1. Decr FiO2 (ie high altitude, tubing not connected,
etc) (will correct with incr O2)
2. Diffusion
disease (ie parenchymal lung disease, etc) (often will correct with incr O2)
3. V/Q
mismatch (perfusing poorly oxygenated lung, or ventilating poorly perfused
lung) (won’t correct with incr O2)
4. Shunt
(mixing of arterial and venous blood) (won’t correct with incr O2)
5.
Hypoventilation (will correct with incr O2)
Check an A-a
gradient (increased in diffusion disease, V/Q mismatch, and
shunt): [(713 mm Hg x FiO2) –
(PaCO2/0.8)] – PaO2
(shortcut:
if on RA and PCO2 ~40, above works out to: 100 – PaO2)
The allowed
A-a gradient is: (Age/4) + 4
Defined as a change in sputum, change in cough, or
increased dyspnea
Etiology
Dx
Presentation:
·
Fever/hypothermia, rigors, sweats, cough (+/-
sputum), increasing sputum, pleuritic pain, dyspnea/tachypnea, fatigue,
myalgias, abdominal pain, anorexia, & headache
Microbes:
·
S. pneumo, H. influenza, L. pneumophila, C.
pneumoniae, M. catarrhalis, virals
·
In HIV Pts, consider: TB, PCP (esp if CD4 <
200), MAI (esp if CD4 < 100), cryptococcus, pseudomonas
·
In CF pts &
Neutropenics: consider pseudomonas
·
Random associations: Pneumonia and GI Sx with
Legionella (also + HypoNa) & Mycoplasma Pneumonia and incr Bili with
Pneumococcus
Dx:
·
Hx,
CXR (focal infiltrate with CAP
except virals/atypicals/PCP, lobar esp w/ Pneumococcus), BCx & expectorated sputum, Heme8,
Urine Legionella Antigen (only tests
for serogroup 1 = 70% of Legionnaire’s disease; path # 3 form test code 6088)
·
Adjuvant studies: HIV, PPD, induced sputum (sensitivity for PCP ~75%), BAL (sensitivity for PCP > 97%), LDH (incr in 90% of HIV pts w/ PCP), ambulatory desaturation (often seen
with PCP), High Res CT if pt very sick and need to assesds burden of disease
·
Prognosis shown by PORT score (N Engl J Med 1997; 336:243-250)
·
Lady
Windemere syndrome: MAI usually in the RML in elderly
women with suppressed coughs
Rx:
·
Macrolide
+ Cephalosporin or gatifloxacin (adjust for renal
insufficiency) for 7-14 days, consider longer if necrotizing bacteria (S.
aureus, P aeruginosa, Klebsiella, anaerobes)
·
If
PCP, give steroids if PaO2 < 70 or A-a > 35 (Prednisone
40 BID x 5 days, QD x 5 days, and 20 mg QD x 11 days) (decreases the initial
inflammatory response in the alveoli to bacteria killed by ABx)
·
Look
out for exudative effusions (ie: empyema/parapneumonic effusions)
(get decub CXR to check for layering, consider U/S to mark fluid, and tap – see
pleural effusions section below for distinctions between transudates and
exudates)
·
If
suspicious of TB – isolate the pt; can d/c isolation
after 3 neg expectorated sputum or 1 induced sputum
·
Keep
an eye out for ARDS, which would require intubation
with low tidal volumes and permissive hypercapnea (ARDS Net: N Engl J Med 2000;
342: 1301-8) (see section below)
Pulmonary Embolus
Etiology:
·
Immobility, hypercoagulable disorders, recent
surgery, cancer,
Dx:
·
Symptoms
include: dyspnea (73%), pleuritic pain (66%), cough (37%), leg
pain or swelling (27%), hemoptysis (13%); tachypnea >20 (70%), rales (51%),
tachycardia (30%), loud P2 (23%)
·
ABG:
look for an A-a gradient (but don’t be fooled; can have nl RA O2 sats and no
A-a gradient even with a saddle PE!)
·
EKG
may show: SI QIII TIII or right axis deviation, but most often
is normal; most often finding is sinus tachycardia
·
Chest
CT: sensitivity 91%, specificity 97% (may not detect
subsegmental, after 3rd division of pulm arteries)
·
V/Q
Scan: low probability V/Q excludes clinically significant
PE while a high probability virtually rules in PE
·
Clinical
probability (as in everything) is HUGE:
If low clinical prob, and D-dimer is neg, no role for further testing (N
Engl J Med 2003; 349:1227-1235) (but note that our assay here at JHH is
slightly less sensitive than the NEJM paper assay)
If more suspicious, will need to consider LE U/S, CT w/ PE protocol, V/Q
scan, Pulm Angiogram; all of these studies, with the exception of angiography,
rely on pre-test probability (ie, if you’re really worried, a neg CT scan will
only cause you to look for another study, not stop treatment)
·
Can
have pleural effusion (usually exudative, can be
transudative; often bloody)
·
If
considering hypercoag d/o: ATIII, factor V leiden, Prot C/S,
anticardiolipin Ab & RVVT
·
The
diagnostic tree is immense, and further discussion here would
be too excessive
Rx:
·
Anticoag:
Enoxaparin (1 mg/kg SQ BID) ‘preferred’ over UFH (bolus 80 mg/kg
then 18 unt/kg/hr, goal PTT 60-80) (Arch Int Med 2000; 160(2):181-8), transition to Coumadin
·
Make
sure to guaiac before starting anticoag!
