Types of ECMO and Indication
The two main subtypes of ECMO, VV-ECMO and VA-ECMO, differ in the configurations of their circuit which allows for them to carry out different functions and enables them to be utilised for respiratory support, cardiac support, or both1,2.
VV-ECMO involves venous cannulation alone- either single dual-lumen cannulation via the right jugular vein to enable removal of blood from the vena cava or right atrium by a pump device for oxygenation via an extracorporeal membrane oxygenator before it is returned to the right atrium, or double cannulation where blood is removed from the inferior vena cava (IVC) vein through cannulation of the right common femoral vein and infused into the right atrium via cannulation of the right internal jugular or vice versa though the former is more commonly used in modern practice (Figure 1)14. This configuration allows for gas exchange to occur but does not provide circulatory support, meaning VV-ECMO is only indicated for isolated respiratory failure and the patient must be haemodynamically stable. By contrast, VA-ECMO involves both arterial and venous cannulation, enabling removal of blood from the right atrium or IVC before it is returned to the arterial system via a peripheral cannula i.e. peripheral VA-ECMO (through the femoral, axillary or carotid artery) or a central cannula i.e. central VA-ECMO via the ascending aorta (Figure 2)15. The setup of the VA-ECMO circuit enables it to carry out gas exchange along with providing circulatory support thus it is indicated for isolated cardiac failure along with combined cardiopulmonary failure. The key differences between the two types of ECMO are summarised in Table 1.
Indications for ECMO are broadly divided according to cardiac support, respiratory support or a combination of both in the case of cardiopulmonary failure1. VA-EMCO is indicated for cardiac failure where there is a low cardiac output (defined as a cardiac index less than 2L/min/m2) and hypotension (defined as a systolic blood pressure of less than 90mmHg) refractory to adequate intravascular volume, high-dose inotropic support and IABP support16-18. Both VV- and VA-ECMO are indicated for acute respiratory failure as rescue therapy to provide gas exchange until the lungs recover or as a bridge to lung transplantation19-21. The main absolute contraindications to ECMO are pre-existing pathologies that make recovery unlikely such as disseminated malignancy and severe neurological injury, or unsuitability for destination therapy. Specific cardiac and respiratory indications, absolute and relative contraindications to for ECMO are summarised in Table 2.