diff --git a/metropolis/proto/api/management.proto b/metropolis/proto/api/management.proto
index ba20cee..b48e2b1 100644
--- a/metropolis/proto/api/management.proto
+++ b/metropolis/proto/api/management.proto
@@ -137,10 +137,10 @@
       HEARTBEAT_TIMEOUT = 3;
     }
     Health health = 5;
-    // heartbeat_timestamp is the duration since the last of the node's
-    // heartbeats was received, expressed in nanoseconds. It equals zero if no
-    // heartbeats were received.
-    int64 heartbeat_timestamp = 6;
+    // time_since_heartbeat is the duration since the last of the node's
+    // heartbeats was received, expressed in nanoseconds. It is only valid with
+    // the health status of either HEALTHY or HEARTBEAT_TIMEOUT.
+    int64 time_since_heartbeat = 6;
 }
 
 message ApproveNodeRequest {