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J Thorac Cardiovasc Surg 2006;131:1099-1107
© 2006 The American Association for Thoracic Surgery
Surgery for Congenital Heart Disease |
Cardiac Program, the Hospital for Sick Children, Toronto, Ontario, Canada
Read at the Eighty-fifth Annual Meeting of The American Association of Thoracic Surgery, San Francisco, Calif, April 10–13, 2005.
Received for publication April 8, 2005; revisions received October 5, 2005; accepted for publication October 10, 2005. * Address for reprints: Glen S. Van Arsdell, MD, Division of Cardiovascular Surgery, The Hospital for Sick Children, 555 University Ave, Toronto, Ontario, Canada M5G 1X8 (Email: glen.vanarsdell{at}sickkids.ca).
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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= arterial oxygen saturation/Sa-VO
2) have been used as indirect indicators to estimate systemic blood flow (Qs) and oxygen delivery (DO
2). We sought to examine the correlation of the indirect indicators to VO
2-derived measures of oxygen transport.
METHODS: Respiratory mass spectrometry was used to continuously measure VO
2 after the Norwood procedure (n = 13). Measured saturations and the direct Fick equation were used to obtain pulmonary blood flow, Qs, DO2, and oxygen extraction ratio (ERO
2) values. Correlations to SVO
2, Sa-VO
2, and were sought.
RESULTS: There was a close correlation of SVO
2, Sa-VO
2, and to ERO
2 (r = 0.92, 0.96, and 0.97, respectively; P < .0001). Correlation to Qs and DO
2 was variable (r = 0.39 to 0.78, respectively; P < .0001). Correlation to VO
2 was poor but significant (r = 0.24 to 0.40, P < .0001). Inclusion of VO
2 improved the correlation to Qs and DO
2 (r = 0.66 to 0.97, P < .0001).
CONCLUSIONS: The close correlation of SVO
2, Sa-VO
2, and to ERO
2 indicates that each is a measure of the balance of DO
2 and extraction. The significant but less reliable correlation to DO
2 and VO
2 indicates the values for SVO
2, Sa-VO
2, and do not discriminate between the contribution of DO
2 and VO
2. Measured VO
2 and hemodynamics may improve the optimization of postoperative management strategy in the individual neonate.
= oxygen excess factor; Qp = pulmonary blood flow; Qs = systemic blood flow; SaO
2
= arterial oxygen saturation; Sa-VO
2
= systemic arterial and venous oxygen saturation difference; SVO
2
= systemic venous oxygen saturation;
VO
2
= oxygen consumption
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Introduction |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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Performance requirements for the single ventricle with a parallel circulation are substantial and are made more physiologically stressful by the recent insult of cardiopulmonary bypass (CPB) and myocardial ischemia. Postoperative management strategies are predicated on balancing pulmonary (Qp) and systemic (Qs) blood flow
6-8
and optimizing systemic oxygen delivery (DO
2).
9-12
These strategies have been designed on the basis of experimental models,
13,14
theoretic models,
15-17
and human studies
3,5,11,18
that have intrinsic limitations. Experimental animal models might not fully reflect human physiology. Mathematic models require assumptions that might differ from those seen clinically. To date, human models have used estimations of, rather than directly measured, oxygen consumption (VO
2) and pulmonary venous saturations.
3,5,11,18
Because of the difficulties in obtaining measured Qs, DO
2, and Qp values, decision making for postoperative management is based on reflectors of DO
2 and Qs, as well as an estimated pulmonary venous oxygen saturation. The reflectors of DO
2 and Qs are superior vena caval oxygen saturation (SVO
2),
5,8,19
the difference between arterial and venous oxygen saturations (Sa-VO
2) and oxygen excess factor ( = arterial oxygen saturation [SaO
2]/Sa-VO
2).
3-7,11,12,18
Qp is generally accounted for by making an estimation that pulmonary venous saturation is greater than 95%.
3,5,11,18,19
This estimation is not always correct.
8
The above-outlined parameters can be obtained with minimal difficulty in clinical practice but have the limitation of not accounting for interindividual and intraindividual variability of VO
2.
20-24
It has been increasingly realized that changes in VO
2 might have an important effect on hemodynamics and oxygen transport.
