Apparatus and methods are described including a catheter, and first and second impellers configured to be inserted into a subject's body via the catheter, the first and second impellers being disposed in series with each other. A first impeller cage is disposed around the first impeller, and a second impeller cage is disposed around the second impeller. A sleeve extends longitudinally along more than 50 percent of a distance between the first and second impellers. Other applications are also described.