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Blood Gases
Blood Gas Basics
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| Blood Gases............................... $49 |  |
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A quick calculation of the Aa gradient tells you that this patient’s “normal” oxygen is not normal at all — it’s depressed for her level of hyperventilation. Suspecting pulmonary embolism, you order a lung scan.
Time 3 p.m. The paramedics bring in a boy found hanging from the swing set. Rhythm is initially asystole, improving to sinus tach with epi and atropine. Pulse is very weak, and he remains unresponsive. Initial ABG’s return with a pH of 6.91. Using the boy’s weight and the base excess on the blood gases, you calculate an accurate and safe acidosis correction dose of sodium bicarbonate.
Time 4:30 p.m. Another tricyclic OD. ABG’s show everything “out of whack.” Using a simple method, you arrive at a diagnosis of “partially compensated respiratory alkalosis.”
This software can help you get more clinical information from arterial blood gas analysis. It’s intended for the physician who must make quick decisions based on ABG’s, or the critical care nurse who wants to expand his or her understanding of blood gases. Our emphasis will be on rapid, easily remembered, “quick ‘n dirty” methods — those methods most useful when the pressure is on and the books aren’t around. Back to Top
Why are blood gases done?
There are four good reasons for obtaining blood gases: 1) to assess the oxygenation capacity of the lungs for diagnostic reasons, 2) to assess the oxygen pressure in the blood for therapeutic reasons, 3) to assess respiratory adequacy, and 4) to assess acid-base status.
1) assessment of oxygenation capacity
An example of this indication for drawing blood gases is the post-op patient with pleuritic chest pain. If the oxygenation capacity (as determined by calculating the arterial-alveolar (Aa) gradient) is absolutely normal, this is very strong evidence against a pulmonary embolism. Under these circumstances, blood gases would be drawn on room air to assure an accurate arterial-alveolar gradient.
Assessment of oxygenation can be valuable for a hyperventilating patient — by proving to the patient that his lung function is normal, i.e. a “therapeutic” blood gas.
2) assessment of oxygen pressure to guide therapy
Oxygen is toxic. High inspired concentrations of oxygen can damage lungs and eyes. For example, in the premature infant with lung disease, repeated blood gas determinations are performed so that the lowest possible inhaled oxygen concentration can be used that maintains the blood oxygen pressure at a level that keeps the infant alive. (In most modern hospitals, a pulse oximeter is used for routine oxygenation monitoring. Blood gases are used as a baseline, and to monitor carbon dioxide retention and acid-base balance.)
Similarly, suppose you have a patient with a weak heart who’s on a ventilator with positive airway pressure for ARDS (adult respiratory distress syndrome). The positive airway pressure gets more oxygen into his blood, but decreases the venous return to his heart. For two days you’ve been fighting a battle between inadequate cardiac output and inadequate oxygenation: increase his CPAP (continuous positive airway pressure), his cardiac output falls, he turns blue, but his oxygen looks great; decrease his CPAP, his oxygen falls down, he turns blue, but his cardiac output is great. Improvement of blood gases may allow you to decrease the airway pressure, getting the patient safely off his dangerous therapeutic tightrope.
3) assessment of respiratory adequacy
When the decision is made to override the body’s respiratory regulation system by intubating and artificially ventilating the patient, the blood gas machine must take the place of the carotid body chemical receptors. The level of arterial carbon dioxide and oxygen provide information on whether the rate or depth of ventilation, ventilator dead space, or airway pressure must be changed to preserve the patient’s normal physiologic balance.
4) assessment of acid-base balance
The measurement of serum pH and carbon dioxide pressure (and subsequent calculation of HCO3-) provide the means for identifying the presence of many diseases, especially when combined with determination of serum electrolytes. For example, the presence of severe acidosis in a comatose patient completely changes the clinical approach and subsequent evaluation. And of course, blood gases are used to monitor the therapy of acidosis in the unstable patient. Back to Top
Why not get blood gases?
There are a lot of good reasons NOT to get blood gases. Perhaps the most important reason NOT to get blood gases is when the results won’t change what you’re going to do. For example, getting blood gases on every young asthmatic who comes to the ER is stupid. What’s it going to change? If you just want to prove he’s wheezing, or “monitor his response to therapy,” get a PEFR (peak expiratory flow rate).
On the other hand, getting blood gases on a young asthmatic after he’s had full doses of every possible therapy and is looking increasingly tired and blue is smart (if only to prove to his lawyers why you undertook the aggressive action of intubation and mechanical ventilation — when he gets a tension pneumothorax in the night and dies), even though your decision is based on the clinical picture.
Another reason NOT to get blood gases is “because the policy says to.” Protocols can be useful in helping you remember a solid, practical approach to a specific problem. But they should remain guidelines. Standard therapies and standard evaluation strategies should not be written into policies that force you to obtain unnecessary tests, and that can be used to “hang” you legally if — for very valid clinical reasons — you do not follow a long-forgotten obscure “policy” buried somewhere in that shelf of three-ring binders in the administration offices.