·
Duration
of Anticoag:
1st
event due to reversible factors: 3-6 mos, 6 mos preferred
Idiopathic 1st event: 6
mos (Ann Int Med 2003;139(1):19-25)
Hypercoagulable
disorder: likely for life, goals per recommendations for the hypercoag d/o
·
O2
as necessary to maintain adequate saturations
Exudate: Pneumonia, TB, PE, Cancer, see: www.utdol.com/application/image.asp?file=pulm_pix/exudativ.gif
Transudate: CHF, PE, hepatic-hydrothorax, etc: www.utdol.com/application/image.asp?file=pulm_pix/transuda.gif
Thoracentesis:
Fever with a pleural effusion
(“Never let the sun set on an infected pleural effusion”)
1cm thick effusion on u/s or lat decub cxr without known cause
Unilateral effusion in chf exacerbation
Effusions in
chf that don't resolve in 3 days c diuresis (75% of chf effusions resolve
within 48hrs of diuresis)
Poor oxygenation due to unresolving
effusion(s)
Diagnosing Exudate vs Transudate:
Light’s Criteria for Exudate sens spec
fluid/serum protein >0.5 98
83
fluid/serum LDH
>0.6 86 84
fluid LDH > 2/3 upper limit nl 82
89
fluid chol
>60 54
92
fluid chol
>43 75 80
fluid/serum chol > 0.3
89
81
serum - fluid albumin < 1.2 87 92
[NEJM
(2002) 346: 1971 – 77]
Acute Respiratory Distress Syndrome (ARDS)
Defined as:
widespread bilat infiltrates, PaO2/FiO2 < 200, no evidence of incr LA
pressure (ie PCWP < 18 mmHg)
See: ARDS
Net: N Engl J Med 2000; 342: 1301-8
Etiology:
·
A
very abbreviated list:
Sepsis, pancreatitis, aspiration, pulmonary infection, drugs, burns, etc
Dx:
·
Hx,
CXR, ABG, eval of LA pressure clinically or invasively
Rx:
·
In contrast to conventional ventilation, use higher PEEP, lower TV (6 cc/kg), and
permissive hypercapnea as long as pH is maintained at a reasonable level (usually anything > 7.15 or so) to
minimize volutrauma and barotrauma to alveoli
·
Treat
the underlying cause
·
“Salvage”
Therapy (little evidence, but the pt is going nowhere good
fast so try something!): Proning
(decr V/Q mismatch), Recruitment
(PEEP 40 x 40 sec to open up some extra alveoli), Inhaled NO (selective vasodilation; goal dose is < 10 ppm), Paralysis (decr O2 requirement), High Frequency Oscillator (the extreme
of high PEEP and low VT)
Indications for intubation:
·
Hypoxemia, Hypercapnea w/acidosis,
Metabolic Acidosis, Apnea, Fatigue, Airway Protection
Most Often Used Modes of Ventilation:
·
Volume Cycled
1) A/C: (MICU likes this)
- Set: rate (min),
FiO2, Vt, PEEP
- Each breath (mandatory or spontaneous) receives the
full Vt
- Bad things: if pt breathing quickly ->
respiratory alkalosis; and hyperinflation ->
autoPEEP & hypotension
2) SIMV: (SICU likes this)
- Set: rate (min), FiO2, Vt, PEEP
- full Vt only for the mandatory breaths, spontaneous
breaths get Vt based on effort
- spont breaths against high resistance (of tubing)
causes incr work of breathing, so set PS if need be
·
Starting Parameters:
FiO2
100%, wean to maintain PaO2 > 60
Vt
6-12 cc/kg (unless worry of ARDS, then 6 cc/kg)
RR
10-15 breaths per min
PEEP
5 cm (make sure you maintain plateau pressures < 30 cm)
Weaning:
·
Predictors include: the original issue
precipitating intubation is resolving/resolved, MV < 15, RR < 35, Vt >
325 cc
·
Tobin Index (RR/Vt) < 105 = 80%
chance of success; > 105 = 90% chance of failure (N Engl J Med
1991; 324: 1445 - 50)
