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Adaptation takes place not only when going to high altitude, as generally accepted, but also when going down to sea level. Immediately upon ascent to high altitude, the carotid body senses the lowering of the arterial oxygen partial pressure due to a diminished barometric pressure. High altitude adaptation is defined as having three stages: 1) acute, first 72 hours, where acute mountain sickness (CMS or polyerythrocythemia) can occur; 2) subacute, from 72 hours until the slope of the hematocrit increase with time is zero; here high altitude subacute heart disease can occur; and 3) chronic, where the hematocrit level is constant and the healthy high altitude residents achieve their optimal hematocrit. In the chronic stage, patients with CMS increase their hematocrit values to levels above that of normal individuals at the same altitude. CMS is due to a spectrum of medical disorders focused on cardiopulmonary deficiencies, often overlooked at sea level. In this study we measured hematocrit changes in one high altitude resident traveling several times between La Paz (3510 m) and Copenhagen (35 m above sea level) for the past 3 years. We have also studied the fall in hematocrit values in 2 low-landers traveling once from La Paz to Copenhagen. High altitude adaptation is altitude and time dependent, following the simplified equation: Adaptation=Time/Altitude where High altitude adaptation factor=Time at altitude (days)/Altitude in kilometers (km). A complete and optimal hematocrit adaptation is only achieved at around 40 days for a subject going from sea level to 3510 m in La Paz. The time in days required to achieve full adaptation to any altitude, ascending from sea level, can be calculated by multiplying the adaptation factor of 11.4 times the altitude in km. Descending from high altitude in La Paz to sea level in Copenhagen, the hematocrit response is a linear fall over 18 to 23 days.

Chronic Mountain Sickness (CMS) patients have repeatedly been found to hypoventilate. Low saturation in CMS is attributed to hypoventilation. Although this observation seems logical, a further understanding of the exact mechanism of hypoxia is mandatory. An exercise study using the Bruce Protocol in CMS (n = 13) compared to normals N (n = 17), measuring ventilation (VE), pulse (P), and saturation by pulse oximetry (SaO(2)) was performed. Ventilation at rest while standing, prior to exercise in a treadmill was indeed lower in CMS (8.37 l/min compared with 9.54 l/min in N). However, during exercise, stage one through four, ventilation and cardiac frequency both remained higher than in N. In spite of this, SaO(2) gradually decreased. Although CMS subjects increased ventilation and heart rate more than N, saturation was not sustained, suggesting respiratory insufficiency. The degree of veno-arterial shunting of blood is obviously higher in the CMS patients both at rest and during exercise as judged from the SaO(2) values. The higher shunt fraction is due probably to a larger degree of trapped air in the lungs with uneven ventilation of the CMS patients. One can infer that hypoventilation at rest is an energy saving mechanism of the pneumo-dynamic and hemo-dynamic pumps. Increased ventilation would achieve an unnecessary high SaO(2) at rest (low metabolism). This is particularly true during sleep.

Chronic mountain sickness (CMS) is a condition in which hematocrit is increased above the normal level in residents at high altitude. In this article we take issue with the "Consensus Statement On Chronic And Subacute High Altitude Diseases" of 2005 on two essential points: using a questionnaire to evaluate the symptoms of CMS to use the term "loss of adaptation" as opposed to "adaptation to disease in the hypoxic environment". We opine that CMS is rather an adaptive reaction to an underlying malfunction of some organs and no specific symptoms could be quantified. To substantiate our line of reasoning we reviewed 240 CMS cases seen at the High Altitude Pathology Institute in La Paz. Patients who had a high hematocrit (<58%) underwent pulmonary function studies in search for the cause of hypoxia: hypoventilation, diffusion alteration, shunts, and uneven ventilation-perfusion. The tests included arterial blood gas tests, chest x-rays, spirometry, hyperoxic tests, flow-volume curves, ventilation studies at rest and during exercise, ECG, exercise testing and doppler color echocardiography to assess heart structure and function. When correlated with clinical history these results revealed that CMS is practically always secondary to some type of anomaly in cardio-respiratory or renal function. Therefore, a questionnaire that tries to catalog symptoms common to many types of diseases that lead to hypoxia is flawed because it leads to incomplete diagnosis and inappropriate treatment. CMS, once again, was shown to be an adaptation of the blood transport system to a deficient organs' function due to diverse disease processes; the adaptation aimed at sustaining normoxia at the cellular level in the hypoxic environment at high altitude.

Pulse oximetry during breath-holding (BH) in normal residents at high altitude (3510 m) shows a typical graph pattern. Following a deep inspiration to total lung capacity (TLC) and subsequent breath-holding, a fall in oxyhemoglobin saturation (SaO(2) is observed after 16 s. The down-pointed peak in SaO(2) corresponds to the blood circulation time from the alveoli to the finger where the pulse oximeter probe is placed. This simple maneuver corroborates the measurement of circulation time by other methods. This phenomenon is even observed when the subject breathes 88% oxygen (PIO(2) = 403 mmHg for a barometric pressure of 495 mmHg). BH time is, as expected, prolonged under these circumstances. Thus the time delay of blood circulation from pulmonary alveoli to a finger is measured non-invasively. In the present study we used this method to compare the circulation time in 20 healthy male high altitude residents (Group N with a mean hematocrit of 50%) and 17 chronic mountain sickness patients (Group CMS with a mean hematocrit of 69%). In the two study groups, the mean circulation time amounted to 15.94 +/-2.57 s (SD) and to 15.66 +/-2.74 s, respectively. The minimal difference was not significant. We conclude that the CMS patients adapted their oxygen transport rate to the rise in hematocrit and blood viscosity.