**5.4.2 CO2**

310 Congenital Heart Disease – Selected Aspects

Fig. 10. The individual (thin line) and mean (bold line) changes in systemic hemodynamics and oxygen transport before and after termination of dopamine following the Norwood

Fig. 11. Examples of the on-line measurement of oxygen consumption (VO2) in three patients showing rapid and (A) small, (B) moderate, and (C) large decreases in VO2 after

procedure.

terminating dopamine.

CO2 has been suggested as a factor increasing DO2 in neonates both before and after the Norwood procedure (Bradley, Simsic et al. 2001; Mora, Pizarro et al. 1994). Consequently, it is a common practice to maintain a relatively high arterial CO2 tension (PaCO2), primarily by hypoventilation. The potent pulmonary vasoconstrictive effect of CO2 was believed to decrease pulmonary blood flow (Qp), thereby increasing Qs (Mora, Pizarro et al. 1994). We studied the effect of stepwise increases in PaCO2 from 40 to 50 to 60 mmHg, and found complex effects of CO2 on systemic and regional oxygen transport (Li, Zhang et al. 2008). Moderate hypercapnia increases Qs as a result of its effect on SVR, rather than via PVR as previously proposed. The increase in systemic blood flow is primarily a consequence of increased cerebral blood flow that compromises splanchnic circulation. Moderate hypercapnia also decreases VO2 and stimulates the release of catecholamines. The decrease in VO2 improves the balance of oxygen transport, but the increase in catecholamines may be undesirable (Figures 12). Clinically, CO2 should be used with caution when the aim is to improve oxygen delivery.

Fig. 12. During stepwise increases in PaCO2 from 40 to 50 to 60 mmHg and after termination of CO2, changes in systemic and total pulmonary vascular resistances (SVR and PVR), systemic and pulmonary blood flow (Qp and Qs), oxygen consumption and delivery (VO2 and DO2), oxygen extraction ration (ERO2), and lactate, cerebral and splanchnic oxygen saturations (ScO2 and SsO2) and in epinephrine and norepinephrine.

#### **5.4.3 Hyperglycemia**

Hyperglycemia has been identified as a risk factor for adverse outcomes in critically ill patients, including those after CPB. Tight glucose control with insulin therapy has been shown to improve outcomes, but is not common practice for children following CPB. In our

Accurate Measurement of Systemic

**5.4.4 Other factors** 

outcomes.

**6. Conclusion** 

**7. References** 

2084.

Oxygen Consumption in Ventilated Children with Congenital Heart Disease 313

negatively associated with systemic hemodynamics and oxygen transport status. Randomized clinical trials of glucose control with insulin therapy are warranted to identify the cause-and-effect relationship and to provide important information regarding

To date, we have found a few factors in current routine postoperative management that have varied effects on oxygen transport. Further investigations are required to identify other factors in clinical management with favourable or adverse effects, and to design new treatment strategies to improve postoperative oxygen transport and clinical

The predictive equations currently in use are unacceptable to measure VO2 in ventilated children with congenital heart disease, particularly in those younger than 3 years of age, and in the early postoperative period after CPB. Direct, continuous, and precise measurement of VO2 is fundamental for accurate assessments of hemodynamics and oxygen transport in children undergoing cardiac catheterization and in the ICU after cardiac surgery. Respiratory mass spectrometry is the 'state-of-the-art' method, allowing highly sensitive and precise measurement of VO2. Measured VO2 and the Fick principle allow the calculation of each parameter of systemic hemodynamic and oxygen transport, in varied circulations in congenital heart defects, both before and after complete surgical repair or palliation. These actual measurements are not only useful in clinical management, but important for bedside physiological studies on the balance of systemic oxygen transport in children after CPB. Some routine treatments in current use are intended to improve the balance of oxygen transport, but may actually worsen it. When considering clinical management of unbalanced oxygen transport, clinicians should choose therapies that address both decreased DO2 and increased VO2. ICU management strategies need to be refined to optimize the balance of systemic oxygen transport in children with congenital heart defects undergoing cardiac surgery. The resultant improved clinical outcomes in the early postoperative period and at long term follow-up

Azakie, T., S. L. Merklinger, et al. (2001). "Evolving strategies and improving outcomes of

Barnea, O., E. H. Austin, et al. (1994). "Balancing the circulation: theoretic optimization of

Bradley, S. M., J. M. Simsic, et al. (2001). "Hemodynamic effects of inspired carbon dioxide

the modified norwood procedure: a 10-year single-institution experience." Ann

pulmonary/systemic flow ratio in hypoplastic left heart syndrome." J Am Coll

after the Norwood procedure." Ann Thorac Surg 72(6): 2088-2093; discussion 2093-

appropriate glucose management strategies for children following CPB.

will improve the quality of life for these vulnerable children.

Thorac Surg 72(4): 1349-1353.

Cardiol 24(5): 1376-1381.

data, elevated glucose level showed negative correlations with CO and DO2, and positive correlations with SVR and ERO2 (Figure 13) (Zhang 2008). Therefore, hyperglycemia is

Fig. 13. Representative regression lines for the model predicting correlations between blood glucose and systemic vascular resistance (SVR), cardiac output (CO), systemic oxygen delivery (DO2), oxygen consumption (VO2), and oxygen extraction (ERO2), at 24, 48, and 72 hours after the Norwood procedure for 17 neonates in the ICU.

negatively associated with systemic hemodynamics and oxygen transport status. Randomized clinical trials of glucose control with insulin therapy are warranted to identify the cause-and-effect relationship and to provide important information regarding appropriate glucose management strategies for children following CPB.
