Graphical Analysis of Systolic Pressure Variations And
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GRAPHICAL ANALYSIS OF SYSTOLIC PRESSURE VARIATIONS AND RELATED NONINVASIVE INDICATORS OF BLOOD VOLUME STATUS. by Richard Stewart Shelton A thesis submitted to the faculty of the University of Utah in partial fulfillment of the requirements for the degree of Masters of Science in Bioengineering Department of Bioengineering University of Utah July 2002 Copyright © Richard Stewart Shelton 2002 All Rights Reserved SUPERVISORY COMMITTEE APPROVAL FORM FINAL READING APPROVAL FORM ABSTRACT Systolic Pressure Variation or SPV is a useful indicator of blood volume status. SPV is characterized as an initial increase in systolic pressure followed by a subsequent decrease in systolic pressure due to positive pressure ventilation. With blood loss, the systolic pressure is further decreased and the overall variation in systolic pressure increases. SPV is similar to the variation of other indicators of blood volume status in that it corresponds to a simultaneous variation in stroke volume. The cause of these variations was studied through graphical analysis using different models of the circulation. These models showed that positive intrathoracic pressure causes an initial increase in stroke volume followed by a decrease in stroke volume, which causes the increase and decrease in systolic pressure observed in SPV. However, these models were not able to predict a greater decrease in stroke volume after blood loss. Therefore, a further decrease in the systolic pressure was not predicted. Nevertheless, some more recent studies of the compensation mechanisms of the body affecting these models suggest that a further decrease in systolic pressure with blood loss is possible. Currently, these compensation mechanisms are not well enough understood to completely explain the further decrease in systolic pressure in SPV after blood loss. Therefore, an animal study is proposed to better understand the compensation mechanisms that cause the further decrease in systolic pressure with blood loss. TABLE OF CONTENTS ABSTRACT………………………………………………………………………….….iv ACKNOWLEDGEMENTS……………………………………………………………..vi Chapter I. SYSTOLIC PRESSURE VARIATIONS………………………………..1 II. PHOTO-PLETHYSMOGRAPHIC PULSE WAVEFORM VARIATIONS ……………………………………………………………………..…….7 III. CARDIOGENIC OSCILLATION VARIATION ……………………..10 IV. THE THEORY OF SPV ..………………………………………………15 V. ∆UP …………………………..………………………………………...17 VI. ∆DOWN……..………………………………………………………….21 Guyton’s Graphical Model …………………………………………….21 SPV is Caused by Positive Pressure Breaths .………………………….28 VII. SPV INCREASES WITH BLOOD LOSS ……………………………..30 The Effect of Blood Loss and Compensation Mechanisms on the Function Curves…………………………………………………………………..31 Will ∆down Increase with Blood Loss According to Guyton’s Model? ………………………………………………………………………….32 What Went Wrong?…………………………………………………….34 VIII. ADDITIONAL STUDIES OF PHOTO-PLETHYSMOGRAPHIC PULSE WAVEFORM VARIATIONS……………………. …………………...38 IX. FURTHER WORK …………………………………………………….45 X. CONCLUSION………………………………………………..………..47 REFERENCES………………………………………………………………….………49 v ACKOWLEDGEMENTS vi CHAPTER 1 SYSTOLIC PRESSURE VARIATIONS What Is It? Systolic Pressure Variation (SPV) is the variation in systolic blood pressure during one positive pressure breath initiated by ventilation [1-13]. Figure 1. Systolic Pressure Variations (SPV). Perel A, Pizov R, Cotev S: Systolic blood pressure variation is a sensitive indicator of hypovolemia in ventilated dogs subjected to graded hemorrhage. Anesthesiology. 67: 499, 1987. SPV is measured by first establishing a baseline systolic pressure during apnea [1]. Then, after initiation of a positive pressure breath, the increase and decrease in systolic pressure from this baseline is measured as ∆up and ∆down respectively [1]. 1 Figure 2. ∆up and ∆down. Pizov R, Ya’ari Y, Perel A: Systolic pressure variation is greater during hemorrhage than during sodium nitroprusside-induced hypotension in ventilated dogs. Anesth Analg. 67:171, 1988. In general, at the initiation of a positive pressure breath, the systolic pressure will initially increase and ∆up is measured [1]. Then, within the same positive pressure breath, the transient increase in systolic blood pressure is followed by a decrease in systolic blood pressure below the baseline systolic blood pressure and ∆down is measured [1]. Figure 3. Isolated breath with ∆up and ∆down. Perel A, Pizov R, Cotev S: Systolic blood pressure variation is a sensitive indicator of hypovolemia in ventilated dogs subjected to graded hemorrhage. Anesthesiology. 67: 499, 1987. 