Section A


Table of Contents

Contents of Section A

Intern's Credo. .

15 Laws of Hospital Practice

10 Laws of Outpatient Practice

5 Rules for Lab Tests

6 Medical Truisms

10 Medical Cliches

1 Useful Mnemonic

15 Once-Difficult Diagnoses

8 Bedrock Lab Tests

10 Rules for Reading Chest X-rays

15 Rules on Oxygen Therapy

6 Signs of Obvious Breathing

10 Occupational Hazards for Physicians

              INTERN'S CREDO                  
              There's no admission like        
              no admission.                     



1. (Wherever you are. . .) If it happens here it happens
2. Money not spent on the terminally ill will not go to feed
hungry children. (This law is a corollary of: money not spent
on military hardware. . .)
3. There is always family. When a patient is admitted with "no
family" you just haven't found them yet.
4. When a consensus is needed for DNR status, there's always
a key family member who is out of town.
5. If you order a CAT scan and a weight measurement on the
same patient at the same time, the former will be done and
charted before a scale is found.
6. Hospitalized patients can invariably be divided into two
distinct groups. One group wants to go home as soon as
possible and the other group never wants to leave.
7. The medical literature deals with patients who have only one
disease. Most of your patients have more than one disease.
8. The number of layers of clothing through which the chest is
auscultated directly correlates with the house officer's level of
9. After a patient is in the hospital for three days no one will
know the patient's complete list of medications.
10. Any chart note worth writing is worth being legible; if it can't
be quickly and easily read by a peer, it might as well not have
been written.
11. If God wanted us to have plastic tubes we'd be born with
them: use it or remove it.
12. Dyspnea in a hospitalized patient is never due to anxiety. If
a physician attributes dyspnea to anxiety the physician is
either a fool or has completed an exhaustive workup for
causes of dyspnea. Most of the time the physician is a fool.
13. The top three causes of fever in a hospitalized patient are:
infection, infection, infection.
14. When you assume another cause of fever remember the
previous law.
15. In 1920 about 90% percent of all diagnoses were made with
just a thorough medical history; in the 1990s the percentage
is the same. Ditto for the next century.


1. A drug proven effective after multi-center trials costing
millions of dollars doesn't work very well if patients don't
take it.
2. No study has ever shown that an outpatient can reliably ingest
more than three drugs for more than three weeks.
3. No study has even shown the efficacy of more than three
drugs taken simultaneously.
4. The drugs you think your patient is taking are never exactly
the same as what your patient is taking.
5. Drugs for which blood monitoring is necessary should be
avoided unless unequivocally needed - they all have a narrow
therapeutic window. Examples: theophylline, digoxin,
coumadin, procainamide.
6. The more complicated your patient, the more likely your
patient is seeing other physicians for the same or related
7. Outpatients can invariably be divided into two groups: those
with no questions, and those who always have one more
8. An appointment every three months for a patient with two or
more diseases taking two or more medications is about as
helpful as no appointment at all.
9. Often the best treatment for a patient with chronic disease is
the doctor's phone number.
10. You will be retired before your hospital's outpatient
department implements a really useful record keeping system.

(NOTE: chest x-rays have their own set of rules)

1. Don't order a test if the outcome will have no effect on the
management of your patient.
2. If the ordering of a lab test is equivocal, pretend you are the
one paying for it. Then make your decision.
3. If a test result surprises you because it is abnormal, consider
repeating it or obtaining a complementary test (e.g., CRP for
ESR). If the test is abnormal a second time believe it.
4. Only people can lie. Lab tests are not people. Lab tests don't
5. A hundred things can screw up a lab test result, from
improper labeling to improper data transcription. The better
you know your patient the better you will recognize a screw up.


1. Not all asthmatics wheeze. However, finding a dyspneic
asthmatic "without wheezing" usually means the patient was
not auscultated during forced expiration.
2. Daily weights are seldom accurate because no one pays for
them. A "weight service" that could charge per patient would
maintain the scales and provide accurate daily weights.
3. Chronic complainers, hypochondriacs and Munchausen
patients can contract the same diseases as everyone else.
4. The most underrated piece of medical equipment is the fax
5. American hospitals have 19th century record keeping systems
because they spend 19th century dollars on the problem.
6. An on-time patient deserves an on-time doctor.

