Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.55 no.1 Campinas Jan./Feb. 2005
Systolic pressure variation as diagnostic method for hypovolemia during anesthesia for cardiac surgery*
Variación de la presión sistólica como método diagnóstico de la hipovolemia durante anestesia para cirugía cardiaca
Ricardo Vieira Carlos, M.D.I; Cristina Salvadori Bittar, M.D.II; Marcel Rezende Lopes, TSA, M.D.III; José Otávio Costa Auler Júnior, TSA, M.D.IV
IMédico Preceptor da Disciplina
de Anestesiologia da FMUSP
IIGraduanda da FMUSP - Bolsista PIBIC
IIIMédico Assistente do Serviço de Anestesiologia e UTI Cirúrgica do Instituto do Coração, HC - FMUSP
IVProfessor Titular da Disciplina de Anestesiologia da FMUSP, Diretor do Serviço de Anestesiologia e UTI Cirúrgica do Instituto do Coração, HC FMUSP
BACKGROUND AND OBJECTIVES: An accurate
predictor of effective intravascular volume is of paramount importance for patients
submitted to major surgical procedures. A new method to evaluate intravascular
volume based on systolic blood pressure variations (SPV), (difference between
the maximum and minimum systolic values during controlled respiratory cycle)
and its variable delta down (dDown) has shown to be a sensitive indicator of
ventricular preload. As SPV is not routinely used in clinical practice our purpose
was to evaluate the accuracy of this parameter in evaluating volume status of
patients submitted to cardiac surgery.
METHODS: As from specially developed software, blood pressure variation was transmitted in real time from operating room monitor to a network-connected computer. After the adaptation of this system, nine patients submitted to cardiac surgery were evaluated. Variables were recorded in two moments: T0 (before volume replacement) and TP (after volume replacement). At the same time, conventional hemodynamic parameters were also studied and compared to systolic pressure variation.
RESULTS: Primary study results have shown that SPV (systolic pressure variation), in its dDown component, presents the best variation consistency after volume replacement with starch. Remaining hemodynamic parameters evaluated, although pointing to clear cardiovascular improvement after replacement, are highly variable among the patients and even on expander's response.
CONCLUSIONS: Results have shown that SPV is a sensitive method to evaluate intravascular volume status in patients under mechanical ventilation, when correlated to central venous pressure, pulmonary capillary wedge pressure and systolic index variations.
Key words: MONITORING: blood pressure, cardiac output, central venous pressure, hemodynamic, pulmonary capillary wedge pressure; SURGERY, Cardiac; VOLEMIA
JUSTIFICATIVA Y OBJETIVOS: La estimativa
perfeccionada del volumen intravascular efectivo es de gran importancia en pacientes
sometidos a procedimientos quirúrgicos de grande amplitud. La evaluación
de la volemia, basada en la variación de la presión sistólica
(VPS), (diferencia entre los valores sistólicos máximos y mínimos
durante un ciclo respiratorio controlado mecánicamente) y su variable delta
down (dDown) se ha mostrado un indicador sensible de la pre-carga, cuando comparados
con parámetros hemodinámicos convencionales. Como la VPS no es un
parámetro utilizado rutinariamente para evaluación de la volemia,
este trabajo tuvo como objetivo introducir la técnica de la medida de la
VPS y verificar su validez en pacientes sometidos a la anestesia para cirugía
MÉTODO: Desde un programa de computadora especialmente desarrollado, se transmitió en tiempo real la variación de la presión arterial desde el monitor de la sala quirúrgica para la microcomputadora conectada en red. Después de la adaptación de este sistema, fueron estudiadas las variaciones de la presión sistólica en nueve pacientes sometidos a la revascularización del miocardio. Las variables fueron registradas en dos momentos, utilizándose la expansión volémica como marcador: M0 (antes de la expansión volémica) y M1 (después de la expansión volémica). También fueron estudiados algunos parámetros hemodinámicos convencionales, confrontados con la variación de la presión sistólica.
RESULTADOS: Los principales resultados de este estudio muestran que la VPS, en su componente dDown, es la que presenta mayor consistencia de variación después de la expansión volémica con almidón.Los demás parámetros hemodinámicos estudiados, aunque apunten para una clara mejoría cardiovascular después de la expansión, poseen alta variabilidad entre los pacientes y mismo en cuanto a la respuesta al expansor.
CONCLUSIONES: Los resultados logrados muestran que la VPS se comporta como un sensible indicador de la volemia, en pacientes bajo ventilación mecánica, cuando correlacionada a las variaciones de la presión venosa central, presión capilar pulmonar e índice sistólico.
