Lab and Arterial Blood Gas Interpretation
Venous Blood Gas
pH .03 lower
PvCO2 5.8 higher
PvCO2 less than 45 on room air rules out hypercarbia (Journal of Emergency Medicine Volume 28, Issue 4 , May 2005, Pages 377-379)
Venous is as good as arterial blood gases in DKA (Emerg Med Austr 2006;18:64)
pH was almost identical, bicarb was close enough
Central (SVC) pH, bicarb, BE, and lactate agrees with arterial (Emerg Med J 2006;23:622)
New method can calculate an ABG from a VBG (Emerg Med J 2009;26:268)
VBG is just fine for COPEDers for pH, HCO3 and CO2 (Eur J Emerg Med. 2010 Oct;17(5):246-8. )
Lactate
A normal venous lactate rules out an elevated arterial, an elevated venous does not necessarily correlate with an elevated arterial. Tourniquet time may contribute to this inaccuracy. If you do venous lactate, draw as first tube and put on ice immediately. (Annals EM 1997. Vol 29. Number 4)
Relative hyperlactatemia is still associated with mortality (>0.75 mmol/L) Crit Care 2010;14:R25
reanaylyzed in healthy subjects. no effect from tourniquet time or room temp holding if analyzed in 15 min or less (Acad Emerg Med 2007;14:587)
both arterial and venous lactates performed within 10 minutes. Collected data included injury mechanism, demographics, admission vital signs, disposition, length of stay, hospital outcomes and injury severity score. The mean arterial lactate concentration was 3.11 mmol/L (SD 3.45, 95% CI 2.67-3.55) and the mean venous lactate concentration was 3.43 mmol/L (SD 3.41, 95% CI 2.96-3.90) demonstrating no significant differences between the two sources of blood lactate. The correlation between venous and arterial lactate levels was 0.94 (Lavery RF, Livingston DH, Tortella BJ, Sambol JT, Slomovitz BM, Siegel JH. The utility of venous lactate to triage injured patients in the trauma center. J Am Coll Surg. 2000;190(6):656-664.)
Adams et al. included all ED patients over a seven month time period in whom a lactate level was measured for any reason. They considered an AG >12 abnormal and conducted sensitivity analyses of the AG for detecting the presence of a lactate >2.5 mmol/L. The AG was 52.8% sensitive, 81.0% specific with a negative predictive value of 89.7% for the prediction of lactic acidosis.(
Adams BD, Bonzani TA, Hunter CJ. The anion gap does not accurately screen for lactic
acidosis in emergency department patients.
Emerg Med J. 2006;23(3):179-182.)
Nguyen HB, Rivers EP, Knoblich BP, et al. Early lactate clearance is associated with improved outcome in severe sepsis and septic shock. Crit Care Med. 2004;32(8):1637-1642.
In ARDS, the lung may be the major source of lactate Intens Care Med 2004;30:817
Whole blood venous point-of-care lactate concentrations in healthy subjects do not change significantly over 15 minutes at either 18C or 238C, and the use of a tourniquet has no appreciable effect on lactate concentrations. (Acad Emerg Med 2007; 14:587591)
the utility of venous lactate to triage pts in trauma (J Am Coll Surg 2000;190(6):656) VLAC equiv to ALAC; >=2 predicts need for ICU, etc. another study correlating lactates with mortality (Inten Care Med 2007;33:1892)
—
if fiO2x5=PaO2 then lungs are fully recruited
Dead Space Ventilation V/Q>1
Venous Admixture V/Q<1
Shunt V/Q=0
Dead Space
Vd/Vt=(PaCO2-PECO2)/PaCO2
PECO2 is the CO2 in expired gas, but not same as ETCO2, it must be measured by resp therapy
Shunt Fraction
Qs/Qt=(CcO2-CaO2)/(CcO2-CvO2)
CcO2=pulmonary capillary blood
Aa Gradient=PA-Pa
PA=PIO2-(PaCO2/RQ)
PIO2=FiO2(760-47) RQ=0.8
Normal Aa rises with Age
20 4-17
30 7-21
40 10-24
50 14-27
60 17-31
70 21-34
80 25-38
Normal Aa gradient increases 5-7 mmHG for every 10% increase in FiO2 due to the overcoming of hypoxic vasoconstriction opening blood flow to poorly ventilated lung areas.