(some of which are true some of the time)

10. Not charted not done.
9. Doctors make the worst patients.
8. Doctors' relatives have the most complications.
7. Doctors are lousy investors.
6. Only nice people get cancer.
5. Nasty people don't get bad diseases.
4. See one, do one, teach one.
3. Doctors bury their mistakes.
2. The patient is the one with the disease.
1. If something can go wrong, it will.

1 Useful Mnemonic

MUDPILES (for causes of increased anion gap)

Diabetic ketoacidosis
Paraldehyde, Phenformin, Propylene glycol
Iron, INH
Lactic acidosis
Ethanol, Ethylene glycol
Salicylate, Starvation


In the 19th and part of the 20th century, astute clinicians argued interminably over the following diagnoses. The reason? There was no readily available or convenient method by which to test their clinical observations. How we do it today (in parentheses) seems so simple!

pneumonia vs. no pneumonia (chest x-ray)
hypo- vs. hyper- glycemia (blood sugar measurement)
low vs. high PaCO2 (blood gas measurement)
low vs. normal PaO2 (blood gas measurement)
hypothyroidism vs. euthyroid state (thyroid function tests)
euthyroid state vs. hyperthyroidism (thyroid function tests)
low vs. high serum potassium (serum electrolytes)
big spleen vs. normal spleen (ultrasound; CT scan)
big kidneys vs. normal kidneys (ultrasound; CT scan)
big liver vs. normal liver (ultrasound; CT scan)
acidosis vs. alkalosis (blood gases and serum electrolytes)
increased intracranial pressure vs. normal ICP (CT scan, LP)
seizures vs. pseudo seizures (EEG)
myocardial infarction vs. no MI (EKG, enzymes, echocardiogram)
cardiogenic shock vs. septic shock (Swan-Ganz catheterization)

And the difficult diagnoses today? Each of the following diagnostic considerations often leads to several tests, which are just as often inconclusive or difficult to interpret.

pulmonary embolism (most difficult of the common diagnoses)
cause of non-essential hypertension
chronic fatigue syndrome
any condition of viral etiology


A few tests are universally available and enormously useful. The better a house officer understands these "bedrock" lab tests, and their underlying science, the better care he or she can give patients. As a minimum, any house officer in the primary care fields should be comfortable with interpreting these tests, i.e., should be able to understand the data and apply it to the patient. Sadly, although each test is predicated on time-honored science and physiology, they are not emphasized in medical school teaching. It would behoove any curriculum to instill in students a true understanding of these tests. They are listed alphabetically.

Arterial blood gases (pH, PaO2, PaCO2, SaO2)
Chest x-ray (PA and lateral)
Complete blood count (hematocrit, WBC, differential, platelet
Electrocardiogram (12-lead)
Serum electrolytes, BUN and blood glucose
Sputum analysis (Wright's stain, Gram's stain)


(Especially for House Officers in Clinical Specialties)
1. The only way to learn to read chest x-rays is to read chest x-rays. Daily.
2. Always check the name and date on the film. Five percent of
the time the film will be of someone else.
3. When confronted with an abnormal chest x-ray, always seek
out prior films for comparison. Unless absolutely, positively
certain, always assume that old films are available
4. Don't ignore the lateral film. It can often clarify the presence
or absence of lower lobe disease.
5. "Technique" accounts for much of the variation in serial chest
x-rays. Don't confuse this change (e.g., the amount of x-ray
penetration, or rotation of the patient) with change in disease.
6. Consider everything on the film that might provide some
potentially useful information: endotracheal tube? (respiratory
failure); surgical clips? (thoracotomy); absent breast shadow?
(mastectomy); chest leads? (AP film); large amount of soft
tissue? (obesity); etc.
7. The presence of clear lung fields (no infiltrate, no vascular
redistribution) and normal-sized heart rules out left-sided
congestive heart failure as a cause of the patient's symptoms.
8. Lung fields can never be called clear on a portable chest x-ray
until you've identified the left hemi-diaphragm and lack of
infiltrate behind the heart.
9. The chest x-ray never lies.
10. Ask for help. Help is always available.

15 Rules on Oxygen Therapy -- What Every House Officer
Should Know
(Trust me: this is important.)


1. PO2, SaO2, CaO2 are all related but different.
2. PaO2 is a sensitive and non-specific indicator of the
lungs' ability to exchange gases with the atmosphere.
3. FIO2 is the same at all altitudes.
4. Normal PaO2 decreases with age.
5. Oxygen from the wall outlet is always 100%.
6. The body does not store oxygen.

