologic alterations that occur during diving result primarily from...
    • The increased pressure that surrounds the chest
    • Increased ambient pressure decreases the size of the chest and gas dissolved within it, as explained by Boyle’s law
  • Boyle's law
    • At a constant temperature, the volume of a gas varies inversely to the pressure applied to it
  • For every 33 ft of seawater, ambient pressure...
    • Increases by one atmosphere (760 mmHg)
  • Lung volume vs. depth
    • Since the gas in the lungs is compressible, lung volume is inversely proportional to the depth attained
  • At a depth of 33 ft,...
    • External pressure is 1520 mmHg
    • Lung volume is cut in half

5

Diving

  • Gas in the compressed lung
  • Partial pressures of gas in the compressed lung
  • Henry’s law

  • Gas in the compressed lung
    • Not changed in composition
    • i.e. air inspired from the surface will still contain 21% oxygen
  • Partial pressures of gas in the compressed lung
    • Since the total pressure of gas in the lung increases at greater depth, the partial pressures of each gas also increase
    • Alveolar PO2 and PCO2 (and PN2) increase progressively with depth
  • Henry’s law
    • At a constant temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas
    • At greater depth the concentration of oxygen, carbon dioxide and nitrogen increases in the blood and tissues

6

Diving

  • The increased ambient pressure that accompanies diving also affects...
  • The extra-thoracic pressure
  • The inspiratory muscles 
  • Below a depth of about 1 meter, it is impossible to...

  • The increased ambient pressure that accompanies diving also affects...
    • The mechanics of the respiratory system
  • The extra-thoracic pressure
    • Opposes the normal outward elastic recoil of the chest wall
    • Leads to a reduction in FRC and TLC
  • The inspiratory muscles
    • Must also generate more force to overcome this pressure and expand the lungs and chest wall
  • Below a depth of about 1 meter, it is impossible to...
    • Breathe through a tube connected to the surface
    • This is because below this depth, the maximum achievable inspiratory pressure (about -100 cmH2O) cannot overcome the pressure surrounding the chest

7

Breath-hold diving

  • To inhibit the urge to draw a breath, breath-hold divers usually...
  • During the dive, as ambient pressure increases,...
  • In addition, during the breath hold,...
  • The increase in PaCO2 stimulates...
  • Breath-hold divers may...
  • Mechanical damage to the lungs

  • To inhibit the urge to draw a breath, breath-hold divers usually...
    • Hyperventilate to an alveolar PO2 of about 120 mmHg and a PCO2 of 30 mmHg
  • During the dive, as ambient pressure increases,...
    • Alveolar PO2 and PCO2 also increase
    • This causes more O2 to diffuse from the alveolar gas into the capillary blood
  • In addition, during the breath hold,...
    • CO2 production continues and cannot be excreted via the respiratory system, further increasing alveolar and arterial PCO2
    • Therefore, significant respiratory acidosis and acidemia may occur
  • The increase in PaCO2 stimulates...
    • Central chemoreceptors, which increase the drive to breathe and limit breath-hold time
  • Breath-hold divers may...
    • Lose consciousness from hypoxemia upon ascent, as the volume of the thoracic cavity increases, and the partial pressure of oxygen plummets, and drown
  • Mechanical damage to the lungs
    • Rare in breath-hold divers since lungs cannot contain more air than would fill them to TLC at surface pressure

8

Breath-hold diving

  • A
  • B
  • C
  • D
  • Abbreviations
    • PAO2
    • PACO2
    • PaO2
    • PaCO2
    • PvO2
    • PvCO2

  • A
    • Breath-hold diver hyperventilates to PAO2 of 120 and PACO2 of 25, with corresponding aterial partial pressures
  • B
    • During descent, lung volume shrinks by 25% and alveolar and arterial PO2 and PCO2 increase
    • In this model, assume that the time of descent is so rapid, that oxygen consumption and CO2 production are insignificant between A and B
  • C
    • During breath-hold time, oxygen consumption lowers PAO2 and carbon dioxide production increases PCO2
  • D
    • Ascent to the surface results in further fall in PAO2 and PACO2, resulting in still lower PaO2
    • In this model, assume that the time of ascent is so rapid, that oxygen consumption and CO2 production are insignificant between C and D
  • Abbreviations
    • PAO2: partial pressure of oxygen in the alveoli
    • PACO2: partial pressure of carbon dioxide in the alveoli
    • PaO2: partial pressure of oxygen in arterial blood
    • PaCO2: partial pressure of carbon dioxide in arterial blood
    • PvO2: partial pressure of oxygen in mixed venous blood
    • PvCO2: partial pressure of carbon dioxide in mixed venous blood