Hypoxemia
Why: Hypoventilation, Pulmonary Disorder, DO2/VO2 Imbalance
In disorders with intrapulmonary shunt (pneumonia, ards), the mixed venous O2 will be a larger determinant of PaO2 and should be evaluated.
Evaluating Hypoxemia
Step I-Aa Gradient
Normal-generalized hypoventilation disorder. Drug induced or neuromuscular,
Increased-V/Q Abnormality and/or systemic supply/demand imbalance (DO2/VO2). Obtain a SvO2 or CvO2.
Mixed Venous PO2
breakpoint is 40 mmHg
Normal-Problem is solely V/Q abnormality.
Low-Systemic DO2/VO2 imbalance. Either decreased oxygen delivery (anemia, Low CO) or increased consumption (hypermetabolic state)
Hypercapnia
PaCO2=k x (VCO2/VA)
k is a constant
VA=VE(1-Vd/Vt)
VE is expired volume
VA is non-deadspace ventilation
Overfeeding can cause hypercarbia
Alveolar Hypoventilation
Resp Muscle Weakness (Get PImax)
Central Hypoventilation Syndromes
Dual Oximetry
SaO2-SvO2=VO2/(Q x Hb)
Normal is 20-30%
30-50 low cardiac output, anemia, hypermetabolism
50-60 High Risk of dysoxia, transfusion trigger.
Hypocapnia
(N Engl J Med, Vol. 347, No. 1 July 4, 2002)
. In such cases, there is a dissociation between the condition of central venous blood, with a high partial pressure of arterial carbon dioxide and a low pH, and that of the systemic arterial blood, with a low carbon dioxide tension and an alkalemic pH; this dissociation is due to the combination of low pulmonary perfusion and normal ventilation, and this condition is called pseudorespiratory alkalosis. 16
Hypocapnia may cause or aggravate cellular or tissue ischemia by both decreasing the cellular oxygen supply and increasing the cellular oxygen demand (Fig. 3). Although hypocapnia induced by hyperventilation may increase alveolar oxygen tension, multiple important pulmonary effects of hypocapnic alkalosis (e.g., bronchoconstriction, 48 attenuation of hypoxic pulmonary vasoconstriction, 49 and increased intrapulmonary shunting 49 ) result in a net decrease in the partial pressure of arterial oxygen.
Because both hypocapnia and alkalosis cause a leftward shift of the oxyhemoglobin dissociation curve, off-loading of oxygen at the tissue level is restricted. 50 In addition, hypocapnia causes systemic arterial vasoconstriction, decreasing the global and regional oxygen supply and compounding the reduction in the delivery of oxygen to tissue
the time required for equillibration of PaO2 after PEEP adjustment is 20 minutes (Crit Care Med 2005;33(5):995)
if fiO2x5=PaO2 then lungs are fully recruited
Dead Space Ventilation V/Q>1
Venous Admixture V/Q<1
Shunt V/Q=0
Dead Space
Vd/Vt=(PaCO2-PECO2)/PaCO2
PECO2 is the CO2 in expired gas, but not same as ETCO2, it must be measured by resp therapy
Shunt Fraction
Qs/Qt=(CcO2-CaO2)/(CcO2-CvO2)
CcO2=pulmonary capillary blood
Place pt on 100% O2 for 5-15 minutes. The Aa Gradient is entirely from shunt. Dive Aa by 20 to get the shunt fraction
Aa Gradient=PA-Pa
PA=PIO2-(PaCO2/RQ)
PIO2=FiO2(760-47) RQ=0.8
Normal Aa rises with Age
20 4-17
30 7-21
40 10-24
50 14-27
60 17-31
70 21-34
80 25-38
Normal Aa gradient increases 5-7 mmHG for every 10% increase in FiO2 due to the overcoming of hypoxic vasoconstriction opening blood flow to poorly ventilated lung areas.
Hypoxemia
Why: Hypoventilation, Pulmonary Disorder, DO2/VO2 Imbalance
In disorders with intrapulmonary shunt (pneumonia, ards), the mixed venous O2 will be a larger determinant of PaO2 and should be evaluated.
