Continuous Venous Oximetry During Cardiopulmonary Bypass: Influence of Temperature Changes, Perfusion Flow, and

Hematocrit Levels

Anis Baraka, MD, Maurice Baroody, MD, Sania Haroun, MD, Maud Nawfal, MD, Alia Dabbous, MD, Abla Sibai, MS, Salim Jamal, BS, and Sana Shamli, BS

This study was performed in 11 patients undergoing cardiac surgery during cardiopulmonary bypass (CPB). A Bentley-10 oxygenator (American Bentley, Irvine, CA) was used during bypass, and I.500 mL of Ringer’s solution was used to prime the oxygenator. A perfusion flow of 2.4 L/min/m* was used, and an equivalent flow of 100% oxygen was added to the oxygenator. The mixed venous oxygen saturation (SVO,) was monitored by continuous in-line venous oximetry using the Bentley Oxy-Stat Meter. Body temperature and SVO, measurements were made at the venous blood port of the oxygenator. The au-

thors investigated the correlation between SGO, and changes of body temperature, perfusion flow, and hematocrit values. SVO, correlated inversely, in a linear fashion, with the body temperature. The SGO, was markedly decreased after rewarming, and was significantly affected during normothermia by changes in perfusion flow rates and hematocrit levels. An adequate GO, was found when the flow was ~2.4 L/min/m* and the hematocrit was 120%. In-line oximetry can help to optimize perfusion dw- ing CPB and to detect episodes of desaturation. 0 7990 by W.S. Saunders Company.

M IXED VENOUS oxygen saturation (SSO,) reflects the overall balance be-

tween tissue oxygen supply and demand.‘*2 There- fore, this parameter can be used during cardio- pulmonary bypass (CPB) to evaluate the adequacy of oxygen transport and the peripheral circulation3v4

The present study used the Bentley Oxy- Stat Meter (American Bentley, Irvine, CA) for continuous in-line venous oximetry during CPB.3” The study correlated SSO, with temperature changes, perfusion flow, and hematocrit levels.

MATERIALS AND METHODS

The investigation was performed in 11 patients, aged 33 to 68 years and weighing 54 to 89 kg, who underwent coronary artery bypass grafting or valve replacement during CPB. The investigation was approved by the Institution Research Committee, and informed consent was obtained.

and the flow was monitored by a flow rate computer (Sams Inc, Ann Arbor, MI). A flow of 100% oxygen at 2.4 L/mitt/m’ was delivered to the bubble oxygenator through- out CPB and was not changed when the perfusion flow was altered. Continuous in-line oximetry of the venous saturation was achieved by the Bentley Oxy-Stat Meter.‘.4 The site for body temperature and SBO, measurements was the venous blood at the entrance to the pump oxygenator.5

During CPB, body temperature was gradually de- creased, and the heart was arrested after aortic cross- clamping with a cardioplegic solution (K+ 30 m&q/L at 4OC). After approximately 10 to 20 minutes of CPB, a steady state of perfusion was achieved, and the mean body tempera- ture was stabilized at 25.4 + 3.3OC. When surgery was completed, the aortic cross-clamp was released, and the patient was warmed to a body temperature of 37OC.

The study of the influence of temperature changes, perfusion flows, and hematocrit level on SiiO, was completed as described below.

The patients were premeditated with 10 mg of mor- phine, 25 mg of promethazine, and 0.4 mg of scopolamine, intramuscularly. Anesthesia was induced with 0.1 to 0.2 mg/kg of midazolam, 20 to 40 pg/kg of fentanyl, and a mixture of 0.25 mg/kg of alcuronium and 0.1 mg/kg of pancuronium. Following orotracheal intubation, ventilation was controlled with 100% oxygen, and anesthesia was main- tained by incremental doses of morphine before sternotomy (total dose, 0.5 mg/kg). Patients were monitored with an electrocardiogram (V,), a radial artery catheter, and a pulmonary artery catheter.

(1) During hypothermia, the SVO, was monitored while the perfusion flow was maintained at 2.4 L/min/m*. The perfusion flow was then changed to 1.2 L/min/m’ and

This article is accompanied by an editorial. Please see Rubsamen DS: Continuous blood gas monitoring during cardiopulmonary bypass-How soon will it bc the standard of care? J Cardiothorac

Before CPB, the patients were given Ringer’s lactate solution at a rate of 10 mL/kg, and an additional 1,500 mL was used to prime the Bentley bubble oxygenator (Bentley- 10, American Bentley). No blood or colloid was added to the prime. During bypass, the hematocrit level was lowered from a mean preoperativevalue of 42.1% + 3.8% to 25.2% f 3.1%. During CPB, the patients were perfused by a roller pump (Sarns 5000, Ann Arbor, MI) at a tlow rate of 2.4 L/min/mr,

From the Department of Anesthesiology, American University of Beirut, Beirut, Lebanon.