Another reason NOT to get blood gases is when something else less expensive or less unpleasant will get you the same information. Pulse oximeters have replaced the need for repeated sampling in cases where the primary interest is adequate oxygenation of the blood. Blood gases should also not be drawn where there’s a possibility of complications that outweighs the benefits of the test. For example, sticking a needle through the artery of a hemophiliac just to show he’s hyperventilating might not be such a good idea. Back to Top
How are blood gases done?
Blood gases are drawn from an artery with a small needle attached to a syringe. The syringe contains a small amount of heparin to prevent clotting of the blood. If possible, the sample is obtained from the radial artery at the wrist. When repeated samples are necessary, an “arterial line” (a small catheter left inside the artery) is used. In newborns, the samples would be obtained through an umbilical artery catheter. When the radial pulse is unobtainable (blood pressure less than 60 systolic) the sample is usually obtained from the femoral artery in the groin.
Prompt flow of blood into the syringe (without pulling back on the plunger) shows that the sample is truly from the artery. However, during CPR the sample must often be aspirated from the femoral artery.
The sample is corked off immediately to prevent exposure to room air, and placed in ice. The site of the arterial puncture is kept under pressure (to prevent hematoma formation) for about ten minutes.
The sample is placed in a machine that measures the pH, partial pressure of oxygen (PaO2), and carbon dioxide (PaCO2). Usually the hemoglobin is measured in a separate machine. These are the “measured values,” from which other values are calculated. “Controls" (solutions of known pH) are used to insure the accuracy of the measurements.
Readings may need to be adjusted for the patient’s temperature. Oxygen, carbon dioxide, and pH are all affected by a change in patient temperature. The machine is calibrated for measurements on a patient with a temperature of 37.0 centigrade. Measurements must be adjusted for temperature because:
1) Increased temperature changes the volume of the dissolved gas (not CO2 that has been converted to HCO3-) in the serum and red blood cells. The ideal gas equasion shows that volume of a gas (V) and temperature (T) are directly proportional.
PV = nRT
2) Increased temperature increases the vapor pressure of water, effectively lowering the partial pressure of oxygen in the blood stream.
3) Increased temperature changes the chemical reactions involving buffer solutions — the pK or dissociation constant is temperature dependant. Back to Top
What values are returned from ABGs?
Only three to five values are actually measured when a blood gas analysis is performed. All remaining values are calculated from these measured values. The complexity of information returned to the clinician depends on the amount of computations performed.
At a minimum, the analysis will list values for pH, PaO2, and PaCO2 (these are measured directly), and serum bicarbonate (which is calculated). Most labs will also return values for hemoglobin and base excess.
Oxygen saturation can also be directly measured. Some labs, however, simply list the calculated saturation. Others will list both a measured and a calculated saturation. A large difference in these two values may indicate carbon monoxide poisoning.
Some labs routinely measure carboxyhemoglobin (carbon monoxide-complexed hemoglobin) with every blood gas.
An arterial-alveolar gradient may also be calculated for you by some newer blood gas software. The critical clinical information to be derived from the typical blood gas is: pH, PaO2, PaCO2, HCO3, base excess, Aa gradient, and sometimes hemoglobin. Back to Top
The myth of normal values
My practice of emergency medicine spans three hospitals. Each has a different list of “normal ranges” on their blood gas slips. The values chosen as limits of normal are often arbitrary.
For example, is a person hyperventilating when he hits an arterial carbon dioxide of 34, or do you call 32 abnormal? Do you decide what’s a normal oxygen by measuring a whole bunch of people and using a standard deviation method? Or do you mathematically determine the lowest oxygen at which a person with a carbon dioxide of 40 could still have a normal arterial-alveolar (A-a) gradient?
Our lab (Salt Lake City) says 70 is the lower limit of normal oxygen. Yet a person with a “normal” A-a gradient of 10 and “normal” carbon dioxide of 40 will have an oxygen of 65! (The arterial-alveolar gradient means that the lungs are normal, making the “abnormal” oxygen meaningless.) Furthermore, a person can have a “normal” carbon dioxide, a “normal” oxygen, and yet have abnormal lungs as determined by the arterial-alveolar gradient calculation.
So what good are normal values? Well, they do provide a quick “ballpark” answer about the patient’s status. But you must always keep in mind that you are evaluating the patient’s respiratory status, not just his oxygen. You are evaluating his acid-base balance, not just his pH. Proper interpretation of the measured values on the Blood Gas test will tell you about the health of the lungs and the adequacy of ventilation, and about the nature of any acid-base disturbance. Back to Top
Back to Blood Gases Manual Index
Copyright 1996 Mad Scientist Software
Citation:
Argyle, B., Blood Gases Computer Program Manual.
Mad Scientist Software, Alpine UT, 1996.