Background: Aromatherapy has been proposed as a non-pharmacological adjunctive intervention in critical care settings; however, evidence regarding its effects on objective outcome measures remains inconclusive. This systematic review aimed to evaluate the effectiveness of aromatherapy in modulating objective physiological and biochemical markers in adult intensive care unit (ICU) patients. We hypothesized that the controlled ICU environment would facilitate rigorous measurement of both physiological parameters and biochemical stress markers. Methods: A systematic search was conducted using a novel approach combining lexical database searching (PubMed) and AI-powered semantic search strategies (Elicit, Undermind). Randomized controlled trials examining aromatherapy effects on physiological parameters in critically ill and cardiac patients were included. Outcomes were categorized as cardiovascular, respiratory, and non-cardiovascular/non-respiratory parameters. Subgroup analyses were performed by essential oil type. Results: Eighteen studies comprising 1236 participants were included. Aromatherapy demonstrated moderate effects on cardiovascular parameters (systolic blood pressure: 50% success rate, 5-22 mmHg reductions; diastolic blood pressure: 50%, 3.7-14 mmHg; heart rate: 44%, up to 20 bpm) but limited effects on respiratory parameters (respiratory rate: 25%; peripheral oxygen saturation: 12.5%), with the exception of eucalyptus oil, which showed 75% success for respiratory outcomes in mechanically ventilated patients. Non-cardiovascular/non-respiratory outcomes demonstrated the highest efficacy: anxiety (86%), sleep quality (100%), pain (100%), and sedation outcomes (100%). Subgroup analysis revealed essential oil blends achieved superior cardiovascular and psychological outcomes (100%) compared to lavender alone (64% cardiovascular; 71% anxiety/sedation). Contrary to our hypothesis, no studies measured biochemical stress markers such as cortisol or catecholamines, representing a significant gap in the evidence base. A notable finding was the disparity between evidence certainty: high for anxiety reduction versus low/very low for physiological outcomes, highlighting inadequate control for ICU-specific confounders in existing trials. Conclusion: Aromatherapy may serve as a safe adjunctive intervention in critically ill patients, with potential cardiovascular benefits requiring cautious interpretation and strong effects on anxiety and sleep outcomes. The absence of biochemical outcome measurements and inadequate control for ICU-specific confounders limits mechanistic understanding and causal inference.