Section H
Hardcore
Table of Contents
Disclaimer
Note: This section is from The House Officer's Survival Guide: Rules, Laws, Lists and Other
Medical Musings, by Lawrence Martin, M.D. It is written for doctors in training, but will also be of interest to
medical students, nurses who work in critical care areas, and respiratory therapists. Unlike other sections of "Survival Guide",
Hardcore will not be of much interest to the general public. Please address any feedback to
martin@lightstream.net
THE FOUR MOST IMPORTANT EQUATIONS IN CLINICAL PRACTICE
If you believe that simple equations can be important for patient
care, then this section is for you. Listed below are what I
consider the Four Most Important Equations in Clinical Practice.
A more detailed paper on this same subject is also posted on the
Mt. Sinai site. It is called -- what else? --
The Four Most
Important Equations in Clinical Practice
1. The PaCO2 equation
PaCO2 = CO2 produced by metabolism (ml/min) x k/alveolar ventilation (L/min)
where
alveolar ventilation = minute ventilation minus dead space ventilation
k = 0.863.
REASON IMPORTANT: This equation explains why one
cannot assess PaCO2 clinically, i.e., with only bedside
observations such as respiratory rate, depth of breathing, level of
discomfort, breath sounds, etc. There are no clinical variables
in this equation. For example, a patient with fast and/or deep
breathing can still be retaining CO2 if most of the air goes to
dead space (either because of shallow tidal volumes or
ventilation-perfusion mismatch). Since there is nothing clinical
in this equation, one cannot use clinical criteria to assess PaCO2
(i.e., adequacy of alveolar ventilation). A patient may appear to
be "hyper-ventilating" but in fact be hypoventilating, i.e., have
a high PaCO2.
2. The alveolar gas equation
PAO2 = FIO2 (B.P. - 47) - 1.2 PaCO2
where
PAO2 = alveolar PO2
FIO2 = fraction of inspired oxygen
B.P. = barometric pressure
47 = water vapor pressure in airway in mm Hg
Alveolar-arterial PO2 difference [A-a gradient] =
calculated PAO2 - measured PaO2
REASON IMPORTANT: You need the calculated PAO2 to
know if arterial PO2 (PaO2) is abnormal or not. Specifically, the
variables in this equation (B.P., FIO2 and PaCO2) must be known
to properly interpret any PaO2 value. Is a PaO 2 of 28 mm Hg
normal? (Yes, if breathing mountain air at the summit of Mt.
Everest, where B.P. is only 253 mm Hg). Is a PaO2 of 100 mm
Hg abnormal? (Yes, if the patient is breathing 100% oxygen).
Does a PaO2 of 50 mm Hg indicate a problem of ventilation-perfusion mismatch? (Yes, if the A-a gradient is increased; no,
if the A-a gradient is normal, in which case the low PaO2 may be
solely from elevated PaCO2).
3. The Henderson Hasselbalch equation
pH = pK + log HCO3/.03(PaCO2)
REASON IMPORTANT: Helps diagnose presence and type of
acid-base disorder, and the body's compensation for it. The
abbreviated version is adequate in most clinical situations.
Equation shows there are only four primary acid-base disorders:
two where the first change is in HCO3- (metabolic alkalosis and
acidosis) and two where the first change is PaCO2 (respiratory
alkalosis and acidosis).
4. The arterial oxygen content (CaO2 ) equation
CaO2 content = amount O2 bound to hgb + amt.O2 dissolved
= (1.34 x hgb x SaO2) + (.003 x PaO2)
where
O2 content = ml O2 /100 ml blood
1.34 = ml O2 that can maximally bind to a gram of hgb
hgb = hemoglobin content, in grams/dl
SaO2 = saturation of hemoglobin with oxygen in arterial blood
.003 = ml O2 that can dissolve in plasma per mm Hg PaO2 per 100 ml blood
REASON IMPORTANT: Incorporates factors for adequate
arterial oxygen content, most important of which is hemoglobin;
over 97% of arterial oxygen content is normally carried by hgb.
Note that oxygen content is the only readily available value that
directly reflects the number of oxygen molecules in the blood.
In assessing degree of hypoxemia (as opposed to the cause),
CaO2 is more useful than either PaO2 or SaO2.
A patient can be profoundly hypoxemic with a normal PaO2 (for example, from
severe anemia or carbon monoxide intoxication) or with a
normal SaO2 (from severe anemia).
5. And the fifth most important equation?
Let me know your opinion at martin@lightstream.net
Some candidates are below:
What is the Single Most Important Skill to Learn
Early in Your Residency?
This is a trick question because it's a "guess what I'm thinking" type. Early in your residency you need to learn so many things it's kind of ridiculous to argue what is the 'most important'. You might legitimately answer: "How to take a history," or "How to talk to patients," or "How to write orders," or "How to obtain DNR," or "How to Interpret a Chest x-ray/EKG/blood smear, etc." All these skills are important, but they are not what I consider the single most important.
My answer is: How to perform advanced cardiac life support or "ACLS." My reasoning is as follows. If you don't know ACLS, basically what to do in a life-threatening cardiac emergency, and how to do it, the patient can die within minutes. All other skills mentioned are obviously important, and in the career of most physicians will no doubt be much more useful than knowing ACLS.
