Patients with mixed lesions or complex CHD present the greatest challenge to primary health care providers. An example is a child with single-ventricle physiology, such as hypoplastic left heart syndrome. Patients with this condition experience cyanosis and ventilation-perfusion mismatch. For these patients, blood returning to the heart from the systemic circulation and blood returning from the lungs mixes before being pumped by a single ventricle to the systemic and pulmonary circulation. Several palliative surgeries often are required to redirect blood flow.