Accurate estimate of effective intravascular volume is a major index to assure adequate heart performance during major surgeries, among them cardiac surgeries. This procedure is associated to major blood losses and direct heart manipulation, which may worsen previous ventricular dysfunction. In addition, due to fasting, diuretics and vasodilators, hypovolemia is frequently seen during anesthetic induction and is a common cause of hemodynamic instability. Consequently, adequate volume status for atrial filling is of paramount importance to maintain cardiac output 1-8.
Classic volume status monitoring through catheters in right atrium and/or pulmonary artery is routinely used in this type of procedure, allowing for the anesthesiologist to determine ventricular filling pressures and helping therapeutic decision process 1-6.
Although many studies question central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP) accuracy in estimating right and left ventricular preload, and as a consequence, cardiac output response to plasma expander infusion, these parameters are still widely used in the perioperative period 6. A recent proposal to evaluate volume status in patients under mechanical ventilation and invasive blood pressure is systolic pressure variation analysis (SPV).
During positive pressure inspiration it is possible to observe a two-phase change in blood pressure tracing. In early inspiration, there is increased blood pressure with posterior decrease which coincides with end of inspiration and beginning of expiration. The difference between maximum and minimum systolic blood pressure (SBP) after a respiratory cycle under mechanical ventilation was called systolic pressure variation (SPV). SPV is divided in two components: delta Up (dUp) and delta Down (dDown), as from systolic blood pressure tracing after a short apnea interval for reference. dDown is the difference between SBP value in apnea and the lowest SBP valve during a respiratory cycle under mechanical ventilation. This magnitude reflects venous return decrease due to increased intrathoracic pressure (Figure 1).
dUp is the difference between maximum SBP and reference SBP during apnea. dUp is a temporary left ventricular systolic volume increase, due to the combination of preload increase as blood is expelled from lungs. Decreased afterload, direct pressure of expanded lungs on heart, and improved left ventricular compliance due to temporary decrease in right heart chambers volume also contribute for dUp (Figure 1). In anesthetized normotensive and normovolemic patients, SPV is approximately 8 to 10 mmHg.
In serial investigations, Perel et al. have supplied the fundamental basis for clinical SPV interpretation 7-9. According to these studies, dDown may increase during hypovolemia, being responsible for almost all systolic pressure variation. As fluids are administered, dDown decreases while in hypervolemia or congestive heart failure dDown is virtually nonexistent.
Tavernier et al. 14, studying a group of septic patients under mechanical ventilation, have observed that dDown component of SPV is highly sensitive to indicate fluid infusion. Major results shown in figure 2 show that SPV component dDown is more accurate to diagnose hypovolemia as compared to PCWP and indexed left ventricular end-diastolic volume (ILVEDV) obtained by echocardiography.
This study aimed at comparing conventional hemodynamic parameters before and after volume replacement and at comparing some of them to systolic blood pressure variation. For such, specific software was developed to simultaneously capture systolic blood pressure variations during respiratory cycles under anesthesia using a conventional multiparameter monitor common in operating rooms.
After their free and informed consent, participated in this study 9 male patients with ventricular ejection fraction above 40% submitted to myocardial revascularization.
Patients were premedicated with oral midazolam (7.5 mg), 45 minutes before anesthetic induction. Anesthesia was induced with fentanyl (5 µg.kg-1), etomidate (0.2 mg.kg-1) and atracurium (0.5 mg.kg-1) and maintained with isoflurane (0.8% ± 0.3%). Tracheal intubation was performed after unconsciousness and muscle relaxation and mechanical ventilation was started with 50% oxygen inspired fraction diluted in air. Tidal volume was standardized in 6 to 8 mL.kg-1 in volume-controlled mode, and respiratory rate was standardized in 10 to 12 breathes per minute to obtain CO2 expired fraction between 35 and 40 mmHg.
The study was performed after general anesthetic induction however before surgical stimulation.
Monitoring and Equipment
ECG with three leads and ST segment analysis, and invasive blood pressure through 20G catheter placed in the radial artery, as well as conventional hemodynamic parameters - central venous pressure (CVP), pulmonary artery pressure (PAP), pulmonary capillary wedge pressure (PCWP) and derived calculations [systemic vascular resistance index (SVRI) and pulmonary vascular resistance index (PVRI)] were obtained from the multiparameter monitor (Siemens INFINITY SC7000 modular monitor, Munich, Germany).
