This technical note studies consensus problems of multiple agents with continuous-time second-order dynamics, where each agent can obtain its positions and velocities relative to its neighbors only at sampling instants. It is assumed that the sampling period of each agent is independent of the others' and the interaction topology among agents is time-varying, where the associated direct graphs may not have spanning trees. If the union graph of all direct graphs has a spanning tree, then there exist controller gains and sampling periods such that consensus is reached. Moreover, two approaches are presented to design such controller gains and sampling periods. Simulations are performed to validate the theoretical results.