20-24
Changes in VO
2 might not be fully reflected or understood for the commonly measured values SVO
2, Sa-VO
2, and . We sought to examine the correlation of SVO
2, Sa-VO
2, and to hemodynamics and oxygen transport parameters obtained by using VO
2 and the direct Fick equation.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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The aortic arch was reconstructed with a homograft patch. The 3.5-mm right modified Blalock-Taussig shunt was completed with anastomosis of the distal conduit to the central pulmonary artery. All patients received methylprednisolone (30 mg/kg) in the operating room at or before CPB. Phenoxybenzamine, 0.25 mg/kg, was administered at initiation of CPB. A bolus of aprotinin, 50,000 KIU, was administered, followed by 100,000 KIU per 100 mL of prime. Our protocol was to administer milrinone (100 µg/kg) before termination of CPB and to initiate dopamine (5 µg · kg–1
· min–1) for the immediate time around cessation of bypass. Dopamine was subsequently discontinued if the hemodynamics and ventricular function were satisfactory. Modified ultrafiltration was performed at the completion of CPB in all patients. A pulmonary venous line was inserted into the orifice of the right upper pulmonary vein. A direct oximetric catheter was inserted in the superior vena cava.
Postoperative Management
Our protocol for management is as follows. The central temperature (esophageal) is maintained between 36°C and 37°C. Postoperative monitoring includes heart rate, end-tidal carbon dioxide, and arterial, superior vena caval, and pulmonary venous pressures. Sedation consists of continuous intravenous infusion of morphine and intermittent injections of a muscle relaxant (pancuronium) and lorezepam. Infants are ventilated with volume control and pressure support. Ventilation volume and rate are adjusted to control PaCO
2. Arterial oxygen saturation is maintained between 70% and 85%. Inotropic agents, vasoactive drugs (milrinone, dopamine, phenoxybenzamine, and vasopressin), and volume infusions (5% albumin or blood) are administered according to our standard protocol.
10
Hemoglobin is maintained between 14 and 16 mg/dL.
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Methods of Measurements |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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was calculated as SaO
2/Sa-VO
2.
V
O
2
. VO
2 was measured continuously with an AMIS2000 mass spectrometer (Innovision A/S, Odense, Denmark). This is a sensitive and accurate method that allows simultaneous measurements of multiple gas fractions. The details are described elsewhere.
20
Hemodynamics and oxygen transport and arterial lactate
Qp and Qs were calculated by using the direct Fick method. DO2 and the oxygen extraction ratio (ERO
2) were calculated by using standard equations (Table 2). All values were indexed to body surface area. Arterial lactate levels were measured.
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Data Analysis
Data are expressed as means ± standard deviation. The generalized linear model for repeated measures was used to determine the nature of any time trend of the measures over the 72-hour study period. Where appropriate for given measures, logarithmic and polynomial transformations were tested for the best fit over time. Correlations between the indirect indicators SVO
2, Sa-VO
2, and and the direct measures of hemodynamics and oxygen transport were sought by using the generalized linear model for repeated measures without regard to time. Logarithmic transformations of both variables being compared was used when necessary to model nonlinear relationships. All data analysis was performed with SAS statistical software version 8 (SAS Institute, Inc, Cary, NC).
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Results |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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Changes of SVO 2, Sa-VO 2, and Over Time
Figure 1
shows the profiles of SVO
2, Sa-VO
2, , hemodynamics, and oxygen transport in 13 patients during the first 72 hours after arrival in the intensive care unit after the Norwood procedure. All measures showed broad interindividual variations.
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. There were wide fluctuations within individuals. SVO
2 and showed an initial rapid increase in the first 12 hours, a subsequent slow increase peaking around 28 to 32 hours, and thereafter a slow decrease in a complex polynomial function (P < .0001). Sa-VO
2 showed the same trend but as a mirror image, with a rapid initial decrease in the first 12 hours and a slow decrease until 28 to 32 hours. Thereafter, there was a slow increase (P < .0001).
Changes of Direct Measures of Hemodynamics and Oxygen Transport
Changes of direct measures of hemodynamics and oxygen transport were highly variable, on an individual basis, as demonstrated by the broad standard deviations. Qp and Qs showed a small but significant linear increase over time (P < .001 for Qp, P < .0001 for Qs). Qs was significantly lower during the first 12 hours compared with the following 60 hours (P < .001). The same analysis for Qp was not significant (P = .62). DO
2 showed a significant change over time (P < .0001), with a slow increase peaking around 28 to 32 hours, followed by a slow decrease. DO
2 was significantly lower during the first 12 hours compared with the following 60 hours (P < .001). VO
2 was related to time after logarithmic transformation (P < .0001). There was a significant decrease in the first 12 hours, as there was with arterial lactate levels (P < .0001). ERO
2 was related to time after logarithmic transformation, with a rapid decrease in the first 12 hours (P < .0001). Central temperature was relatively stable (36.6°C ± 0.5°C), with a small linear increase over time (P = .0002).
