This study demonstrates the correlations and associations between several hemodynamic parameters and oxygen delivery in children with parallel circulation. Factors significantly associated with the oxygen extraction ratio were identified as being systemic blood flow, SVC saturation, and Qp:Qs. Factors significantly associated with SVC saturation were found to be systemic blood flow, systemic arterial saturation, and hemoglobin. Factors significantly associated with Qp:Qs were found to be systemic vascular resistance, hemoglobin, pulmonary vascular resistance, and pulse pressure. Higher systemic blood flow, higher SVC saturation, lower Qp:Qs, lower systemic vascular resistance, higher systemic arterial saturation, higher hemoglobin, lower pulmonary vascular resistance, and higher pulse pressure were significantly associated with improved oxygen delivery.
More specifically, these analyses were able to identify some cutoff points that may be helpful in general clinical practice for improving oxygen delivery: systemic blood flow greater than 2.62 l/min, total cardiac output greater than 5.60 l/min, Qp:Qs less than 1.03, systemic arterial saturation greater than 75.50%, SVC saturation greater than 49.00%, hemoglobin greater than 12.5 g/dl, and systemic vascular resistance less than 15.69 woods units (Table 4).
Proposed clinical targets for hemodynamic variables found to be associated directly or indirectly with oxygen extraction ratio
Systemic blood flow
Greater than 2.62 L/min
Total cardiac output
Greater than 5.60 L/min
Pulmonary to systemic blood flow ratio
Less than 1.03
Systemic arterial saturation
Greater than 75.5%
Superior caval vein saturation
Greater than 49.0%
Greater than 12.5 g/dl
Systemic vascular resistance
Less than 15.69 woods units
Parallel circulation represents a unique circulation in which the total cardiac output gets divided into the pulmonary and systemic circulations. Without an increase in total cardiac output, any increase or decrease in either pulmonary or systemic blood flow must be accompanied by an obligatory change in the opposite direction by an equal magnitude. Additionally, the systemic arterial saturation in this circulation is a weighted average of the pulmonary venous and systemic venous saturations. This direct link of flow and saturation between the pulmonary and systemic circulations makes this circulation potentially more tenuous and requires care to balance the two circulations in regards to flow and saturation .
The circulatory system exists for being able to deliver oxygen. Blood and hemoglobin simply act as the transportation system for oxygen, and the heart is the pump that drives forward flow of blood. The heart is normally septated such that the deoxygenated and oxygenated blood pools coming from the systemic venous and pulmonary venous circulations are separated. In parallel circulation, however, this septation is lost. The effect of the mixing of the two blood pools is that the systemic arterial saturation is lower than in a fully septated circulation. The assessment of cardiac output, however, can still be done utilizing the Fick equation, simply understanding that the arteriovenous difference still provides data about the adequacy of systemic oxygen delivery. As always, the adequacy of systemic oxygen delivery can be impaired by either increased oxygen consumption or decreased systemic oxygen delivery. Blood pressure is the product of cardiac output and systemic vascular resistance, and maintenance of blood pressure in and of itself isn’t directly linked to oxygen delivery. Even if mean arterial blood pressure is maintained, increasing cardiac output or decreasing systemic vascular resistance or or both can lead to better systemic oxygen delivery.
The arteriovenous oxygen difference or oxygen extraction ratio require simultaneous monitoring of the systemic arterial and systemic venous saturation. Maintaining an adequate systemic venous saturation has been shown to help detect early hemodynamic decline and improve outcomes in those with parallel circulation [2–6]. Monitoring regional near infrared spectroscopy, similarly, has also been demonstrated to help improve outcome [6, 7].
An increase in the SVC saturation was noted to have an independent association with a decrease in the oxygen extraction ratio. This makes intuitive sense as the increase in the SVC saturation in and of itself reflects adequacy of systemic oxygen delivery, reflecting either a decrease in systemic oxygen consumption or an increase in oxygen delivery. Maintaining a SVC saturation of over 49.00% seems to help optimize systemic oxygen delivery.
A decrease in the Qp:Qs was noted to have an independent association with a decrease in the oxygen extraction ratio. A decrease in Qp:Qs without a concomitant change in total cardiac output would imply an increase in systemic blood flow and consequently an increase systemic oxygen delivery. Maintaining a Qp:Qs of less than 1.03 seems to help optimize systemic oxygen balance. This finding is similar to findings in previous studies [3, 8, 9].
An increase in systemic blood flow was noted to have an independent association with a decrease in the oxygen extraction ratio. Maintaining systemic blood flow greater than 2.62 L/min seems to help mediate increased systemic oxygen delivery and likely leads to the subsequent decrease in the oxygen extraction ratio.