9

Non-breath-hold diving

  • Since breathing air from the surface is not an option, ventilation during diving...
  • The most commonly used breathing support system 

  • Since breathing air from the surface is not an option, ventilation during diving...
    • Must be supported by a pressurized system that forces gas into the lungs
  • The most commonly used breathing support system
    • The open circuit SCUBA (Self-Contained Underwater Breathing Apparatus)
    • This device delivers ambient pressure gas only when inhalation is initiated
    • Use of open circuit SCUBA is limited by the amount of compressed gas available in the cylinder
    • A typical cylinder can supply approximately 2100 L of gas at the surface (1 atm), but at a depth of 66 feet (3 atm), the effective volume of gas is decreased to 700 L. With a VE of 10 L/min
      • This gas supply would last 70 minutes, and with a VE of 20 L/min, the gas would last 35 minutes

10

Complications of diving:
Barotrauma

  • This term refers to...
  • Barotrauma is most likely to occur...
  • As ambient pressure falls, lung volume...
  • Pneumothorax 

  • This term refers to...
    • Lung injury caused by high pressure
    • Second most common cause of death in SCUBA divers (after drowning)
  • Barotrauma is most likely to occur...
    • When a diver who is breathing pressurized gas holds his breath during an ascent
  • As ambient pressure falls, lung volume...
    • Increases and may lead to alveolar rupture, resulting in pneumothorax, alveolar hemorrhage, or air embolism
    • The latter occurs when pressurized alveolar gas enters the alveolar capillaries and then the arterial circulation
    • These air bubbles may then occlude flow to vital organs and tissues
  • Pneumothorax
    • Relatively uncommon
    • Subjects with a history of spontaneous pneumothorax, bullous or cystic lung disease are at increased risk, and should be cautioned against diving

11

Complications of diving:
Decompression illness

  • During a dive, the PO2, PCO2, and PN2 of the extracellular and intracellular, as well as the volume of dissolved gas...
  • During a rapid ascent,...
  • The liberated gas bubbles can...
  • Bubbles that enter the blood may...
  • Bubbles in joints may cause...
  • The treatment for this life-threatening condition

  • During a dive, the PO2, PCO2, and PN2 of the extracellular and intracellular, as well as the volume of dissolved gas...
    • Increase
  • During a rapid ascent,...
    • Gas pressure and solubility decrease
    • Bubbles (especially nitrogen) may form in blood vessels and tissues
  • The liberated gas bubbles can...
    • Alter organ function by blocking vessels, rupturing or compressing tissue, or activating clotting and inflammatory cascades
  • Bubbles that enter the blood may...
    • Impair pulmonary blood flow and cause chest pain, dyspnea, and cough (the chokes)
    • Impair cerebral blood flow causing stroke
  • Bubbles in joints may cause...
    • Pain (the bends) or osteonecrosis
  • The treatment for this life-threatening condition
    • Immediate re-compression, which forces the gas back into solution, followed by very slow decompression, ideally accomplished in a hyperbaric oxygen chamber
    • Administration of 100% oxygen can widen the pressure gradient for nitrogen between the trapped bubble and the circulation, thus hasten absorption of the gas

12

Complications of diving:
Nitrogen narcosis

  • Rapture of the deep
  • At high ambient pressures, very high PN2 can alter CNS function and lead to euphoria, amnesia, clumsiness, and irrational behavior

13

Altitude

  • As we ascend,...
    • Total barometric pressure...
    • The fractional concentration of oxygen in the atmosphere...
  • The PO2 of dry air at any altitude
  • The partial pressure exerted by water vapor in the air entering the alveoli
  • PIO2 equation
  • PAO2 equation

  • As we ascend,...
    • Total barometric pressure decreases
    • The fractional concentration of oxygen in the atmosphere does not change
  • The PO2 of dry air at any altitude
    • ~21% of total barometric pressure
  • The partial pressure exerted by water vapor in the air entering the alveoli
    • Fixed at 47 mmHg
  • PIO2 equation
    • PIO2 = 0.21 X (PB – 47 mm Hg) (PB = barometric pressure)
  • PAO2 equation
    • PAO2 = PIO2 – ( PACO2/ R) (R= respiratory exchange ratio)

14

Altitude

  • As altitude increases,...
  • At the summit of Mt. Everest (8848 m) the barometric pressure is...
  • This means that a person breathing without supplemental oxygen will have a PIO2 of about...
  • If PaCO2 were 10 mmHg, this would lead to a PAO2 of...
  • Assuming a normal PA-aO2 of 8 mmHg, the PaO2 would be...
  • This explains why...

