The physiological calculation application in the HP Component Monitoring System

Hewlett-Packard Journal, Oct, 1991 by Steven J. Weisner, Paul Johnson

This application converts raw real-time data into derived values the clinician can use to assess the patient's hemodynamic, oxygenation, and ventilatory condition. The HP Component Monitoring System bedside monitor provides the clinician with a variety of vital-sign parameters such as heart rate and respiration rate. These raw values and the associated alarms are very important in monitoring the patient. However, the human body is not a collection of independent physiological systems. Rather, all major body systems interact in a variety of ways, many of which can be calculated by combining the raw parameter values into meaningful indicators.

Physiological calculations are used routinely by many hospitals as part of their normal assessment and record-keeping process. Calculations provide a way of quickly reducing a large number of variables into a single number that represents a comprehensive physiological function. For example, to measure the load applied to the left ventricular heart muscle during the period of the heartbeat when the blood is ejected from the heart into the rest of the body ventricular ejection), a variable called systemic vascular resistance (SVR) can be calculated from measurements of the mean arterial blood pressure (ABPm), central venous pressure (CVP), and cardiac output (CO).[1]

Studies have shown that calculated values such as pulmonary vascular resistance (PVR) and left and right cardiac work (LCW/RCW) are good predictors of major malfunctions or mortality in intensive care patients.[2] Other studies have validated the efficiency of using calculations such as stroke index (SI) and left and right ventricular stroke work (LVSW/RVSW) for preoperative assessment of unacceptable risks for major surgery.[3]

The Typical Calculation

Poiseuille's law describes the laminar, constant flow of Newtonian liquids through rigid cylindrical tubes. According to this law, the ratio of pressure drop to the rate of flow is a function of all of the forces that retard this flow (i.e., radius, length, and viscosity). Blood does behave as a Newtonian fluid in blood vessels that are greater than 0.5 mm in diameter. Blood flow through these vessels is generally laminar, although the arterial tree exhibits more pulsatile behavior. Although blood vessel radii do vary slightly because of the applied pressure of the blood, Poiseuille's law can be used to calculate a first-order approximation of resistance by applying Ohm's law for electrical circuits.

Just as resistance in a circuit is equal to the voltage difference divided by the current flow, vascular resistance (R) can be approximated by dividing the pressure difference between the inlet of the vascular bed (Pl) and the outlet of the bed (P2) by the blood flow (Q).

R = (P1 - P2)/Q.

In medical terms, we measure the difference between the mean arterial (ABPm) and venous (CVP) pressures and divide by the cardiac output (CO). The resultant value is converted from units of mmHg/1 to units of dyne-s/cm[sup.5] by multiplying times 79.97. This value is called systemic vascular resistance (SVR).

SVR = 79.97(ABPm - CVP)/CO.

With this value, the clinician can get a measure of the constriction of blood vessels vasoconstriction) or expansion of the blood vessels vasodilation). Changes in SVR are related to other cardiac failures such as hypovolemic shock, left ventricular failure, cardiogenic shock, and hypovolemic.[4]

Other calculations used to assess the state of the cardiovascular system are shown in Fig. 1.

The two pressure measurements ABP and CVP are acquired through invasive pressure catheters attached to the patient and monitored through the Component Monitoring System parameter modules. The cardiac output parameter is obtained through a CO parameter module and measured using a monitoring procedure, which requires the clinician to interact with the Component Monitoring System. The acquisition of the output value (SVR in this case) and the presentation of the calculations to the clinician are described in the following sections.

Data Management Package

The calculations package in the Component Monitoring System is a subset of a more general data management package. This package consists of seven Component Monitoring System application software modules, as shown in Fig. 2. The data acquisition module acquires and averages raw parameter data (e.g., heart rate) over a one-minute period. This raw data is available as a broadcast message on the Component Monitoring System's internal message passing bus. The one-minute-average data is stored in a buffered RAM database. The database module provides 24 hours of data storage for 16 continuously monitored parameters with one-minute resolution. Parameters that are measured intermittently or as part of a procedure, such as noninvasive blood pressure or cardiac output, are referred to as aperiodic parameters. The database module allows storage of 36 aperiodic parameters, each containing up to 96 measurement points. All retrieval of the data is mediated by request messages and return-data messages sent across the message passing bus.