Address reprint requests to Anis Baraka. MD. Depart- ment of Anesthesiology. American University of Beirut, 850 Third Ave. 18th Floor, New York, NY 10022.

o 1990 by W. B. Saunders Company. 0888-6296/90/0401-0&36$03.&7/0

Joornsl of C~&%~KY~C~C Anesthesia, Vol4, No 1 (Fobmar& 1990: pp 35-38 35

36 BARAKA ET AL

100,

502%

t . 8 . .

. . ill 30

TEMPERATURE OC

4.6 L/min/m*. The resulting GO, was recorded approxi- mately 5 minutes after each perfusion flow change.

(2) The original perfusion flow of 2.4 L/mitt/m’ was then restored and maintained throughout the period of rewarming. The SVO, was monitored at each degree of temperature change during rewarming.

(3) When the venous temperature reached 37V, the effects of the different perfusion flows (1.2, 2.4, and 3.6 L/min/m’) on SQ weremonitored, similar to step 1.

(4) The SVO, ievels achieved while using a perfusion flow of 2.4 L/min/m* were correlated with the hematocrit level during CPB.

Correlation and regression analysis was conducted between SiiO, and body temperature, perfusion how, and hematocrit. Statistically significant effects were identified when mean slopes deviated from zero, the null hypothesis of no dependence. Other data are presented as mean + SD. A value of P < 0.05 was considered significant.

RESULTS

Eflect of Temperature Changes on SVO,

As shown in Fig 1, there was an inverse linear correlation between the venous body tem- perature and SVO,. The mean SVO, was 91.9% f 6.5% during hypothermia (25.4 f 3.3”(I), and was significantly decreased to 64.8% + 9.5% after rewarming to 37°C.

Effect of Perfusion Flow on SSO,

As shown in Fig 2, increasing the perfusion flow from 1.2 to 2.4 and 3.6 L/min/m’ was associated with a linear increase of SVO,. The increase of SVO, was more significant after rewarming than during hypothermia.

E#ect of Hematocrit Level on SVO,

As shown in Fig 3, there was a significant positive relationship between SVO, and the hema-

4il

Fig 1. A diagram show- ing a statistically significant in- verse relationship between body temperature and GO, (r = -0.79, P < 0.01; b = 2.4, P < 0.01).

tocrit level after rewarming. However, no signifi- cant correlation was observed during hypother- mia.

DISCUSSION

SSO, reflects the overall balance of oxygen supply and demand’T2; hence, SSO, can be used during CPB to evaluate the adequacy of oxygen transport. 3,4 A decrease in SSO, may indicate

100

svo, %

so

” 1; 2:4 3.;

PERFUSION FLOW INDEX L/min/n?

Fig 2. A diagram showing the relationship between different perfusion flows and mean SgO,. During normother- mia (closed dots). there was a strong positive linear correlation (r approached one. b = -9.9, P < 0.001). Dur- ing hypothermia (open dots), correlation was not statisti- cally significant.

CONTINUOUS VENOUS OXIMETRY DURING BYPASS 37

100

1

0 0 0

0 0

0 0

0

.

siiop .x !I0

i

?? .

L 1 1

20 25 30

Hct %

Fig 3. Adiagram showing the relationship between hematocrit levels and SiiO,. There was no significant correlation during hypothermia (open dots). During normo- thermia (closed dots), the b value was 1.02 (P -C 0.02).

inadequate availability of oxygen or increased oxygen consumption. The association between mixed venous hypoxia (PVO* < 27 mm Hg) and lactic acidosis has been established.‘** Thus, oxygen transport is likely to be compromised when SVO, is less than 60%, and the probability of tissue hypoxia increases when SVO, falls below 55%.

