The Fizz of CO2 - Respiratory Acid Base Disorders

Описание: Carbon Dioxide is a gas that is produced by the mitochodria and passes through the cell membrane into the extracellular fluid and blood. There it dissolves, attaches to hemoglobin or, under the influence of carbonic anhydrase, hydrates with water to generate carbonic acid - which rapidly dissociates to release hydrogen (bound to hemoglobin) and bicarbonate. Carbon Dioxide obeys Dalton's law and Henry's law. The latter determines that the PCO2 is directly proportionate to the CO2 content. Carbon Dioxide becomes more soluble in the blood as temperature falls. Hence measuring gaseous CO2 requires the blood gas machine to be set at 37 degrees.
The body produces, at rest, 200ml per minute of CO2. The body excretes 200ml per minute of CO2. As metabolism increases, respiratory excretion of CO2 increases. This results in a PaCO2 of 40mmHg or 5.1kPa. There is a 3-4mmHg or 0.5kPa difference between the PaCO2 and the etCO2. Because the body exists, usually, is steady state, the etCO2 can be used to estimate the PaCO2 (most of the time).
In apnea, the PaCO2 rises rapidly - it doubles in 8 minutes. When PaCO2 rises, [HCO3-] rises also - and in a very predictable way. So, when a patient develops acute respiratory failure, or underventilates (for example under anesthesia), pH falls, predictably, the PaCO2 rises, predictably and the Bicarbonate rises, predictably. This is acute respiratory acidosis - and in this tutorial I will explain how and why this occurs. It is imperative to understand that CO2 and [HCO3-] are different versions of the same thing in the body and the rise in bicarbonate in respiratory disorders is not some form of "compensation" it is physiology. Indeed in chronic respiratory failure, the increase in respiratory acids (Chronic respiratory acidosis) is counterbalanced by a fall in the plasma Chloride levels.
Acute respiratory alkalosis is associated with pain, anxiety, agitation or over ventilation and is associated with a modest fall in Bicarbonate.
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Contents
00:13 Introduction, Review and Preview
01:45 Clinical Scenario - Apneic Patient
02:45 Henry's Law and CO2
03:50 CO2 and Temperature
05:47 You Must Measure PCO2 at 37 Degrees Celcius
08:00 CO2 and Bicarbonate Production
08:42 CO2 Content of Blood
09:30 PCO2 and Minute Ventilation
11:35 The Rise in PaCO2 in Apnea
12:52 Respiratory Acid Base Disorders
14:10 Respiratory Acidosis
14:52 Hypercarbia, Rise in [H+] and Fall in pH
15:22 Clinical Scenario Revisited
16:20 Actual Bicarbonate (i.e. the calculated Bicarbonate)
17:50 How to Calculate Bicarbonate in Acute Respiratory Failure (hypercarbic acidosis)
18:35 Henderson Hasselbalch
19:56 Rule of Thumb for Hypercarbia-Bicarbonate in Acute Respiratory Acidosis
21:11 How to Calculate Bicarbonate in Chronic Respiratory Failure (hypercarbic acidosis)
22:53 Rule of Thumb CO2-Bicarbonate in Chronic Respiratory Acidosis
24:30 Hypochloremia and Chronic Respiratory Failure
24:50 Standard Bicarbonate
25:30 Respiratory Alkalosis
26:13 Rule of Thumb - Bicarbonate in Acute Respiratory Acidosis
27:17 Review and Preview