Correlations Among SVO 2, Sa-VO 2, and Direct Measures of Hemodynamics and Oxygen Transport
Figure 2
and Table 3
show the generalized linear model correlation among SVO
2, Sa-VO
2, and and the direct measures of hemodynamics and oxygen transport. For significant correlations (P < .05), we have classified an r value of less than 0.30 to be a poor correlation, an r value of 0.31 to 0.70 to be a variable or moderate correlation, and an r value of greater than 0.70 to be a close correlation.
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with ERO
2 (r = 0.92, 0.96, and 0.97, respectively; P < .0001 for all). SVO
2, Sa-VO
2, and were variably correlated with Qs (r =0.39, –0.65, and 0.66, respectively; P < .0001 for all) and variably or closely correlated with DO
2 (r = 0.57, –0.70, and 0.78, respectively; P < .0001 for all). SVO
2, Sa-VO
2, and were significantly but poorly to moderately correlated with Qp/Qs (r = 0.21, 0.34, and 0.35, respectively; P < .0001 for all). Inclusion of Qp and Qs as separate variables in the Qp/Qs correlation demonstrated no correlation (P > .05 for all). SVO
2, Sa-VO
2, and were poorly and variably correlated with VO
2 (r = 0.40, 0.24, and 0.25, respectively; P < .0001 for all). Inclusion of VO
2 to the Qs and DO
2 analysis significantly improved the correlations with SVO
2 (increased r from 0.39 to 0.66 for Qs and increased r from 0.57 to 0.84 for DO
2), Sa-VO
2 (increased r from –0.65 to –0.86 for Qs and from –0.70 to –0.88 for DO
2), and (increased r from 0.66 to 0.87 for Qs and from 0.78 to 0.97 for DO
2). All P values for the inclusion of VO
2 were less than .0001. SVO
2 and Sa-VO
2 were poorly correlated with Qp (r = 0.18, P = .03 and r = 0.08, P < .01, respectively), and showed no correlation with Qp (r = 0.01, P = .36). |
Discussion |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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As such, measures thought to reflect systemic cardiac output have become increasingly used. Low SVO
2 has been shown to correlate with a base deficit.
5
Sa-VO
2 reflects the adequacy of DO
2,
4,11,12
and is a ratio of available oxygen to extracted oxygen (SaO
2/Sa-VO
2). Each of these is thought to be a surrogate measure reflecting DO
2. Present management strategies have focused on maintaining physiologic conditions whereby these surrogate measures have appropriate values. By using respiratory mass spectrometry to measure VO
2, we now have the ability to directly calculate DO
2, Qp, and Qs using the Fick equation and to test the validity of the surrogate measures using correlations to the measured and calculated parameters.
Correlation of Surrogate Measures and ERO 2
There was a significant and close correlation of ERO
2 to SVO
2, Sa-VO
2, and (r = 0.92, 0.96, and 0.97, respectively). ERO
2 is an oxygen transport measure that provides a ratio that incorporates the 3 major components of oxygen transport: oxygen content, DO
2, and VO
2. This study demonstrates that each of the surrogate measures used is most accurately a reflector of ERO
2. The very close reciprocal correlation curve seen with (SaO
2/Sa-VO
2) is explained by the fact that it is intrinsically the ratio of DO
2 to VO
2 and thus the reciprocal of ERO
2.
However, ERO 2 might not fully account for the adequacy of DO 2 because of the potential for changes induced by the phenomenon of DO 2-dependent oxygen extraction. Paradoxically, in the presence of poor or worsening DO 2, a disproportionate decrease in VO 2 might yield an adequate or improving SVO 2 (a close surrogate for ERO 2), as shown in a specific example of a patient (Figure 3) who had a normalizing lactate level and normal base deficit despite a decreasing DO 2.
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to Qs was significant but variable. There was correlation to a moderate degree (r = 0.39, 0.65, and 0.66, respectively). Correlation to DO
2 is moderate to close for the same 3 variables (r = 0.57, 0.70, and 0.78, respectively). The variability and failure to achieve a uniformly close correlation can be explained by the fact that each of the surrogate measures is a single measure or ratio that is dependent on changes in oxygen content, DO
2, and VO
2 (ie, all the components of oxygen transport). As seen in Figure 2, Qs and DO
2 are marginal in the first 24 hours and subsequently gradually increase, a profile that has been seen in other studies after CPB.
