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Stroke Volume Variation
“Can We Use Fluid to Improve Hemodynamics?”
Introduction:
In clinicians’ quest to achieve optimal oxygen delivery (DO2) they are often faced with imprecise, non-specific information in which to guide their therapy. Traditional hemodynamic monitoring parameters (HR, MAP, CVP, and PAOP) are often insensitive and sometimes misleading in the assessment of circulating blood volume. However, the appropriateness of their interventions is often crucial to avoid the deleterious effects of over-, under-, or inappropriate resuscitation. Volume is one of the first therapeutic interventions clinicians turn to when optimizing DO2. The question that is often difficult to answer “Can we use fluid to improve hemodynamics?” and one might add, “Is it the appropriate intervention?” Stroke volume variation (SVV) as displayed on the Edwards Vigileo monitor when used with the Edwards FloTrac sensor may help answer this question.
What causes stroke volume variation?
Stroke volume variation is a naturally occurring phenomenon in which the arterial pulse pressure falls during inspiration and rises during expiration due to changes in intra-thoracic pressure secondary to negative pressure ventilation (spontaneously breathing). Variations over 10mmHg have been referred to as pulsus paradoxus. The normal range of variation in spontaneously breathing patients has been reported between 5-10mmHg.
Reverse pulsus paradoxus is the same phenomenon with controlled mechanical ventilation, however, in reverse. Arterial pressure rises during inspiration and falls during expiration due to changes in intra-thoracic pressure secondary to positive pressure ventilation. In addition to reverse pulsus paradoxus, it has also been referred to as paradoxical pulsus, respiratory paradox, systolic pressure variation and pulse pressure variation. Traditionally SVV is calculated by taking the SVmax - SVmin / SV mean over a respiratory cycle or other period of time.
SVV and assessing fluid response
SVV and its comparable measurement, pulse pressure variation (PPV), are not indicators of actual preload but of relative preload responsiveness. SVV has been shown to have a very high sensitivity and specificity when compared to traditional indicators of volume status (HR, MAP, CVP, PAD, PAOP), and their ability to determine fluid responsiveness. The following table of studies demonstrates SVV sensitivity and specificity in predicting fluid responsiveness against a specified infused volume and defined criteria for a fluid responder.
| Study | Michard2
| Berkenstadt,
et al.1 | Reuter,
et al.3 | | Patients | Sepsis | Neuro Surgery | Cardiac | | Volume | 500 ml | 100 ml | 10 x BMI | Tidal Volume
ml/Kg | 8 to 12 | 10 | 10 | Parameters
Tested (Artery) | ΔPP (R or F) | ΔSVV | ΔSVV | | R2 | 0.85 | 0.53 | 0.64 | Def. of
Responder | ΔCO≥15% | ΔSV≥5% | ΔSV≥5% | | Sensitivity | 94 | 79 | 79 | | Specificity | 96 | 93 | 85 |
How can I use SVV?
Normal SVV values are less than 10-15% on controlled mechanical ventilation. The figures below demonstrate using SVV as a guide for volume resuscitation with a goal SVV of <13%. SVV increased to 19% with a stroke volume (SV) of 43 ml/beat, blood and saline were given to obtain a SVV of 6% and a SV of 58 ml/beat.
What are the limitations and effects of other therapies on SVV? - Mechanical Ventilation
Currently, literature supports the use of SVV only on patients who are 100% mechanically (control mode) ventilated with tidal volumes of more than 8cc/kg and fixed respiratory rates. - Spontaneous Ventilation
Currently, literature does not support the use of SVV with patients who are spontaneously breathing due to the irregular nature of rate and tidal volumes. - Arrhythmias
Arrhythmias can dramatically affect SVV values. Thus, the utility of SVV as a guide for volume resuscitation is greatest in absence of arrhythmias. - PEEP
Increasing levels of positive end expiratory pressure (PEEP) may cause an increase in SVV, the effects of which may be corrected by additional volume resuscitation if warranted. - Vascular Tone
The effects of vasodilatation therapy may increase SVV and should be considered before treatment with additional volume. 
Summary
When used within its limitations SVV is a sensitive tool that can be used to guide the appropriate management of the patient’s preload to achieve optimal DO2 and answer the question “Can we use fluid to improve hemodynamics?” SVV is an available parameter with the FloTrac sensor and the Vigileo monitor.
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