Practicing critical care entails understanding human physiology, pharmacokinetics and molecular pathways in concert with adherence to evidence based literature. Some may say combining all of these entities into practice creates the "art" of critical care medicine. One strategy to gain proficiency in the practice of critical care medicine is to simulate what you would do in specific problem-based scenarios. That is the aim of this textbook aptly titled" "What Do I Do Now?" This text focuses on Cardiothoracic Critical Care and will cover guidelines for evidence-based practice, respiratory and metabolic physiology, common hemodynamic perturbations, ventricular failure and mechanical circulatory support devices. All clinicians who care for cardiothoracic patients who are critically ill can find pearls of practice wisdom complemented by literature citations within this text. So go ahead, place yourself at the foot of the bed and try to think through, "What Would I Do Now" when presented with each patient within these pages of your handheld cardiothoracic intensive care unit