The Highs and Lows of Blood Pressure

OBJECTIVE: Measurement of blood pressure is fundamental for the management of patients in shock, yet the physiological basis and meaning of blood pressure measurements are complex and often not well understood. This article is in two parts: part 1 deals with the mechanical and physiological aspects of blood pressure and its measurement and part 2 deals with the role of changes in regional resistances in the determination of tissue perfusion and bedside approaches to management of shock. DATA SOURCE AND SELECTION: This review is based on physiological principles from texts and experimental studies which elucidate some of the key principles. The views expressed are the author's synthesis of the views of others and his own opinions. DATA SYNTHESIS: Arterial blood pressure is a major determinant of regional flow and is often used as a surrogate indicator of tissue perfusion, but in reality, it is a poor indicator of blood flow. Blood pressure is determined by cardiac output (total flow) and total vascular resistance. Distribution of flow for a given blood pressure is dependent on the relative values of resistances in different vascular beds. If this distribution of resistances were known, this would be the ideal guide to therapy. Unfortunately, regional resistances cannot be assessed in the clinical setting, and so we are left with blood pressure as a guide to therapy. CONCLUSIONS: This article discusses the implications of these points and explores factors that need to be taken into account when designing empiric trials to determine appropriate blood pressure targets for patients in shock. Even if well-studied empirically developed guidelines become available, it likely still will be important to individualize patient management and approaches for this are discussed too.