An increase in the systemic arterial saturation was noted to have an independent association with an increase in the SVC saturation. This likely represents an increase in systemic oxygen content which then, if the arteriovenous difference is maintained, will lead to a higher systemic arterial saturation in the next cardiac cycle. The notion that increased oxygen is inherently harmful in parallel circulation has been demonstrated to be false, and while increased fraction of inspired oxygen may be detrimental when left unchecked, this can easily be monitored via multiple ways including following the venous saturation . If an increase in the fraction of inspired oxygen leads to a worsening in the oxygen extraction ratio, then this is an indication that the fraction of inspired oxygen delivered to the patient may have passed a situation-specific threshold to negatively impacted systemic oxygen delivery. In the presence of alpha-blockade such as with phenoxybenzamine or phentolamine, systemic arterial saturation and oxygen extraction ratio seem to be linearly correlated, and no increase in oxygen extraction ratio is usually seen with increasing systemic arterial saturation . These concepts support the findings of the current analyses which suggest maintaining a systemic arterial saturation of greater than 75.50% seems to help optimize systemic oxygen balance.
An increase in hemoglobin level was noted to have an independent association with an increase in SVC saturation and a decrease in Qp:Qs. Higher hemoglobin increases oxygen delivery if cardiac output is maintained. If oxygen delivery increases, then the SVC saturation also increases in the absence of increased oxygen extraction. Maintaining a hemoglobin of over 12.5 g/dl seems to help optimize systemic oxygen delivery. Previous studies have demonstrated a similar hemoglobin cutoff [12, 13]. The benefit of hemoglobin may be secondary not only to the increased oxygen delivering capacity but also to the rheological effects of packed red blood cell on circulatory viscosity. Lister and colleagues demonstrated that packed red blood cell transfusion decreased the Qp:Qs in the setting of ventricular septal defects . While the transfusion increased both the pulmonary vascular resistance and the systemic vascular resistance, the increase in pulmonary vascular resistance was greater. The risks versus benefits of transfusions are beyond the scope of this manuscript, but it should be noted that packed red blood cell transfusion is not without documented risks .
An increase in systemic vascular resistance was noted to have an association with an increase in Qp:Qs. This association has been previously described and is relatively intuitive. If systemic vascular resistance increases but myocardial contractility remains the same, effective cardiac output will subsequently fall due to an increased percentage of stroke volume flowing to the pulmonary circuit. If systemic vascular resistance increases and myocardial contractility increases to maintain similar cardiac output, myocardial oxygen consumption will have to increase. In the first scenario the decrease in stroke volume flowing to the systemic circuit will result in lower systemic oxygen delivery, while in the second scenario the increase in myocardial oxygen consumption at an equal cardiac output will also lead to a decrease in systemic oxygen delivery. Thus, maintaining systemic vascular resistance at low levels is important in the setting of parallel circulation and may be achieved by agents such as phenoxybenzamine, phentolamine, sodium nitroprusside, nicardipine, or milrinone [16–19].
An increase in pulmonary vascular resistance was also noted to have an association with a decrease in Qp:Qs. This is fairly intuitive as the Qp:Qs is inversely related to the pulmonary vascular resistance.
Along with systemic vascular resistance, an increase in systemic blood flow was also noted to have an association with a decrease in Qp:Qs. This should come as no surprise as increasing cardiac output while keeping oxygen content the same results in greater systemic oxygen delivery as systemic oxygen delivery is the product of these two. Enhancing total cardiac output can be done by increasing circulating volume or by increasing contractility with agents such as milrinone, low dose epinephrine, or calcium . Apart from their effects on contractility, the vascular effects of these vasoactive agents must be taken into consideration as these agents can increase systemic vascular resistance and subsequently myocardial oxygen consumption [21, 22].
Lastly, a higher pulse pressure was noted to have an association with a lower Qp:Qs. This is particularly of note as many use the pulse pressure as a surrogate marker of Qp:Qs. Anecdotally, most associate a high pulse pressure with higher Qp:Qs assuming that the pulse pressure is widened because of pulmonary steal. This should not be of particular surprise as pulse pressure is directly related to cardiac output. In the setting of parallel circulation this means that pulse pressure is directly related to systemic flow. While some level of steal from the systemic circulation can occur, it must be kept in mind that systemic flow does seem to mediate the pulse pressure more. A vast majority of children in this study had right ventricle to pulmonary artery conduits (Sano modification) to provide pulmonary blood flow. Hence the reduction in diastolic blood pressure due to steal from the systemic circulation in diastole is not present. Thus, while a greater pulse pressure may be associated with increased Qp:Qs in those with Blalock-Taussig-Thomas shunts, this does not seem to be the case in the setting of right ventricle to pulmonary artery conduits.
There have been very few studies that have focused on the hematologic and physiologic associations between parallel circulation and systemic oxygen delivery. The values identified in this study are simply representative of numbers that have resulted from analyses of a group of patients from a single center. What is more important is the underlying physiologic principles rather than the absolute numbers. As parallel circulation has greater inherent risk of systemic venous desaturation and hemodynamic compromise, it is important to elucidate factors associated with a low-risk state in this circulation.
While these analyses are additive to the literature, they are not without their limitations. This is a single center, retrospective study. Due to varying surgical and medical strategies across different institutions, these findings may not be perfectly applicable to other patient cohorts. The physiologic principles should remain the same, although there may be some difference in the actual cutoff values. These should, however, be minimal. The data here are cardiac catheterization data. Thus, the patients were intubated and sedation for the procedure. This must be kept in mind as this data may not be as applicable to a free-breathing patient off all sedatives. Nonetheless, the underlying physiologic principles related to the adequacy of systemic oxygen delivery and its associated factors should not be dramatically different.