RecruitingDrug trialOpportunistic PK/PD Trial in Critically Ill Children (OPTIC)OPTIC is a prospective, open-label, non-randomized study of multiple medications administered to approximately 2000 children in the pediatric cardiac intensive care unit (PCICU) per routine clinical car by their treating provider. The purpose of this study is to characterize the PK of drugs routinely administered to children per standard of care using opportunistic and scavenged samples. The prescribing of drugs to children will not be part of this protocol. After the child/adult (\<21 years of age) is consented/enrolled, demographic and clinical data will be extracted from the EHR. Biospecimen information (including date and time of sample collection) will be collected. Data analysis will be conducted on all participants with at least 2 evaluable samples. The protocol represents minimal risk to the children/adults who provide body fluid for this study, including potential loss of confidentiality (samples will be assigned a unique accession number) and risks associated with blood draws. Adverse Events (AEs)/Serious Adverse Events (SAEs) caused by the study specimen collections will be monitored and recorded in the Electronic Data Capture (EDC) system.