But as a hospital resident ACLS is the one thing you must know, because it is you who will be called to the cardiac arrest, not your attending physician or the sub-specialist. "Chest team!," "Code blue!" and "Dr. Heart!" all mean the same thing you come. If you don't know what to do when you get there, the patient doesn't have much of a chance.
As a resident you can make mistakes in history-taking or writing orders or chest x-ray interpretation. In a good training program someone will likely bail you out, or you can bail yourself out with a strategic call for help. Time will be on your side. But in the few minutes immediately after a cardiac arrest code is called, your knowledge and skills may be all that stands between the patient surviving and dying. You won't have time to "look it up" or confer with your attending.
ACLS, taught in an intensive one or two day course, is now a requirement of virtually all hospital residents (it is also required of all ICU and CCU nurses). ACLS certification is good for two years, at which time the course is taken again. While the various cardiac treatment algorithms change somewhat over time, the basics do not (AIRWAY, BREATHING, CIRCULATION, DIAGNOSIS).
(In the most recent ACLS manual, the following changes are noted: Bretylium is no longer used; high doses epinephrine are no longer recommended; amiodarone is now recognized as an important anti-arrhythmic drug; and a one-time dose of Vasopressin is now recommended in VF/pulseless VT as an alternative to epinephrine 1 mg.)
You need to know how to quickly insert a peripheral intravenous line and ventilate an apneic patient with a bag-valve mask device (AMBU bag). You need to know how to intubate, should that become necessary. (In many hospitals chest team patients are intubated by an always-available, in-house anesthesiologist. If your hospital is set up this way, fine.)
You need to know where the defibrillator is kept, how to use it, what drugs to use for which arrhythmias and, not least important, when to stop. You need to feel comfortable when a chest team is called. You need to know enough so that if the patient doesn't survive, it is not because you didn't know something, but because there was nothing else to do.
An excellent web site for ACLS review is ACLSnet.
Also useful is:
Resuscitation, Airway Management and Acute Arrhythmias (U. Iowa Virtual Hospital)
Following is a short true-false quiz based on current ACLS teaching.* Answer whether each of the statements is true (T) or false (F). Answers are given later in this section. A passing score is 8 out of 10. Following the quiz are ACLS algorithms for the three types of cardiac arrest: VF/pulseless VT, pulseless electrical activity and asystole.
Abbreviations
ABCs = airway, breathing, circulation (assessment preparatory to basic CPR)
VF = ventricular fibrillation
VT = ventricular tachycardia
AV = atrio-ventricular
PEA = pulseless electrical activity
___________
* ACLS Provider Manual, American Heart Association, 2001.
ACLS QUIZ
T F 1. For VF or pulseless VT, the first treatment is to defibrillate the patient up to three times if either rhythm persists before any drugs are given.
T F 2. Treatment of the ventricular tachycardia rhythm Torsades de Pointes includes quinidine or procainamide.
T F 3. For wide complex tachycardia of uncertain origin, verapamil can be used to separate out a supraventricular from a ventricular focus.
T F 4. Adenocard (adenosine) can be used for wide complex tachycardia of uncertain origin, because it will slow a supraventricular focus and will not harm if the rhythm is ventricular.
T F 5. Pulseless electrical activity is the presence of some type of electrical activity other than VF or VT when a pulse cannot be detected by palpation of any artery.
T F 6. The recommended dose of epinephrine in VF, PEA, and asystole is 1 mg IV push every 3-5 minutes.
T F 7. In VF, unsynchronized (as opposed to synchronized) cardioversion should be avoided since it may precipitate cardiac standstill.
T F 8. The dose of atropine when treating bradycardia is .03 to .04 mg/kg, or approximately 0.5 to 1.0 mg, in repeat doses every 3 to 5 minutes.
T F 9. First degree AV block is a delay in passage of the cardiac impulse from atria to ventricles. Block usually occurs at the level of the AV node but may be infranodal.
T F 10. Unlike Mobitz Type I second degree heart block (Wenckebach), Mobitz Type II second degree heart block most commonly occurs at the level of the bundle branches and is associated with an organic heart lesion.
Cardiac Arrest -- VF & Pulseless VT*
Primary ABCD Survey - Basic CPR & Defibrillation
A. Airway: Open the airway
If no rhythm after 3 shocks:
Secondary ABCD Survey
A. Airway: intubate as soon as possible
Epinephrine 1 mg IV push; repeat every 3-5 min
Resume attempts to defibrillate: 360 J within 30 to 60 seconds
If still no rhythm:
Consider antiarrhythmics:
Resume attemps to defibrillate
Continue: Drug-Shock-Drug-Shock, etc.
___________
*Algorithms from ACLS Provider Manual,
American Heart Association, 2001.
|
Cardiac Arrest -- Pulseless Electrical Activity*
Primary ABCD Survey - Basic CPR & Defibrillation
A. Airway: Open the airway
Secondary ABCD Survey
A. Airway: intubate as soon as possible
5 H'S & 5 T'S
Hypovolemia
Tablets (drug OD, accidents)
Transcutaneous pacing: If considered, perform immediately. Not recommended
for out-of-hospital aystolic cardiac arrest.