Therapy & Diagnosis

7. Supplemental O2 is an FIO2 > 21%.
8. Supplemental O2 is a drug.
9. Supplemental O2 is the most commonly-prescribed
drug in hospitals.
10. A reduced PaO2 is a non-specific finding.
11. A normal PaO2 and alveolar-arterial PO2
difference (A-a gradient) do not rule out
pulmonary embolism.
12. High FIO2 doesn't affect COPD hypoxic drive.
13. A given liter flow rate of nasal O2 does not
= any specific FIO2.
14. Face masks cannot deliver 100% oxygen unless
there is a tight seal.
15. Oxygen masks migrate.

1. PO2 , SaO2 , CaO2 are all related but different.

The house officer who understands the difference between

PaO2, SaO2 and CaO2
understands much about gas exchange physiology.

PaO2, the partial pressure of oxygen in the arterial blood, is determined solely by the pressure of inhaled oxygen pressure (the PIO2), the PaCO2, and the architecture of the lungs. The most common physiologic disturbance of lung architecture is ventilation-perfusion (V-Q) abnormality; less commonly, there can be diffusion block or anatomic right to left shunts. If the lungs are normal, then PaO2 is affected only by the alveolar PO2, which in turn is determined by the fraction of inspired oxygen, the barometric pressure and the PaCO2 (see Four Most Important Equations in this book in Section H or at").

PaO2 is a major determinant of SaO2, and the relationship is the familiar sigmoid-shaped oxygen dissociation curve. SaO2 is simply the percentage of available binding sites on hemoglobin that are bound with oxygen in arterial blood. The O2 dissociation curve (and hence the SaO2 for a given PaO2) is affected by PaCO2, body temperature, pH and other factors. However, SaO2 unaffected by the content of hemoglobin, so anemia does not affect SaO2.

CaO2 is arterial oxygen content. Unlike either PaO2 or SaO2, the value of CaO2 directly reflects the total number of oxygen molecules in arterial blood, both bound and unbound to hemoglobin. CaO2 depends on the hemoglobin content, SaO2, and the amount of dissolved oxygen. Units for CaO2 are ml oxygen/100 ml blood (see Four Most Important Equations in Section H).

2. PaO2 is a sensitive and non-specific indicator of the lungs' ability to exchange gases with the atmosphere.
In patients breathing ambient or "room" air (FIO2 = .21), a decreased PaO2 indicates impairment in the gas exchange properties of the lungs, usually signifying V-Q imbalance. PaO2 is a very sensitive indicator of gas exchange impairment; it can be reduced from virtually any parenchymal lung problem, including asthma, chronic obstructive pulmonary disease, and atelectasis that doesn't show up on a chest x-ray.

3. FIO2 is the same at all altitudes.
The percentage of individual gases in air (oxygen, nitrogen, etc.) doesn't change with altitude, but the atmospheric (or barometric) pressure does. FIO2, the fraction of inspired oxygen in the air, is thus 21% (or .21) throughout the breathable atmosphere. PaO2 declines with altitude because the inspired oxygen pressure declines with altitude (inspired oxygen pressure is fraction of oxygen times the atmospheric pressure). Average barometric pressure at sea level is 760 mm Hg; it has been measured at 253 mm Hg on the top of Mt. Everest.

4. Normal PaO2 decreases with age.
A patient over age 70 may have a normal PaO2 around 70-80 mm Hg, at sea level. A useful rule of thumb is normal PaO2 at sea level (in mm Hg) = 100 minus the number of years over age 40.

5. Oxygen from the wall outlet is always 100%.
As oxygen leaves the outlet the FIO2 at that point is 100%. However, unless the patient is connected to a ventilator, or has a tight-fitting face mask, the appliance used to deliver oxygen to the face will always result in an inhaled FIO2. less than 100%.

6. The body does not store oxygen.
Athletes who inhale a few minutes of oxygen, and then return to the playing field, are not benefitted in any physiologic way. If a patient needs supplemental oxygen it should be for a specific physiologic need, e.g., hypoxemia during sleep or exercise, or even continuously (24 hours a day) as in some patients with severe, chronic lung disease.