Evaluating Hypoxemia
Step I-Aa Gradient
Normal-generalized hypoventilation disorder. Drug induced or neuromuscular,
Increased-V/Q Abnormality and/or systemic supply/demand imbalance (DO2/VO2). Obtain a SvO2 or CvO2.
Mixed Venous PO2
breakpoint is 40 mmHg
Normal-Problem is solely V/Q abnormality.
Low-Systemic DO2/VO2 imbalance. Either decreased oxygen delivery (anemia, Low CO) or increased consumption (hypermetabolic state)
Hypercapnia
PaCO2=k x (VCO2/VA)
k is a constant
VA=VE(1-Vd/Vt)
VE is expired volume
VA is non-deadspace ventilation
Overfeeding can cause hypercarbia
Alveolar Hypoventilation
Resp Muscle Weakness (Get PImax)
Central Hypoventilation Syndromes
Dual Oximetry
SaO2-SvO2=VO2/(Q x Hb)
Normal is 20-30%
30-50 low cardiac output, anemia, hypermetabolism
50-60 High Risk of dysoxia, transfusion trigger.
Hypocapnia
(N Engl J Med, Vol. 347, No. 1 July 4, 2002)
. In such cases, there is a dissociation between the condition of central venous blood, with a high partial pressure of arterial carbon dioxide and a low pH, and that of the systemic arterial blood, with a low carbon dioxide tension and an alkalemic pH; this dissociation is due to the combination of low pulmonary perfusion and normal ventilation, and this condition is called pseudorespiratory alkalosis. 16
Hypocapnia may cause or aggravate cellular or tissue ischemia by both decreasing the cellular oxygen supply and increasing the cellular oxygen demand (Fig. 3). Although hypocapnia induced by hyperventilation may increase alveolar oxygen tension, multiple important pulmonary effects of hypocapnic alkalosis (e.g., bronchoconstriction, 48 attenuation of hypoxic pulmonary vasoconstriction, 49 and increased intrapulmonary shunting 49 ) result in a net decrease in the partial pressure of arterial oxygen.
Because both hypocapnia and alkalosis cause a leftward shift of the oxyhemoglobin dissociation curve, off-loading of oxygen at the tissue level is restricted. 50 In addition, hypocapnia causes systemic arterial vasoconstriction, decreasing the global and regional oxygen supply and compounding the reduction in the delivery of oxygen to tissue
the time required for equilibration of PaO2 after PEEP adjustment is 20 minutes (Crit Care Med 2005;33(5):995)
Lab Tubes
What’s in those vacuum-filled blood sample tubes?
Worth knowing:
- .
Red-top: Nothing. The blood will clot, and we’ll extract the serum. Used for most routine chemistries. Nowadays most “red top” tubes are red-and-gray top tubes, with a silicone separator
Purple-top: EDTA (calcium-chelating anticoagulant). Best for blood cell counting.
Blue-top: Citrate (calcium-chelating anticoagulant, readily neutralized). Best for routine coagulation studies.
Gray-top: Fluoride-oxalate. Inhibits glycolytic enzymes. Best for glucose and routine toxicology.
Green-top: Heparin anticoagulant. Less popular than the others.
*Navy-blue top: Trace-metal-free
*Yellow top: Acid citrate dextrose. Tests that must be kept at room temperature and shipped to a reference lab. PCR’s and reference-immunology especially. (NOTE: Some yellow-top tubes are silicone-barrier-gel only).
Healthy serum lacks clotting factors I, II, V, VII, and XIII
“Routine Screening”
Contents of tubes
The tubes may contain additional substances that preserve the blood for processing in the medical laboratory. Using the wrong tube may therefore make the blood sample unuseable.
The substances may include anticoagulants (EDTA, lithium citrate , heparin) or a gel with intermediate density between blood cells and blood plasma. When the tube is centrifuged, the blood cells sink to the bottom of the tube, are covered by a layer of the gel, and the plasma (or serum) is left on top. The gel enables the tube to be tipped upside-down, and transported without the blood cells remixing with the plasma.
