If their BPs are soft, you figure out why. You don't always go to pressors. In fact, you should always think about fluid resuscitation first. Get large bore IVs in and bolus fluids. A good framework to think about this goes back to physiology. What determines BP? The good old Ohm's Law. P = CO x R. CO = SV x HR. So there are three things that can affect pressure - stroke volume, heart rate, and resistance. Decreases in each of these can drop your blood pressures.
A useful framework is to think about causes of shock. Because that's why you want to treat soft BPs in the first place. If your BPs go too low, your organs aren't perfused and you go into shock. There are four big categories of shock, namely hypovolemic, distributive, cardiogenic, and obstructive.
In hypovolemic shock, you have too little circulating volume. Thus, your SV will drop. HR will rise to compensate but you can only get tachy to so much before your heart doesn't have time to fill completely. This results in decreasing BPs. In distributive shock, your vessels massively vasodilate, resulting in your resistance plummeting. This also drops your BP drastically. Your HR will respond and you will become tachy (exception is neurogenic shock, in which case the problem is a failure of sympathetic regulation so you get an inappropriate bradycardia). In both of these cases, you should start by bolusing them with fluids. If they're in hypovolemic shock, this (almost) treats the etiology. It should fix the problem unless they're actively bleeding or still losing fluids. If they're in distributive shock, you also treat the cause. In sepsis, that involves treating the infection. In anaphylaxis, you reverse the anaphylaxis. In cases of distributive shock, you think about pressor therapy if they're not responding or inadequately responding to the fluids.
The other two types of shock are different in that there's nothing wrong with your volume status or vascular resistance. The problem is in (or near) the heart. Cardiogenic shock is exactly as it sounds. Your heart is failing to do its job for whatever reason and the forward flow of blood out of the heart is impaired. Here, you do fluid resuscitation and then you give inotropes to help your heart pump. Finally, obstructive shock is when there is a physical obstruction to forward blood flow. One example is tamponade. There is an obstruction of blood filling in the heart. The definitive treatment for obstructive shock is the relieve the obstruction, e.g. pericardiocentesis.
UptoDate has a great algorithm for initial and continued management of shock.