2 Measurement of SPV has been successfully used as a minimally invasive indicator of blood volume status for over a decade [1-9]. Most of the initial successful studies where SPV was used as an indicator of blood volume status were performed on dogs [1-3]. In these studies done by Perel and associates, SPV was measured at normal blood volume, and after blood loss of 5, 10, 20, and 30% of their estimated blood volume. Then, SPV was measured again after retransfusion. As blood volume was decreased, SPV and especially the ∆down portion of SPV, increased [1]. Figure 4. SPV at normal and decreased blood volume. Rooke GA, Schwid HA, Shapira Y: The effect of hemorrhage and intravascular volume replacement on systolic pressure variation in humans during mechanical and spontaneous ventilation. Anesth Analg. 80: 926, 1995. In these first studies, increase in SPV was found to be a more accurate indicator than other commonly used indicators of blood volume status [1-2]. Below are the results of a study comparing SPV and ∆down with other indicators of blood volume status. %SPV, shown below in the results, is the measured SPV divided by the systolic pressure at apnea. 3 Figure 5. SPV Results. Perel A, Pizov R, Cotev S: Systolic blood pressure variation is a sensitive indicator of hypovolemia in ventilated dogs subjected to graded hemorrhage. Anesthesiology. 67: 501, 1987. Similar studies have confirmed the usefulness of SPV in monitoring blood volume status in varied circumstances [4-13]. Some studies have even demonstrated the usefulness of automating the measurement of SPV where the systolic pressure at the end of a positive pressure breath was approximated as the baseline blood pressure at apnea [14]. 4 Figure 6. Schwid study comparison of end-expiration pressure to apnea pressure. Schwid HA, Rooke GA: Systolic blood pressure at end-expiration measured by the automated systolic pressure variation monitor is equivalent to systolic blood pressure during apnea. Journal of Clinical Monitoring and Computing. 16: 118, 2000. As a partial explanation of the origin of these variations in systolic pressure, Perel and associates performed a study in which they concluded that variations in systolic pressure were closely related to variations in stroke volume [15]. Furthermore, these decreases in systolic pressure in ∆down were accompanied by simultaneous decreases in stroke volume [15]. These results imply that the variations in systolic pressure are a direct result of the variations in stroke volume beat to beat. Below are the results showing the correlation between the variations in systolic pressure and stroke volume measured through velocity time index (VTI) detected by a doppler probe. 5 Figure 7. Correlation of SPV and Stoke Volume Variation or % change in VTI. Beaussier M, Coriat P, Perel A, Lebret F, Kalfon P, Chemla D, Lienhart A, Viars P: Determinants of systolic pressure variation in patients ventilated after vascular surgery. Journal of Cardiothoracic and vascular anesthesia. 9(5):550, 1995. 6 CHAPTER 2 PHOTO-PLETHYSMOGRAPHIC PULSE WAVEFORM VARIATIONS These variations in stroke volume can also be detected through monitoring variations in other indicators of hemodynamics. One of these is photo-plethysmographic pulse waveform variations (PWV) or the variations of the waveforms of pulse oximetry. The first study to show the usefulness of PWV in monitoring blood volume was performed by Partridge and associates [16]. They showed that an increase in PWV was a sensitive indicator of blood loss or hypovolemia [16]. An example of the change in PWV during blood loss and fluid resuscitation is shown below. After blood loss, PWV increased; and after fluid resuscitation, the PWV decreased to near its original variation. Figure 8. Examples of PWV in Partridge study. Partridge BL: Use of pulse oximetry as a noninvasive indicator of intravascular volume status. J Clin Monit. 3: 265, 1987. In addition, Perel and associates also found PWV to be effective as an indicator of blood volume status [17]. PWV was measured by Perel and associates in a similar way 7 to SPV [17]. The photo-plethysmographic pulse waveform was first recorded during apnea [17]. From this waveform the signal strength of the peak plateau was recorded as a baseline [17]. Next, at the initiation of a positive pressure breath, the signal strength from the maximal peak to the apneic plateau was measured as the ∆up [17]. Finally, the signal strength from the minimal peak to the apneic plateau was measured as the ∆down [17]. An example of PWV compared to SPV during normal blood volume and during hypovolaemia is shown below. Figure 9. Examples of PWV in Shamir study. Shamir M, Eidelman LA, Floman Y, Kaplan L, Pizov R: Pulse oximetry plethysmographic waveform during changes in blood volume. British Journal of Anaesthesia. 82(2): 180, 1999. The results of their studies matched closely with the results of SPV studies. They show that PWV is an effective monitor of blood volume status. However, PWV has several advantages over SPV. Because pulse