Through pulmonary artery catheter (Swan-Ganz CCOmbo V - Edwards Lifesciences LLC, Irvine, Califórnia, USA) inserted in right internal jugular vein after anesthetic induction, vascular pressures and cardiac output were obtained. Cardiac output was continuously obtained by means of Vigilance monitor (Edwards Lifesciences LLC, Irvine, Califórnia, USA). Anesthesia machine was model Cicero (Cicero Drägerwerke, Lübeck, Germany), which has supplied capnography, pulse oximetry, gases analysis, airway pressure and tidal volume data. These two latter were necessary for SPV and were transferred from the ventilator to the multiparameter monitor by the MIB system (protocol converter-medical information bus; Siemens INFINITY SC7000 modular monitor, Munich, Germany).
Capture of Blood Pressure Signals Simultaneously to Airway Pressure
To analyze SPV and by definition obtain dDown (difference between systolic blood pressure in apnea and minimum systolic blood pressure during one mechanical respiratory cycle), a method was jointly developed with the Informatic Division to record patients' data using the Electronic Patient Record (EPR) system. EPR has allowed the recording of all available parameters in a frequency of up to 100 times per second. Parameters were stored in Excel spreadsheet (Microsoft Company), using a conventional PC. Parameters of interest were transferred from the operating room multiparameter monitor to the PC through the Intranet network. Values were captured once per second and apnea length was 20 seconds to determine reference systolic blood pressure (SBP). To reach signals agreement, beginning and end of apnea were recorded according to multiparameter monitor time supplied by the internal computer network (Intranet).
As from data presented in table format, SBP in apnea was measured using as reference respiratory parameters with zero value (expired CO2 fraction, airway pressure curve and tidal volume) and apnea time. dDown and dUp values were calculated during post-apnea respiratory cycle.
After system's adjustment, variables were recorded: SPV and its components, and remaining hemodynamic parameters in two moments: M0 (moment zero) before volume replacement, approximately 20 minutes after anesthetic induction and hemodynamic stabilization, and M1, immediately after volume replacement with 7 mL.kg-1 of 6% hydroxyacetylamide (hydroxyethyl starch, 200/0.5 Haes Steril 6%; Fresenius-Kabi AG, Bad Hombourg, Germany). Volume replacement was performed in 30 minutes through peripheral venous catheter.
Study Objectives and Statistical Analysis
The objective was to observe SPV and its dDown component behavior before and after fluid infusion, as well as variation of hemodynamic parameters obtained together with SPV. Student's t paired test was used to compare both moments, and Pearson's correlation was used to check variation magnitude of some parameters considered of higher impact (cardiac index, systolic volume index, pulmonary capillary wedge pressure, and central venous pressure) which were correlated to SPV. The objective was to determine which variable had the most effective response to replacement. To accept or reject hypothesis, p equal to or below 0.005 was considered significant.
Absolute SPV, CO, CI, PCWP, PVRI, SVRI and HR values obtained for each patient before and after volume replacement are shown in table I. Table II shows mean, maximum and minimum value and SD of some selected variables with significant changes in M1 as compared to M0. As expected, when volume was replaced, all variables presented changes indicating cardiac function improvement in all patients. CI, represented by SVI and considered the result of heart pump function, was below expectations in most patients. Since there has been SVI increment after volume replacement, this may be characterized as hypovolemia correction.
dDown was within acceptable ranges for all individuals and in line with the literature. Its decrease after volume replacement reflects hemodynamic improvement, especially volume deficit correction.
For CVP and PCWP, it has been observed that 5 and 2 patients, respectively, were below normal values with 100% increase after volume replacement. Systemic vascular resistance index (SVRI) and pulmonary vascular resistance index (PVRI) showed lower values even after replacement, what could be inferred by increased cardiac output.
Since changes in variables after hydration were in different scales, proportional variations from M0 to M1 in each variable were studied as follows:
VM1 - VM0
where VM0 is variable value in M0 and M1 is variable value in M1.
Table III shows that CVP was the parameter with highest mean proportional changes. However, standard deviation of this variable was relatively high, which could be explained by individual variations and confirms the concept that CVP is not an accurate guide for volume replacement. Most consistent proportional change was seen in dDown because it has lower standard deviation and proportional mean changes are close to changes in remaining variables. This is also in line with the literature in terms of its major specificity regarding volume status.
Correlations between proportional changes of variables two by two are shown in figure 8, while numeric values are shown in table IV. Dispersion matrix in figure 8 may be then interpreted. dDown variable has poor correlation with all other variables. This means that when compared two by two, they do not equally respond in different patients, indicating that response is not uniform between them and the same intervention.