21,26,27
The improvements in SVO
2, Sa-VO
2, and in the first 12 to 24 hours occurred during the period when Qs and DO
2 were most decreased. This finding is explained by the improvement in VO
2, as discussed below. In previous analysis, based on an estimated and fixed VO
2, the improvement was attributed to favorable changes in Qs and DO
2.
3,4,19
Correlation of Surrogate Measures to VO 2
There was significant but moderate-to-poor correlation of VO
2 to SVO
2, Sa-VO
2, and (r = 0.40, 0.24, and 0.25, respectively). The most rapid improvement in all the indirect indicators was seen in the first 12 hours after the Norwood procedure and can be explained by the slope of the decrease in VO
2 during the first 12 hours. The VO
2 profile is similar to previously reported findings for other cardiac procedures.
21
The addition of VO
2 to the regression analysis increases the correlation of all 3 variables, SVO
2, Sa-VO
2, and , to r values of 0.66 to 0.92, all of which are significant. Qs and DO
2 are contributors to the changes in SVO
2, Sa-VO
2, and , particularly over the later time frame, when there are less changes in VO
2.
Present strategy to treat postoperative Norwood patients has been to compartmentalize Qp (a contributor to oxygen content) and Qs (the major contributor to DO
2). Because there have been little clinical data on VO
2, manipulation of VO
2 has not gone beyond theoretic concerns. Major contributors to VO
2 are temperature,
20
inflammatory mediators,
21,24
and inotropic agents.
28
The addition of VO
2 as a third compartment for a targeted treatment strategy, particularly in the first 12 hours, when there is a significant increase, remains to be explored both for the ability to manipulate VO
2 and for its effect on hemodynamic stability.
Correlation of Surrogate Measures and Qp
There was little correlation of Qp to SVO
2 and Sa-VO
2 (r = 0.18 and 0.08, respectively) and no correlation to (r = 0.01). These findings suggest that Qp estimates require standard measurement of Qp/Qs or Fick equation calculations. Surrogate measures of DO
2 might reflect the adequacy of oxygenation for the ranges of systemic arterial saturation seen in this study (55% to 86%).
Limitations
SVO
2 was used as an estimate of the mixed venous saturation for the calculations of Qs and DO
2. This measure does not account for potential differences in the inferior vena caval saturation.
29,30
Further investigations analyzing both superior and inferior vena caval oxygen saturation are warranted.
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Conclusions |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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do not completely reflect the changes in Qs and DO
2 because of the variability of VO
2. SVO
2, Sa-VO
2, and are measures of both DO
2 and VO
2, as demonstrated by their close correlation to ERO
2. Measured oxygen transport and hemodynamics might improve the optimization of a postoperative management strategy for individual patients. |
Discussion |
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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One of our beliefs is that a mixed venous oxygen saturation that is higher and an arteriovenous oxygen saturation difference that is lower are desirable. This is because they correlate with higher Qs and DO 2. Your study confirms this belief. Although some of the correlations were rated as moderate, the directly measured values of VO 2 actually improved the correlation. These findings therefore affirm postoperative management aimed at maximizing mixed venous oxygen saturation.
In contrast, this report challenges our understanding of the physiology of the first 48 hours. Immediately after surgical intervention, mixed venous saturation is low, and arteriovenous saturation difference is high. They then improve over the next 48 hours. As you noted, this has been observed in previous studies and interpreted to indicate low systemic cardiac output. However, your study shows that low cardiac output plays only a minor role. The major reason that mixed venous saturation is low is that VO 2 is high immediately after surgical intervention and then decreases markedly over the next 48 hours. Like many interesting findings, this one is somewhat counterintuitive because this is exactly the time VO 2 might be expected to be low because many of the patients are sedated, paralyzed, and mechanically ventilated at this time.
The first question then is this: Why is VO 2 high early after surgical intervention? Is it a generalized inflammatory response to bypass, or is it due to the particular CPB technique used in these patients? Your article indicates that your patients underwent CPB with phenoxybenzamine with low-flow rather than high-flow bypass, as recommended by Roger Mee. Does this create a whole-body oxygen debt in the operating room that must then be repaid in the intensive care unit? Could you give us your group's thoughts on the explanation for this very interesting finding.
