The importance of the muscle and ventilatory blood pumps during exercise in patients without a subpulmonary ventricle (Fontan operation)

Abstract

Objectives: The aim of this study was to determine the relative contribution of the muscle and ventilatory pumps to stroke volume in patients without a subpulmonic ventricle.

Background: In patients with Fontan circulation, it is unclear how venous return is augmented to increase stroke volume and cardiac output during exercise.

Methods: Cardiac output (acetylene rebreathing), heart rate (electrocardiography), oxygen uptake (Douglas bag technique), and ventilation were measured in 9 patients age 15.8 ± 6 years at 6.1 ± 1.8 years after Fontan operation and 8 matched controls. Data were obtained at rest, after 3 min of steady-state exercise (Ex) on a cycle ergometer at 50% of individual working capacity, during unloaded cycling at 0 W (muscle pump alone), during unloaded cycling with isocapnic hyperpnea (muscle and ventilatory pump), during Ex plus an inspiratory load of 12.8 ± 1.5 cm water, and during Ex plus an expiratory load of 12.8 ± 1.6 cm water.

Results: In Fontan patients, the largest increases in stroke volume and stroke volume index were during zero-resistance cycling. An additional increase with submaximal exercise occurred in controls only. During Ex plus expiratory load, stroke volume indexes were reduced to baseline, non-exercise levels in Fontan patients, without significant changes in controls.

Conclusions: With Fontan circulation increases in cardiac output and stroke volume during Ex were due to the muscle pump, with a small additional contribution by the ventilatory pump. An increase in intrathoracic pressure played a deleterious role in Fontan circulation by decreasing systemic venous return and stroke volume.

Figure 1. Experimental Protocol

The experimental protocol was divided into 2 days. Day 1 consisted of a maximal exercise test. Day 2 consisted of a series of conditions separating the various components of Ex on the basis of ventilatory effort. EL = expiratory load; IL = inspiratory load; Max = maximum; 0W = zero-resistance cycling; 0W+H = zero-resistance cycling plus isocapnic hyperpnea.

Figure 2. Oxygen Uptake

The largest increase in oxygen uptake was seen with submaximal exercise. All exercise conditions were statistically different from baseline (rest). EL = expiratory load; IL = inspiratory load; 0W = zero-resistance cycling. *p < 0.05 compared with baseline (rest) within groups; †p < 0.05 between groups within condition.

Figure 3. Heart Rate

The change in heart rate mimics the change seen in oxygen uptake. The heart rate change was minimal in the Fontan patients with zero-resistance cycling (0W) but increased in the controls in this condition. EL = expiratory load; IL = inspiratory load. *p < 0.05 compared with baseline (rest) within groups; †p < 0.05 between groups within condition.

Figure 4. Minute Ventilation

Minute ventilation was adequately matched in the condition of zero-resistance cycling (0W) plus isocapnic hyperpnea to submaximal exercise and was relatively constant across the remaining conditions. EL = expiratory load; IL = inspiratory load. *p < 0.05 compared with baseline (rest) within groups.

Figure 5. Cardiac Index

The largest increase in cardiac index was seen with submaximal exercise in both groups. All conditions were statistically different from baseline in the Fontan patients. EL = expiratory load; IL = inspiratory load; 0W = zero-resistance cycling. *p < 0.05 compared with baseline (rest) within groups; †p < 0.05 between groups within condition.

Figure 6. SVI

The largest increase from baseline (rest) in the Fontan patients was seen with the zero-resistance cycling (0W) (muscle pump) condition. The stroke volume index (SVI) in Fontan patients returned to near baseline values with the expiratory load (EL). IL = inspiratory load. *p < 0.05 compared with baseline (rest) within groups; �p < 0.05 between groups within condition.

Figure 7. Individual Stroke Volume Index by Condition

The changes with exercise and with expiratory load (EL) were consistent in each individual Fontan patient.

Figure 8. SVI: Contribution in Patients on the Basis of Condition

When taken as a percent of total increase stroke volume index (SVI), the muscle pump contributed the largest increase in the Fontan patients. The relative contributions in the control patients were similar with each condition.