Which of the following is used to decrease blood pressure and respiratory rate?

Barbiturates are central nervous system depressants that have sedative and anxiolytic properties. When used in a clinical setting, they can lead to a decrease in both blood pressure and respiratory rate as part of their sedative effects. By depressing the central nervous system, barbiturates can also slow down cardiac function and diminish the body's natural response to stress, which often includes increased heart rate and respiratory effort. This makes them effective in certain situations where calming a patient is necessary, though their use must be carefully monitored due to the potential for respiratory depression and other side effects.

The other options do not share the same dual effect on blood pressure and respiratory rate as barbiturates. Cardiac stimulants generally serve to increase heart rate and improve contractility, which would not decrease blood pressure. Antipyretics are primarily designed to reduce fever and do not have a direct effect on blood pressure or respiratory rate. Diuretics help to lower blood pressure by promoting fluid loss but do not directly impact respiratory rate. Therefore, barbiturates stand out as the appropriate choice for this question.