Two other very tight correlations should be stressed: between cardiac index (CI) and systolic volume index (SVI), and between PCWP and CVP variables. The former, for being the same parameter because SVI is cardiac index by beat. As to both filling pressures, CVP and PCWP, both respond similarly to volume replacement. This is in line with statements that in most clinical situations, CVP alone could be used to evaluate volume status, eliminating the need for PCWP. None of the three variables dDown, CVP and PCWP is correlated to SVI. However, variable with most indications of association to SVI is dDown.
Table III shows that variable with more proportional mean changes was CVP.
Paired t Tests
Table V shows results of paired t tests for each variable of interest, showing that all had significant changes from M0 to M1. Confidence intervals of this table show significant differences for variables: dDown, CI, PVRI and SVRI. Variables CVP, PCWP and SVI, although being modified after volume replacement, were not statistically significant.
Only dDown has shown absolute variation above the respective significant value after volume replacement. It has to be stressed that these tests were performed with a small sample size. It is possible that with a larger sample other parameters would show absolute variation above the significant value.
Major results of this study show that SPV, in its dDown component has the highest variation consistency after volume replacement with starch. Remaining hemodynamic parameters, although pointing to clear cardiovascular improvement after replacement, are highly variable among patients and even in response to expander.
Blood Pressure Variation during Mechanical Ventilation
Fluid replacement monitoring is critical to prevent complications caused by overload or lack of intravascular space volume 15. Parameters such as PCWP and CVP are used, respectively, by 93% and 58% of physicians to decide for volume replacement 16. However, studies have shown that such values are somewhat inaccurate to evaluate volume status, especially in patients under mechanical ventilation 17-19. This is because preload-induced changes in systolic volume also depend on contractility and afterload, which are not evaluated by such parameters 20.
So, due to limitations of the above-mentioned indicators, which are primarily based on cardiac chambers end-diastolic pressure, discussions are appearing in the literature on different methods to better evaluate ideal preload for heart function 20.
Primarily based on end-diastolic volume and not on pressure, there are other methods such as transesophageal echocardiography and transesophageal Doppler which, although considered less invasive, are far from being ideal. Both require the presence of experienced operator 21,22, and echocardiography does not supply continuous information 22-24. Aortic flow and end-diastolic volume measurements by esophageal Doppler should also not be taken for long periods of time 25-27.
Current literature has shown the influence of mechanical ventilation on systolic volume, which may be expressed in the blood pressure curve as an alternative to evaluate volume status 7-14,28. According to the literature, the magnitude of systolic volume changes during mechanical ventilation may be a good indicator of cardiac output dependence on preload 16.
Although accurate blood pressure variations analysis requires sedated or anesthetized patients under mechanically controlled ventilation, regular cardiac rhythm and one arterial line, the method seems to be practical and very attractive for intensive care units physicians and anesthesiologists.
Airflow movement generated by the ventilator inward and outward lungs determines major hemodynamic influences, being most of them related to increased or decreased blood flow to cardiac chambers with ejected systolic volume as final common pathway. Magnitude of systolic volume changes is a consequence of tidal volume generated by the ventilator and by increased pressure in the chest.
Systolic volume variation during one mechanical respiratory cycle, which can be identified in blood pressure tracing, also depends on myocardial contractility and volume status. For clarity reasons, actions of positive pressure pulmonary inflation on right and left ventricles will be commented in separate.
The movement of airflow under pressure inside the alveoli promotes right ventricular preload decrease and afterload increase. Both effects are transitory and determined by pleural pressure variations. Decreased preload may be explained by increased pleural pressure leading to momentary venous return decrease. Increased afterload is related to transpulmonary pressure increase (alveolar pressure less pleural pressure). Both effects contribute to right ventricular systolic volume decrease.
Ventilation inspiratory peak coincides with right ventricular systolic volume decrease which will be transmitted to the left ventricle, however with the delay of some heart beats due to time taken by the blood to cross pulmonary vessels 29-31. Consequently, left ventricular filling and systolic volume are also affected. The effect on both ventricles is cyclic, since filling pressures and systolic volumes will be restored during expiration 31. It is interesting to note that a mild left ventricular systolic volume increase may be observed during early inspiration. Two mechanisms may explain this: initially, during pulmonary inflation, blood is expelled from pulmonary vessels to left ventricle by alveoli distension during inspiration. Then, there is left ventricular afterload decrease promoted by pleural pressure increase 29-31.
And the question is: how could ventilation effects on blood pressure tracing be identified and used as a guide for fluid therapy?