The use of continuous in-line venous oxime- try during CPB, rather than periodic gas analy- sis, enables the perfusionist to quickly reveal any imbalance of oxygen supply and demand, and to make a rapid adjustment in response to any change in the patient’s physiologic status.3’4 In the present study, the Bentley Oxy-Stat Meter was used for continuous in-line venous oximetry. Both the SSO, and body temperature were mea- sured at the venous entrance to the pump oxygen- ator, since this site most closely represents the mixed venous blood and reflects the mean temper- ature of all the tissues of the patient.5

SVQ2 reflects the overall balance of oxygen supply (DO,)/demand (90,) according to the following formula:

co, SS02 = Sa02 - -

DO,

During CPB, administering 100% oxygen to the oxygenator is associated with arterial PO,

levels above 100 mm Hg6; hence, the arterial blood is usually fully saturated. Thus, SaO, may be considered as 1 .O, and

.

svo* = 1 - 2 CO,

The present study investigated the correla- tion between SVO, and its individual determi- nants. During CPB, the body temperature is the main determinant of q02, while perfusion flow and hemoglobin level are the main determinants of Do,.

The results show that change of body temperature is the most important factor that affects SVO,. During hypothermic CPB, a perfu- sion flow of 2.4 L/min/m* resulted in a mean SVO, of 91.9% + 6.5%, indicating an increase of oxygen delivery relative to consumption. Hypo- thermia reduces the oxygen consumption and increases the oxygen in physical solution; there- fore, most of the metabolic need for oxygen can be met from the dissolved oxygen without using the hemoglobin transport system. The “Qto rule” means that there will be an appropriate 50% decrease in oxygen demand for every 10°C de- crease in body temperature.7 The present report showed that lowering the perfusion flow from 2.4 to 1.2 L/min/m* during hypothermia (mean temperature, 25.4 -c 3.3V) did not lower the SVO, below 70% in any of the patients, suggest- ing adequate oxygenation. Previous reports have also shown that using such low flow rates during CPB did not alter whole-body oxygen consump- tion at steady-state hypothermia.*

During rewarming, the SVO, decreased in a linear fashion with increases in body tempera- ture. When a venous blood temperature of 37% was achieved, the mean SVO, was significantly decreased to 64.8% + 9.5%. Rewarming is associ- ated with increased oxygen consumption, dis- solved oxygen does not contribute significantly to oxygenation, and hemoglobin desaturation be- comes the predominant source of oxygen supply. Since the available oxygen equals perfusion flow x hemoglobin oxygen content, any decrease of perfusion flow or hemoglobin concentration during normothermia will markedly decrease the available oxygen, and results in a marked de- crease of SVO, secondary to increased oxygen extraction.

38

Perfusion flow rates during CPB have been controversial since the earliest days of cardiac surgery. The initial flow rates used were approxi- mately 2.4 L/min/m’, which matches the car- diac output in resting adult humans.g The present study showed that the effect on SSO, of changing the perfusion flow was more significant after re- warming than during hypothermia (Fig 2). At low temperature, a perfusion flow as low as 1.2 L/min/m2 was enough to ensure optimal oxygen- ation. Following rewarming, higher perfusion flow was necessary to provide optimal oxygen- ation. Decreasing the perfusion flow to less than 2.4 L/min/m’ was associated with marked venous desaturation after rewarming.

The correlation between hematocrit level and SVO, was also more significant after rewarm- ing than during hypothermia (Fig 3). Optimal oxygen transport during normothermia may oc- cur at a hematocrit level less than “normal.” At a hematocrit of 25%, oxygen transport is still 90% of that at a “normal” hematocrit level.” Thus, moderate hemodilution to a hematocrit of 25% to 30% does not greatly impair the transport of oxygen in the resting basal state, provided that measures are taken to preserve the normal blood volume and perfusion flow. However, increased

BARAKA ET AL

oxygen extraction by tissues during isovolemic hemodilution will occur when the hematocrit is reduced to less than 20%.” Whenever hemodilu- tion decreases the hematocrit to less than 20%, increasing the perfusion flow and/or adding blood or packed cells to the prime may be necessary to achieve adequate ,550, levels.

In conclusion, the present study shows that continuous monitoring of SSO, using in-line oximetry can help to optimize perfusion during CPB. It may also be used as a criterion for predicting cardiac output during emergence from bypass. l2 SSO, reflects the overall balance of oxygen supply and demand. This study showed that SVO, is inversely correlated with the body temperature. During hypothermia, an SVO, of 70% could be achieved despite lowering of the perfusion flow to 1.2 L/min/m’. In contrast, rewarming to 37°C was associated with a signifi- cant decrease of SVO,. During normothermia, the SVO, was significantly influenced by changes in perfusion flow and hematocrit. Adequate SSO, could only be achieved when the perfusion flow was ~2.4 L/min/m’, and the hematocrit level was 120%. When lower perfusion flows and hematocrit levels were used, marked hemoglobin desaturation occurred.

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