Epinephrine 1 mg IV push, repeat every 3-5 minutes
Atropine, 1 mg IV (if PEA rate is SLOW), repeat every 3 to 5 minutes
as needed, to a total dose of 0.04 mg/kg.
Aystole persists. Withhold or cease resuscitative efforts?
___________
*Algorithms from ACLS Provider Manual,
American Heart Association, 2001.
|
1.T 2.F 3.F 4.F 5.T 6.T 7.F 8.T 9.T 10.T
The Great Masquerader (poem)
She was just 17.
Some chest pain
And "my breath is short."
But the lung scan was normal
(Later, it was hedged -- "well, maybe not normal after all.")
Story so good we
Didn't believe the scan.
Squirted dye in her lungs.
Normal result, too.
(Later it was reviewed - "you know, not an optimal study.")
So can't be lung clots
With normal scan
and dye study.
We invented other reasons
For pain and breath shortness.
Sheer invention, that.
She died quick.
The next day, in fact,
Of clots in her lungs.
We should have listened.
To the patient,
And ourselves.
Units Quiz
A personal survey shows that a substantial minority of house
officers are confused by. . . units of measurement. Many house
officers are so accustomed to spouting off numbers without
units that, for want of use, the units are forgotten or never
learned. Below are values exactly as they were reported on
rounds. All values are within normal limits except for the
alcohol level. (In the case of digoxin and theophylline, values
are in the therapeutic range). See how many proper units you
know. Write in the units before checking the answers (on next
page).
A) Hematocrit 40 _________.
B) Sodium 145 __________.
C) Platelet count 260 __________.
D) White cell count 8.4__________.
E) Serum protein 6.3 __________.
F) PaO2 88 ____________.
G) pH 7.39 ___________.
H) BUN 12 ___________.
I) Serum theophylline 12 __________.
J) Cardiac output 4.7 _________.
K) Pulmonary artery pressure 25/12 __________.
L) Central venous pressure 10 ____________.
M) Amylase 45 __________.
N) Alcohol level 226 ___________.
O) Forced vital capacity 3.4 ____________.
P) Hemoglobin 13.6 ___________.
Q) Barometric pressure 760 ____________.
R) Sedimentation rate 46 _____________.
S) Digoxin 1.2 ___________.
T) INR 2.5 ___________.
U) SaO2 98 ___________.
V) CO2 24 __________.
W) PaCO2 55 __________.
X) PTT 32 ________.
Y) Bleeding time 8 _________.
Z) Peak Flow 400 _________.
ANSWERS: UNITS QUIZ
A) Per cent. B) mEq/L or mM/L. C&D) thousand per cubic ml (per
cc). E) grams/dl. F) mm Hg. G) unitless. H) mg/dl. I) mg/L or
ug/ml. J) L/min. K&L) mm Hg or cm H20 (depending on calibration
of monitoring equipment). M) units/L. N) mg/dl. O) liters. P)
grams/dl. Q) mm Hg. R) mm/hour. S) ng/ml. T) unitless ratio. U)
%. V) mEq/L. W) mm Hg. X) seconds. Y) minutes. Z) L/min.
QUIZ: GENERIC VS. BRAND NAMES
House officers commonly order well-known drugs by their brand
name only, but medical articles (not to mention board exam
questions) invariably mention only the generic name. Match the
generic drug with the correct brand name.
Generic Names
1. omeprazole
2. metronidazole
3. clarithromycin
4. cimetidine
5. nizatidine
6. famotidine
7. cisapride
8. nifedipine
9. beclomethasone
10. enalapril
11. flumazenil
12. alendronate
13. flurazepam
14. midozalam
15. albuterol
16. lorazepam
17. diltiazem
18. lovastatin
19. sertraline
20. fluoxetine
------
Brand Names -- match one of the lettered brand names with a generic
name above
A. Inderal--- T. Prozac
B. Versed--- U. Risperdol
C. Brethine--- V. Romazicon
D. Azmacort--- W. Vasotec
E. Biaxin--- X. Relafen
F. Proventil--- Y. Rifampin
G. Ativan--- Z. Flagyl
H. Carafate--- AA. Dalmane
I. ReVersed--- BB. Lasix
J. Cardizem--- CC. Nizoral
K. Prilosec--- DD. Isoptin
L. Vanceril--- EE. Propulsid
M. Haldol--- FF. Axid
N. Azactam--- GG. Elavil
O. Procardia--- HH. Aldomet
P. Tagamet--- II. Pepcid
Q. Klonopin--- JJ. Effexor
R. Catapres--- KK. Mevacor
S. Fosomax--- LL. Zoloft
ANSWERS: 1-K; 2-Z; 3-E; 4-P; 5-FF; 6-II; 7-EE; 8-O; 9-L; 10-W; 11-V; 12-S; 13-AA; 14-B; 15-F; 16-G; 17-J; 18-KK; 19-LL; 20-T
END OF SECTION H
Table of Contents