Venous Blood Gases ================== 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). (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.