15 The Effects of Pregnancy, Altitude, and Diving on the Respiratory System Flashcards
1
Q
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
A
- 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
Q
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
A
- 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
Q
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
A
- 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
Q
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
A
- 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
Q
Diving
- Gas in the compressed lung
- Partial pressures of gas in the compressed lung
- Henry’s law
A
- 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
Q
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…
A
- 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
Q
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
A
- 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
Q
Breath-hold diving
- A
- B
- C
- D
- Abbreviations
- PAO2
- PACO2
- PaO2
- PaCO2
- PvO2
- PvCO2
A
- 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
Q
Non-breath-hold diving
- Since breathing air from the surface is not an option, ventilation during diving…
- The most commonly used breathing support system
A
- 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
Q
Complications of diving:
Barotrauma
- This term refers to…
- Barotrauma is most likely to occur…
- As ambient pressure falls, lung volume…
- Pneumothorax
A
- 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
Q
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
A
- 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
Q
Complications of diving:
Nitrogen narcosis
A
- 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
Q
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
A
- 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
Q
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…
A
- 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
Q
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…
A
- 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)