  • As altitude increases,...
    • PAO2 and PaO2 fall
    • The drop in PaO2 stimulates chemoreceptors, and this causes...
      • Minute ventilation to increase
      • PaCO2 to fall
      • Arterial pH to rise (respiratory alkalosis)
  • At the summit of Mt. Everest (8848 m) the barometric pressure is...
    • 253 mm Hg
  • This means that a person breathing without supplemental oxygen will have a PIO2 of about...
    • 43 mmHg
  • If PaCO2 were 10 mmHg, this would lead to a PAO2 of...
    • About 30 mmHg
  • Assuming a normal PA-aO2 of 8 mmHg, the PaO2 would be...
    • 22 mmHg
  • This explains why...
    • Supplemental oxygen is needed

15

Altitude

  • Acute exposure to hypobaric hypoxia of altitude induces many physiologic changes involving multiple organ systems that together act to...
  • These physiologic changes
  • Adaptation 
  • Healthy unacclimatized individuals may develop several medical conditions at altitude, including...

  • Acute exposure to hypobaric hypoxia of altitude induces many physiologic changes involving multiple organ systems that together act to...
    • Reduce the gradient between PIO2 and tissue PO2
    • Optimize delivery and utilization of oxygen at the cellular level
  • These physiologic changes
    • Known as acclimatization
    • Regulated by the transcription factor hypoxia-inducible factor-1-alpha (HIF-1- α)
    • Begins within minutes of ascent
    • Requires several weeks to complete
  • Adaptation
    • The physiologic changes in response to hypobaric hypoxia over generations
    • Observed in some populations living permanently at high altitude
  • Healthy unacclimatized individuals may develop several medical conditions at altitude, including...
    • Acute mountain sickness (AMS)
    • High altitude cerebral edema (HACE)
    • Periodic breathing of altitude
    • High altitude pulmonary edema (HAPE)

16

Acute mountain sickness (AMS) and high altitude cerebral edema (HACE)

  • Result from...
  • Lead to...
  • The “tight fit” hypothesis 

  • Result from...
    • Hypoxia-induced cerebral vasodilation
  • Lead to...
    • Cerebral hyperperfusion and edema
  • The “tight fit” hypothesis
    • Individuals with less space for cerebrospinal fluid are at higher risk for symptoms with mild edema

17

Acute mountain sickness (AMS)

  • Experienced when...
  • Altitude at which symptoms occur
  • Major symptoms
  • Risk factors

  • Experienced when...
    • An unacclimatized person ascends to a moderate altitude
  • Altitude at which symptoms occur
    • There is significant intra-individual variability
    • The most susceptible individuals may be affected as low as 8000 feet
    • Nearly half of lowlanders will develop AMS at 14,000 ft
  • Major symptoms
    • Headache, dizziness, dyspnea at rest, weakness, nausea, and sleeplessness
  • Risk factors
    • Residence at less than 3000 feet
    • Age < 50
    • Vigorous physical exertion during or after ascent
    • Rapid ascent and obesity
    • Substances that interfere with sleep (such as alcohol, sedative-hypnotic medications or primary sleep disorders) or respiratory function
    • Subjects who have had altitude-induced illness are at high risk for recurrence

18

Acute mountain sickness (AMS)

  • Can be prevented by...
  • What may accelerate acclimatization
  • Treatment 

  • Can be prevented by...
    • Slow ascent (no more than 1000 ft/day once you reach 8000 – 10,000 feet)
  • What may accelerate acclimatization
    • Daytrips to higher elevation with return to lower elevation at night (“climb high, sleep low”)
    • Adequate hydration may also be helpful
  • Treatment
    • Descent
    • Dexamethasone (a corticosteroid which reduces cerebral edema)
    • Acetazolamide (a carbonic anhydrase inhibitor)
      • Probably exerts its beneficial effects by inhibiting sodium and bicarbonate reabsorption in the proximal tubule, thereby promoting bicarbonate and sodium excretion
      • Bicarbonate wasting improves serum alkalemia, and sodium excretion reduces brain edema
    • These medications are also effective prophylaxis in subjects with prior AMS who are unable to ascend slowly
    • Acetazolamide, but not dexamethasone, accelerates acclimatization in addition to improving AMS symptoms