Dr Li. Thank you for your comments. It is well documented that VO 2 increases after CPB. The increase in VO 2 has been attributed to several factors, including rewarming from hypothermic CPB, repayment of oxygen debt, and, most importantly, systemic inflammatory response. As shown in our previous study in a group of older children after complete repair of congenital heart disease, the initial postoperative VO 2 was also high, and similarly to our current study, it decreased significantly over the first 12 hours. The decrease in VO 2 coincided with the decrease in circulating cytokine levels.
Dr Bradley. Is there any relationship, for example, between the level of VO 2 coming out of the operating room and the length of CPB?
Dr Li. In this particular group of patients, the longer CPB time was associated with a higher VO 2.
Dr Bradley. The second question is this: Can this phenomenon be avoided? Specifically, were anti-inflammatory strategies, such as modified ultrafiltration, preoperative steroids, or aprotinin, used in your patients? Alternatively, should we be keeping our patients deliberately cool for the first 48 hours after surgical intervention? That might be an unfair question to ask you.
Dr Li. I will try.
Dr Bradley. Do you know whether your patients had modified ultrafiltration?
Dr Li. Yes, they all had modified ultrafiltration. Modified ultrafiltration has been reported to filter out some cytokines. Again, in our previous study in the older group of children with the use of modified ultrafiltration, the postoperative cytokine levels were generally lower compared with data in the literature in children without the use of modified ultrafiltration. This might reduce the systemic inflammatory response, potentially resulting in a decrease in VO 2.
All children received at least intraoperative steroids. Steroids have been reported to reduce the systemic inflammatory response and thus might reduce VO 2. In fact, we are proposing a research project to investigate the effect of steroids on VO 2 in children during and after CPB. All children received aprotinin. The central body temperature was maintained at euthermia for the first 48 hours. In our previous studies we have demonstrated that the increase in VO 2 during the early post-CPB period is mainly attributed to the increase in central body temperature: for a 1°C increase in central body temperature, VO 2 increases by 11%, and simply maintaining body temperature at euthermia results in a continuous decrease in VO 2.
Indeed, manipulating VO 2 is increasingly realized to be an important alternative component to optimize the balance of oxygen transport in children with limited reserve of cardiac function early after CPB.
Dr Bradley. Perioperative steroids or aprotinin use?
Dr Li. Yes, we all used steroids. I think it might contribute, too. It might indeed reduce VO 2. In fact, we are thinking of proposing a study to compare—to investigate this effect on VO 2.
Dr Bradley. Thank you. I think it is a very interesting and important study.
Dr Antonio Corno (Liverpool, United Kingdom). Surgeons are generally very simple-minded. Do you have a practical suggestion for the surgeon in the postoperative care of this patient? If we want to improve the oxygen saturation, that means the balance between DO 2 and VO 2. What we can do as surgeons is to provide a higher total Qp. Should we modify our surgical technique to increase the total Qp knowing that this will cause an increase of the total VO 2 because of increased myocardial consumption? How can you maintain a positive balance to provide a better DO 2?
Dr Li. In this group of patients, we have also analyzed the relationships between pulmonary vascular resistance, systemic vascular resistance, and DO 2. Our data showed that varying Qp had little contribution to systemic DO 2, largely because of the mechanical limitation from the Blalock-Taussig shunt. The best way to improve systemic DO 2 is to maintain a relatively low and stable systemic vascular resistance and, additionally, to maintain a relatively high level of hemoglobin.
Dr Carl L. Backer (Chicago, Ill). Was there any difference in the management strategy as far as the ventilator that you use in the operating room compared with the one that you use in the intensive care unit?
One of the problems we have with our patients is that we have a completely different ventilator system in the operating room than we have in the intensive care unit. There is always a transition phase after transport, with hand ventilation to the intensive care unit ventilator, and I just wondered whether you thought that played any role in the VO 2 in the immediate time period when the patient arrives in the intensive care unit?
Dr Li. The ventilator systems are different in the operating room and intensive care unit, but our ventilation management protocols are the same for these patients in the 2 settings. Yes, there is a transition phase after transport, with hand ventilation to the intensive care unit ventilator, and certain adjustment of ventilation was often required to achieve adequate levels of arterial carbon dioxide and oxygen because they might have profound effects on both DO 2 and VO 2. Yet we have found the hemodynamic variability to ventilation issues to be minimized by aggressive afterload reduction with phenoxybenzamine.
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TopAbstractIntroductionMaterials and MethodsMethods of MeasurementsResultsDiscussionConclusionsDiscussionReferences |
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