Venous Blood Gases
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þ Normal VBG values:
 - pH - 0.03 lower than arterial pH unless low-flow state (basically the same)
   pO2 - 40-50
   pCO2 - 5.7 higher than arterial pCO2 (46 rather than 40)
    venous pH was lower by 0.03 unit, and venous PCO2 was higher by 0.8 kPa
    (5.7 mm Hg

 - Normal mixed venous pO2: 40-50 mmHg (Schwartz 3E p 15)
   to remember:  venous pO2 same (40) as the arterial pCO2 (40).
 - Normal venous pH

þ VBG and Bicarb:
 - if VBG pH < 7.05, and VBG pCO2 <= 40, give bicarb
   [Gennis PR; Skovron ML; Aronson ST; Gallagher EJ The usefulness of
   peripheral venous blood in estimating acid-base status in acutely ill
   patients. Ann Emerg Med, 14: 9, 1985 Sep, 845-9]

þ VBG and Pulse Oximetry:
  - The close approximation of arterial and venous pCO2 and pH (except in
    cardiac arrest) is actually an advantage in this era of pulse oximetry.
    It allows one to use venous gases to exclude the presence of significant
    CO2 retention while at the same time relying upon the pulse oximeter to
    gauge if arterial saturation is adaquate. A normal venous CO2 and pH rules
    out significant arterial CO2 retention (1) in non-arrested patients. On
    the other hand large, discepencies in arterial and venous indices occur in
    cardiac arrest and, in those circumstances, the venous values more
    accurately reflect tissue levels (2,4).  This relationship of a widened
    AVCO2 gradient appears to hold as well in nonarrest situations where
    cardiac output is low (3).
Malatesha, G., N. K. Singh, et al. (2007). "Comparison of arterial and venous
pH, bicarbonate, PCO2 and PO2 in initial emergency department assessment."
Emerg Med J 24(8): 569-71.
        OBJECTIVE: To determine the agreement between arterial and venous
samples in a pathologically diverse patient population presenting at an
emergency department (ED) with a view to obviating the need for arterial blood
gas (ABG) analysis in initial ED evaluation. METHODS: Prospective study of 95
patients (69 males, 26 females, mean (SD) age 52 (1.6) years) with diverse
medical conditions, presenting at a tertiary health centre ED and deemed by
the treating physician to require an ABG analysis. Arterial and venous samples
for gas analysis were taken as close in time to each other as possible. The
data thus obtained were analysed for agreement between pH, Pco(2), Po(2) and
bicarbonate using the Bland-Altman method. RESULTS: The arterial and venous
values of pH, bicarbonate and Pco(2) show acceptably narrow 95% limits of
agreement using the Bland-Altman method (0.13 to -0.1, 4.3 to -5.8 and 6.8 to
-7.6, respectively). Agreement in Po(2) measurements was poor (95% limits of
agreement 145.3 to -32.9). CONCLUSION: Venous blood gas analysis for pH,
bicarbonate and Pco(2) may be a reliable substitute for ABG analysis in the
initial evaluation of an adult patient population presenting to the ED.

Razi, E. and G. A. Moosavi (2007). "Comparison of arterial and venous blood
gases analysis in patients with exacerbation of chronic obstructive pulmonary
disease." Saudi Med J 28(6): 862-5.
        OBJECTIVE: To investigate whether venous blood gases (VBG) test can be
replace by an arterial blood gases (ABG) in exacerbation of chronic
obstructive pulmonary disease (COPD). METHODS: From October 2005 to March
2006, at the Emergency Room of Kashan Beheshti Hospital, the data of 107
patients with exacerbation of COPD were assessed. Arterial blood gases and VBG
samples were obtained simultaneously, and indexes of pH, carbon dioxide
partial pressure (PCO2), bicarbonate (HCO3), oxygen partial pressure (PO2) and
Oxygen (O2) saturation level were analyzed. RESULTS: The mean +/- SD of
indexes in ABG and VBG samples were as follows: pH = 7.37 +/- 0.47 versus 7.34
+/- 0.047; PCO2 = 53.88 +/- 7.63 mm Hg versus 59.55 +/- 8.96 mm Hg, HCO3=
30.66 +/- 4.49 mEq/L versus 31.94 +/- 4.39 mEq/L; PO2 = 55.37 +/- 11.19 mm Hg
versus 43.08 +/- 10.54 mm Hg. The average difference between indexes in ABG
and VBG samples were as follows: pH = 0.0241 +/- 0.004, p<0.001, r = 0.864;
PCO2 = 5.673 +/- 1.126 mm Hg, p<0.001, r = 0.761; HCO3 = 1.279 +/- 0.604
mEq/L, p<0.001, r = 0.749; and PO2 = 12.294 +/- 2.115 mm Hg, p<0.001, r =
0.702. CONCLUSION: Venous blood gases, especially pH and PCO2 levels have
relatively good correlation with ABG values. In view of the fact that, this
correlation is not close, VBG cannot be substitute for ABG in exacerbation of
COPD.


(1)
Gennis PR; Skovron ML; Aronson ST; Gallagher EJ
The usefulness of peripheral venous blood in estimating acid-base status in
acutely ill patients.
Ann Emerg Med, 14: 9, 1985 Sep, 845-9

The usefulness of peripheral venous sampling in determining acid-base status
in acutely ill patients was studied. A total of 171 nonarrest patients and
12 patients in cardiac arrest had paired samples of arterial and venous
blood compared for correlation of blood gas results. Linear equations
relating arterial and
venous values of pH, PCO2, and bicarbonate were developed in both groups of
patients; however, the accuracy of predicting arterial values from venous
values was limited. Severe acid-base disturbances were essentially ruled out
by normal or nearly normal venous blood gases. Extremely abnormal venous
levels reliably reflected comparable arterial abnormalities. The results
suggest that immediate intravenous bicarbonate therapy should be considered
for patients with pH less than or equal to 7.05 and PCO2 less than or equal
to 40 torr despite the possibility of inadvertent venous sampling. A larger
series is needed to verify these results in the setting of cardiac arrest.