This is a very important question since clinical application of SPV depends on the transfer of experimental methods to monitors at the bedside. Physiologically, left ventricular systolic volume is the major blood pressure determinant and systolic pressure variation analysis may allow the identification of the influence of fluid infusion on cardiac performance of mechanically ventilated patients.
Systolic pressure variation (SPV) is defined as the difference between maximum and minimum systolic volumes during one mechanically controlled respiratory cycle.
In anesthetized patients with adjusted volume status, normal SPV value is approximately 8 to 10 mmHg, being made up of two segments called "delta Up" (dUp) and "delta Down" (dDown), calculated using as reference a baseline during some seconds in apnea 7-12. Figure 1 shows that dUp is the difference between maximum systolic pressure and baseline during one respiratory cycle. dDdown is defined as the difference between baseline and minimum systolic pressure during one respiratory cycle.
It has been shown in a group of septic patients that, in the presence of dDown close to 5 mmHg, there should be response to fluid infusion with indexed systolic volume increase of approximately 15%. Conversely, if dDown is below 5 mmHg, response to fluid administration will be unlikely 14.
Our results have shown that, in anesthetized patients under mechanical ventilation, systolic pressure variation and dDown have provided adequate evaluation of systolic volume and cardiovascular system response to fluid infusion. This is shown in table I and table V. Table I shows absolute change in values of all hemodynamic parameters, including dDown. Table III shows that variable with highest mean proportional changes was CVP.
However, standard deviation of this variable is higher than its mean, which could be a consequence of discrepant values among individuals. Most consistent proportional change was seen in dDown, because it has the lowest standard deviation and, in mean, its proportional change is close to changes in other variables. This indicates that this parameter has consistent proportional increment with low variability margin, what speaks to its good specificity and more uniform response.
Table V shows significant differences in variables: dDown, CI, PVRI and SVRI. Variables CVP, PCWP and SVI, although changed after volume replacement, are not statistically significant. It is possible that small sample size and individual variability could explain this fact.
Paiva Filho et al. 28, in 2003, have shown in experimental model with dogs that SPV and especially its component dDown, are early hypovolemia indicators and sensitive volume replacement guides, being better than some hemodynamic parameters such as pulmonary capillary wedge pressure (PCWP) and cardiac index (CI). This is in line with results of Michard and Teboul 16, who have shown that systolic pressure variation analysis leads to accurate evaluation of preload dependence.
Reuter et al. 33, in 2002, have used systolic pressure variation to study left ventricular systolic volume variations in patients submitted to cardiac surgery and have shown good correlation between them.
All these studies have shown that systolic blood pressure variation and, consequently, its component dDown, are sensitive to estimate volume status and check circulatory response to fluid infusion. So, dDown may increase during hypovolemia, being responsible for almost all systolic pressure variation. dDown decreases with fluid administration, while in hypervolemia or congestive heart failure it is virtually nonexistent 7,9,16.
There are two major limitations to our study: first is the small sample size, which in our understanding was enough to test the technique but which might have impaired correlations. The second and primary limitation was the difficulty of developing an application program for simultaneous systolic pressure and respiratory cycle data collection and storage as from conventional multiparameter monitors used in anesthesia. For such, monitor data were recorded and transferred to an external computer for further analysis.
The ideal method to simplify the access to patient's hemodynamic status and help therapeutic approach would be direct systolic pressure variation and dDown data collection as from the cardiovascular monitor at the bedside, which is our future goal. For such, an application in the monitor would supply in real time systolic blood pressure variation and dDown numeric values.
Other limitation of SPV as diagnostic method for hypovolemia is that it cannot be applied to patients with arrhythmia or without arterial catheter. Mechanical ventilation is also implicit because it is not possible to significantly correlate SPV to volume status during spontaneous ventilation 12.
Systolic blood pressure variation and its variable dDown are a sensitive volume status indicator during positive pressure mechanical ventilation. Its negative deflection shows good correlation with fluid infusion and simultaneous cardiac output improvement and atrial filling pressures increase. In patients under mechanically controlled ventilation and systolic pressure variation monitoring, dDown seems to be a valid indicator during the difficult task of evaluating volume status during cardiac surgery.
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Prof. Dr. José Otávio Costa Auler Júnior
Address: Av. Dr. Enéas de Carvalho Aguiar, nº 44 Cerqueira César
ZIP: 05403-900 City: São Paulo, Brazil
Submitted for publication May 20, 2004
Accepted for publication September 14, 2004
* Received from Instituto do Coração InCor do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo (HC FMUSP), São Paulo, SP