11. Altitude Flashcards
What is affect on Oxygen
changes secondary tho this
what is blood gas status
At high altitude the partial pressure of oxygen (pO2) is reduced but the percentage of oxygen in the air is unchanged. Thus the inspired oxygen concentration is unchanged at 21%.
Compensatory changes secondary to hypoxia include:
Hyperventilation
Polycythaemia
Increased 2,3-diphosphoglycerate (2,3-DPG)
Increased cardiac output
An alteration in the intracellular oxidative enzymes.
The initial alkalaemia (respiratory alkalosis) is reduced due to increased renal bicarbonate loss, which returns the pH of the cerebrospinal fluid to normal.
Hypothermia may occur at high altitudes because the ambient temperature is considerably lower.
Why does exposure to high concetrations of oxygen cause seixure
how would you calculate vary concentrations of oxygen at diff pp
Exposure to high concentrations of oxygen at high pressure has the potential to cause seizures. The exact underlying pathophysiology is not fully understood. It appears to be the result of direct oxygen toxicity. The increased reactive oxygen species (ROS) and free radical intermediates interact with the neuronal cell plasma membrane. This causes lipid peroxidation at the plasma membrane, resulting in characteristic changes in the EEG.
Assuming that there is minimal alveolar (A) arterial (a) partial pressure difference, one can calculate the approximate PaO2 using the alveolar gas equation.
PAO2 = FiO2 x [Patm - PH2O] - (PaCO2/RQ)
Where: PAO2 = alveolar partial pressure of oxygen, Patm = atmospheric pressure, PH2O = vapour pressure of water, PaCO2 partial pressure of carbon dioxide and RQ = respiratory quotient
Patm = 100 kPa
PCO2 = 5.3 kPa
PH2O = 6.3kPa
PAO2 = FiO2 x [Patm - 6.3kPa] - (5.3kPa/0.8) PAO2 = FiO2 x [Patm - 6.3kPa] - (6.63)
Substituting the values the approximate values for PAO2 and therefore PaO2:
A 0.1 x [240- 6.3kPa] - (6.63) =PAO2 = 16.7 kPa
B 0.15 x [200- 6.3kPa] - (6.63) =PAO2 = 22.43 kPa
C 0.18 x [100- 6.3kPa] - (6.63) =PAO2 = 10.24 kPa
D 0.6 x [120- 6.3kPa] - (6.63) =PAO2 = 61.59 kPa
E 0.92 x [160- 6.3kPa] - (6.63) =PAO2 = 134.8 kPa
Most important change to hypoxia at altitude initially
General changes upon acute exposure to altitude include:
A sudden increase in resting and submaximal heart rate
Increased resting and submaximal ventilation
Increased blood pressure
Increased catecholamine secretion, and
Decreased VO2 max.
These changes result in:
Increased oxygen transport to the tissues
Increased alveolar PO2 with a concomitant decrease in carbon dioxide (CO2) and hydrogen ions (H+)
Increased vascular resistance
Increased lactate production, and
Decreased work capacity.
The stroke volume progressively decreases over a period of a week (reduced plasma volume and left ventricular dysfunction secondary to hypoxia).
The sensors of a reduced PaO2 are the carotid bodies.
Acute exposure to lowered PaO2 and subsequent tissue hypoxia results in a number of haematological changes.
Haemoglobin (Hb) concentration and haematocrit (Hct) have been shown to increase within 24 hours of exposure to altitude. The stimulation of red blood cell (RBC) production occurs as the juxtaglomerular cells within the kidneys stimulate the release of erythropoietin (EPO). However, with a reduction in plasma volume and the lag between EPO secretion and new RBC production, the true initial increases in Hb and Hct actually occur after approximately three to four days of exposure.
In susceptible individuals, acute hypoxia causes cerebral vasodilatation and a rise in intracranial pressure. This can be associated with altitude mountain sickness (AMS) particularly if the ascent is rapid.
Increases in renal bicarbonate excretion in response to a respiratory alkalosis from hyperventilation are less acute than the cardiovascular changes (hours-days).
Increase in cardiac and skeletal muscle capillary density is a chronic change.
At what barometric pressure is insp pp of O2 0
key to survival is
how is breathing aided at high altititudes
<6.3KPa
Hyperventilation
init respons
What reamins constant with increasing altitude
FoO2
Svp of water at body temp
Insp po2 - .21
.21 x baro-6.3
19000m - bar pressure 6.3 = Po2 0
3000% rise in EPO @ 4500
Unwell sob person at altitude
fall in paco2 main effect
shift odc left - increase affinity - more taken up in lungs
Fall in PACo2 =
fall cbf = vascon
