<Tr> <Td> Grade </Td> <Td> Description </Td> </Tr> <Tr> <Td> Grade 1 </Td> <Td> Very faint, heard only after listener has "tuned in"; may not be heard in all positions . Only heard if the patient "bears down" or performs the Valsalva maneuver . </Td> </Tr> <Tr> <Td> Grade 2 </Td> <Td> Quiet, but heard immediately after placing the stethoscope on the chest . </Td> </Tr> <Tr> <Td> Grade 3 </Td> <Td> Moderately loud . </Td> </Tr>

The function of the valves in the chest is what