The meaning of the different colors are standardized across manufacturers. For more details on the meaning of these different colors, see [1], [2], or the bottom of [3].
The Order of Draw refers to the sequence in which these tubes should be filled. The needle which pierces the tubes can carry additives from one tube into the next, and so the sequence is standardized so that any cross-contamination of additives will not affect laboratory results [3].
[edit ]
Containers containing coagulants
- Gold – A serum-separating tube (SST). These contain particles that cause blood to clot quickly, as well as a gel to separate blood cells from serum. (Because the blood has clotted before it has been centrifuged, the liquid part is called serum not plasma.)
- Orange – These tubes contain thrombin which makes the blot clot extremely rapid. This allows the serum to be analysed in a shorter time.
[edit ]
Containers containing anticoagulants
- Green – Contains the lithium salt of heparin, an anticoagulant. Also may contain ammonium or sodium salts of heparin.
- Purple or lavender – contains EDTA. This is a strong anticoagulant and these tubes are usually used for full blood counts and blood films. Can also be used for blood bank.
- Grey – These tubes contain fluoride and oxalate. Fluoride prevents enzymes in the blood from working, so a substrate such as glucose will not be gradually used up during storage. Oxalate is an anticoagulant.
- Light blue – Contain a measured amount of citrate. Citrate is a reversible anticoagulant, and these tubes are used for coagulation assays. Because the liquid citrate dilutes the blood, it is important the tube is full so the dilution is properly accounted for.
- Dark Blue – Contains the sodium salt of heparin, an anticoagulant. Also can contain EDTA as an additive or have no additive. These tubes are used for trace metal analysis.
- Pink – Similar to purple tubes (both contain EDTA) these are used for ABO grouping and cross-matching.
[edit]
Other
- Red – Contains no additives. Tests for antibodies and drugs often require these.
- Light yellow – Used in HLA phenotyping. Also contains SPS, used for blood cultures.
- Speckled top — no anticoagulant. Also called a “tiger top” tube. Contains clot activator.
[edit]
Miscellaneous
The purple top tube became an issue in the O. J. Simpson murder trial when the defense alleged that small droplets of blood found at the crime scene contained the preservative EDTA; had this been true, it would have meant that the droplet might have been taken from the purple top tube used to collect Simpson’s blood and planted at the crime scene. [4]
MGH Lab Callback (Am J Clin Pathol 125(5) 2006)
Table 1. Massachusetts General Hospital Critical Value List
Test Critical Values Chemistry (blood gas) Bilirubin, total, 0-3 mo old, mg/dL (µmol/L) >20 (>342) Calcium, ionized, mg/dL (mmol/L) <3.20 or >6.16 (<0.8 or >1.54) Hemoglobin, g/dL (g/L) <6.5 (<65) pco2, mm Hg <20 or >75 pH <7.10 or >7.59 po2, mm Hg <40 Chemistry (main laboratory) Calcium, mg/dL (mmol/L) <6.5 or >14.0 (<1.63 or >3.53) Carbon dioxide, total, mEq/L (mmol/L) <11 (<11) Glucose, CSF, mg/dL (mmol/L) <40 (<2.2) Glucose, plasma, mg/dL (mmol/L) <45 or >500 (<2.5 or >27.8) Magnesium, mEq/L (mmol/L) <1.0 or >4.9 (<0.50 or >2.45) Osmolality, plasma or serum, mOsm/kg H2O (mmol/kg H2O) <250 or >335 (<250 or >335) Phosphorus, mg/dL (mmol/L) <1.1 (<0.36) Potassium, mEq/L (mmol/L) <2.8 or >6.0 (<2.8 or >6.0) Sodium, mEq/L (mmol/L) <120 or >160 (<120 or >160) Hematology All hematocrit values, % >56% (>0.56) δ values Various δ checks for platelet and hematocrit values Differential Presence of blasts on initial smear Initial hematocrit, % <20 (<0.20) Initial platelet count, × 103/µL (× 109/L) <50 or >999 (<50 or >999) Initial WBC count, /µL (× 109/L) <2,000 or >50,000 (<2.0 or >50.0) Partial thromboplastin time, s >100 Prothrombin time, s >30
CSF, cerebrospinal fluid.
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