(2)
Weil MH, Rackow EC, Trevino R, Grundler W, Falk JL, Griffel MI
Difference in acid-base state between venous and arterial blood during
cardiopulmonary resuscitation.
N Engl J Med 1986 Jul 17;315(3):153-6

We investigated the acid-base condition of arterial and mixed venous blood
during cardiopulmonary resuscitation in 16 critically ill patients who had
arterial and pulmonary arterial catheters in place at the time of cardiac
arrest. During cardiopulmonary resuscitation, the arterial blood pH averaged
7.41, whereas the average mixed venous blood pH was 7.15 (P less than
0.001). The mean arterial partial pressure of carbon dioxide (PCO2) was 32
mm Hg, whereas the mixed venous PCO2 was 74 mm Hg (P less than 0.001). In a
subgroup of 13 patients in whom blood gases were measured before, as well as
during, cardiac arrest, arterial pH, PCO2, and bicarbonate were not
significantly changed during arrest. However, mixed venous blood
demonstrated striking decreases in pH (P less than 0.001) and increases in
PCO2 (P less than 0.004). We conclude that mixed venous blood most
accurately reflects the acid-base state during cardiopulmonary
resuscitation, especially the rapid increase in PCO2. Arterial blood does
not reflect the marked reduction in mixed venous (and therefore tissue) pH,
and thus arterial blood gases may fail as appropriate guides for acid-base
management in this emergency.

(3)
Durkin R, Gergits MA, Reed JF 3rd, Fitzgibbons J
The relationship between the arteriovenous carbon dioxide gradient and
cardiac index.
J Crit Care 1993 Dec;8(4):217-21

It has been reported that under normal conditions, mixed venous blood gases
have approximated arterial samples; however, during cardiac arrest or severe
cardiogenic shock, marked differences between arterial and venous blood
gases have been noted. To further assess the relationships between arterial
and mixed venous blood gases and cardiac index, a study population was
chosen consisting of patients with less severe states of cardiac impairment.
The differences between arterial and mixed venous PCO2s and pHs were
compared with cardiac indexes (CI) of 44 patients in an intensive care unit
with arterial lines and Swan-Ganz catheters in place. Twenty-six patients
with normal CIs (2.6 to 4.1 L/min/m2) had a mean difference in mixed
venous-arterial PCO2 (delta PCO2) of 4.88 +/- 0.40 mm Hg. In patients with
low CIs (< 2.6), the delta PCO2 was 7.44 +/- 0.63 mm Hg (P = .001). The
difference of mixed venous and arterial pH (delta pH) was 0.027 +/- 0.004 pH
units for patients with normal CIs and 0.04 +/- 0.003 pH units for those
with low CIs (P < .002). When the CIs of all patients were plotted against
the delta PCO2s, there was an inverse linear relationship wherein delta PCO2
increased as CI decreased (r = -.47, P = .0011). There is an inverse
relationship between delta PCO2 and CI that has not been previously
described. An elevated delta PCO2 may be a marker of a low cardiac index.

(4)
Adrogu‚ HJ; Rashad MN; Gorin AB; Yacoub J; Madias NE
Assessing acid-base status in circulatory failure. Differences between
arterial and central venous blood [see comments]
N Engl J Med, 320: 20, 1989 May 18, 1312-6

To assess arteriovenous differences in acid-base status, we measured the pH
and partial pressure of carbon dioxide (PCO2) in blood drawn simultaneously
from the arterial and central venous circulations in 26 patients with normal
cardiac output, 36 patients with moderate and 5 patients with severe
circulatory failure, and 38 patients with cardiac or cardiorespiratory
arrest. The patients with normal cardiac output had the expected
arteriovenous differences: venous pH was lower by 0.03 unit, and venous PCO2
was higher by 0.8 kPa (5.7 mm Hg). These differences widened only slightly
in those with moderate cardiac failure. Additional simultaneous
determinations in mixed venous blood from pulmonary arterial catheters were
nearly identical to those in central venous blood. In the five hypotensive
patients with severe circulatory failure there were substantial differences
between the mean arterial and central venous pH (7.31 vs. 7.21) and PCO2
(5.8 vs. 9.0 kPa [44 vs. 68 mm Hg]). Large arteriovenous differences were
present during cardiac arrest in patients whose ventilation was mechanically
sustained, whether sodium bicarbonate had been administered (pH, 7.27 vs.
7.07; PCO2, 5.8 vs. 8.6 kPa [44 vs. 65 mm Hg]) or not (pH, 7.36 vs. 7.01;
PCO2, 3.7 vs. 10.2 kPa [28 vs. 76 mm Hg]). By contrast, in patients with
cardiorespiratory arrest, large arteriovenous differences were noted only
when sodium bicarbonate had been given (pH, 7.24 vs. 7.01; PCO2, 9.5 vs.
16.9 kPa [71 vs. 127 mm Hg]). We conclude that both arterial and central
venous blood samples are needed to assess acid-base status in patients with
critical hemodynamic compromise. Although information about arterial blood
gases is needed to assess pulmonary gas exchange, in the presence of severe
hypoperfusion, the hypercapnia and acidemia at the level of the tissues are
detected better in central venous blood.