7. Supplemental O2 is an FIO2 > 21%.
Supplemental oxygen means an FIO2 greater than the 21% oxygen in room (ambient) air. When you give supplemental oxygen you are raising the patient's inhaled FIO2 to something over 21%; the highest FIO2 possible is 100%. To give more oxygen requires a hyperbaric chamber, an expensive piece of equipment found in relatively few hospitals.

8. Supplemental oxygen is a drug.
Like any other drug, it has indications, contra-indications, and side effects. Unlike the situation with most drugs, there are also easily measurable "levels" of oxygen (either PaO2 with a blood gas measurement or SaO2 with a pulse oximeter).

9. Supplemental oxygen is the most commonly prescribed drug in hospitals.
An estimated 1/4 to 1/3 of all patients admitted to a hospital will receive supplemental oxygen at some point. It is the only prescription drug in common use on all inpatient services (except perhaps Psychiatry).

10. A reduced PaO2 is a non-specific finding.
It can occur from any parenchymal lung problem, and only signifies a disturbance of gas exchange (usually due to V/Q imbalance). A low PaO2 should not be used to make any particular diagnosis, including pulmonary embolism.

11. A normal PaO2 and Alveolar-arterial PO2 difference (A-a gradient) do not rule out pulmonary embolism.
About 5% of confirmed cases of PE manifest a normal A-a gradient.

12. High FIO2 doesn't affect COPD hypoxic drive.
The reason a high FIO2 may raise PaCO2 in a patient with COPD is not because the extra oxygen cuts off the hypoxic drive. Modest rise in PaCO2 occurs mainly because the extra oxygen alters V/Q relationships within the lungs, creating more physiologic dead space.

13. A given liter flow rate of nasal O2 does not = any specific FIO2.
The oft-quoted rule that 2 l/min =an FIO2 of 24%, 3 l/min = 28%, etc., is an illusion, based on nothing experimental or scientific. The actual FIO2 with nasal oxygen depends on the patient's breathing rate and tidal volume, i.e., the amount of room air inhaled through the mouth and nose that mixes with the supplemental oxygen.

14. Face masks cannot deliver 100% oxygen unless there is a tight seal.
So-called non-rebreather face masks can deliver an FIO2 up to around 80%. It is a mistake to label a patient with any loose-fitting face mask as receiving an "FIO2 of 100%." (Again, 100% oxygen can only be delivered with a ventilator or tight-fitting face mask.)

15. Oxygen masks migrate.
Face masks are useful for delivering a precise FIO2 (e.g., "venturi" type masks) and a high FIO2 (e.g., non-rebreathing masks), but outside of the ICU or ER (where the patient can be closely watched) masks tend to migrate - either to the patient's forehead, around the neck, to the bed sheets or the floor. For oxygen therapy, use a nasal cannula whenever feasible; it is more apt to deliver oxygen to the patient continuously.


If a patient's breathing is obvious on initial contact (for example, when you first see the patient on walking into the room) it is abnormal. Normal breathing at rest is simply not obvious; one has to look very closely for chest movement to appreciate breathing. Six signs that may make someone's breathing obvious to the observer - all abnormal - are listed below.

10 Occupational Hazards for Physicians Practicing in the United States
(Other countries may present different infectious disease risks; hazards are listed alphabetically)

rare, but AIDS transmission from a patient's blood has been documented

Assault, including murder -
particularly at risk are ED physicians and psychiatrists who see patients with psychosis or a character disorder

Asthma -
mainly from formaldehyde, among pathologists

Infectious hepatitis -
particularly among surgeons

Non-infectious hepatitis -
mainly from halothane, among anesthesiologists

Sclerotherapist's conjunctivitis -
endoscopists occasionally squirt their eyes with sclerotherapy chemical

Stress and burnout -
all physicians at risk, and of course the problem is not unique to the medical profession

Substance abuse -
risk increased because of easy access to drugs

Suicide -
rate is definitely higher in some medical specialities (e.g., psychiatry) than in the general population

Tuberculosis -
particularly among physicians who work with multi-drug resistant TB


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The material at this site is intended for educational purposes and should not be construed as medical advice or instruction.
Consult your health professional for advice relating to a medical problem or condition.
Copyright © 1996 Lawrence Martin, M.D.
Revised: November 17,1996