19

High altitude cerebral edema (HACE)

  • HACE 
  • Symptoms 
  • Therapy

  • HACE
    • Least common form of high altitude illness
    • Life threatening if not rapidly recognized and treated
  • Symptoms
    • May begin several days after ascent
    • Include headache, loss of coordination, confusion and coma
    • Autopsy studies show cerebral edema, microhemorrhages and brain herniation
  • Therapy
    • No good studies of therapy exist
    • Oxygen and dexamethasone are commonly prescribed
    • Descent as soon as possible is the most important intervention

20

Periodic breathing of altitude

  • Both alkalosis and hypoxia contribute to high-altitude periodic breathing during sleep, a form of Cheyne-Stokes respiration
  • Overstimulation of the carotid chemoreceptors leads to hyperpnea (hyperventilatory period) followed by compensatory apnea which may awaken the individual from non-REM sleep

21

High altitude pulmonary edema (HAPE)

  • Non-cardiogenic pulmonary edema is frequently associated with...
  • HAPE
  • HAPE vs. AMS and HACE
  • Risk factors for HAPE

  • Non-cardiogenic pulmonary edema is frequently associated with...
    • Rapid ascents above 12,000 feet.
  • HAPE
    • Can be fatal
    • Responsible for the majority of deaths due to high altitude disease
  • HAPE vs. AMS and HACE
    • Approximately half of subjects with HAPE also have symptoms of AMS
    • The hypoxia associated with HAPE worsens AMS and may predispose to progression to HACE
  • Risk factors for HAPE
    • Male gender
    • Cold ambient temperatures
    • Vigorous exertion
    • Conditions associated with pre-existing pulmonary blood flow abnormalities
      • Primary pulmonary hypertension
      • Left to right intracardiac shunts

22

High altitude pulmonary edema (HAPE)

  • HAPE is characterized by...
  • Together, these lead to...
  • Symptoms
  • Physical findings 
  • Most susceptible

  • HAPE is characterized by...
    • Markedly elevated pulmonary artery pressures
    • Exaggerated and uneven pulmonary vasoconstriction
    • Inadequate production of endothelial nitric oxide
    • Over production of endothelin
  • Together, these lead to...
    • Regional overperfusion
    • Breakdown of the alveolar capillary barrier
    • Patchy pulmonary edema
  • Symptoms
    • Typically appear two to four days after arrival at a new altitude
    • Include...
      • Dyspnea out of proportion to exertion or that doesn’t improve with rest
      • Cough
      • Production of frothy or rusty sputum
  • Physical findings
    • Tachypnea, crackles, jugular venous distention, and in severe cases cyanosis
  • Most susceptible
    • Children and people with prior episodes

23

High altitude pulmonary edema (HAPE)

  • Genetic factors that influence susceptibility to HAPE
  • Prophylactic therapy

  • Genetic factors that influence susceptibility to HAPE
    • Differences in the structure or function of sodium channels expressed on type II pneumocytes, which transport fluid out of the alveolar space
    • Impaired sodium channel function makes it more difficult for sodium (and thus water) to traverse the alveolar epithelium and be absorbed into the blood
    • Polymorphisms in the endothelial nitric oxide synthase gene, the angiotensin converting enzyme gene and certain human leukocyte antigens
  • Prophylactic therapy
    • Beta-agonists, which increase sodium channel expression in lung epithelium

24

High altitude pulmonary edema (HAPE)

  • Best preventive measure
  • Current therapy of HAPE 
  • Medications

  • Best preventive measure
    • Slow ascent (as with AMS)
  • Current therapy of HAPE
    • Descent
    • Portable hyperbaric oxygen chamber (Gamow bag)
    • Supplemental oxygen, if available
  • Medications
    • Nifedipine (a calcium channel blocker), tadalfil or sildenafil (phosphodiesterase-5 inhibitors) and beta-agonists are all used empirically, since they would be predicted to decrease PA pressures in HAPE-susceptible individuals, though few clinical trials of treatment exist
    • These same medications have proven helpful for prophylaxis of HAPE, although studies are generally small

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