As we know, there are many factors affecting SVV during anesthesia, mainly: end-systolic volume, end-diastolic volume, systemic vascular tone (afterload), total blood volume, heart muscle contractility, heart rate (determining duration of ventricular contraction), etc. many of these factors could be affected by the anesthetics, and I think there are many confounding factors that may complicate the assessment.

PPV seems to be a more reliable parameter and some works have been done to use it as a blood volume determinant.

from a new angle, SVV and PPV may be used simultaneously to gain a more reliable result (?) I am not sure...

PPV Has been appeared to be superior to SVV in recent studies and some noninvasive monitoring such as LiDCO measure SVV/PPV with high accuracy.

However, there are factors that hinder the accuracy of PPV monitoring, most notably ventilator-patient dyssynchrony, arrhythmia (particularly atrial fibrillation), low-tidal-volume

ventilation, altered chest wall and pulmonary compliance, pulmonary

hypertension, and increased intra-abdominal pressure. So, we should be careful and note that the interpretation of any finding on the monitor needs anesthesiologist's vigilance. Hope this helps.

SVV and PPV will assess fluid responsiveness and NOT blood volume. An example to demonstrate this would be the patient who had a normal SVV of 12 who was then given a large dose of spinal anaesthetic. We would see the blood pressure fall and the SVV rise to say 20. In this example the blood volume has remained the same but the elevation of SVV indicates that the patient would be fluid responsive. Giving fliud would increase the Blood pressure and reduce the SVV to the normal range.

Whether a patient is a fluid responder or not depends upon where they are on the Frank Starling Curve. If they are on the steep part they are fluid responsive. If they are on the flat part they are not fluid responders.

Reference: http://www.edwards.com/products/mininvasive/Pages/strokevolumevariationwp.aspx

If you have a critical patient who has a decreased blood pressure, the question would be, is volume loading is suitable for this patient? Or could this loading harm the patient and cause pulmonary edema and other fluid overload complications, so, we need to try other treatment e.g inotropes.

So, we need to determine if this patient is fluid responder or not.

If the patient cardiac function is on the ascending part of frank starling curve he would respond to fluid loading but if the heart is on the flat portion of the curve then he will not respond, and fluid loading will be harmful.

Many studies have shown a poor value of static tests like CVP, PCWP to predict which patients may respond to fluid. However, many studies have shown the ability of dynamic tests like SVV and PPV to predict volume responders.

For example, if you have a critical patient with decreased Bp and his SVV is more than 14, most probably he or she will respond to fluid loading, but if his or her SVV is less than 14 then most probably he or she is not going to respond to fluid loading and needs other treatment. Nevertheless, it is a matter of clinical judgment by using all the available data. Please for more details and references see the attached PPt presentation.

Very interesting, more interesting is the PVI or pleth variability index because it is non invasive. actually, these tools may be necessary as fluid replacement tend to move towards restrictive policy. Goal directed fluid therapy appear to be the future trend in fluid replacement. More recently is R- wave of ECG changes and changes in end tidal CO2 with respiration.

Recently published study in the BJA 2013; 100(3): 374-80 stated that 'the dynamic indices SVV and PPV add little value to a fluid optimisation protocol and should not replace SV measurments ...'. I very much agree with this conclusion. I think that we are trying to read too much from the values that are not directly measured but computed by the software of the device that is being used for CO measurment.

I'm sure that the only reason why we don't use this measurements as much as we should is the great prize of the devices which get us these values. We need cheaper devices!

To expand on Paul Stewart's example (spinal anaesthesia increases PPV without decreasing blood volume): it is worth noting that the PPV could also be decreased back towards normal by giving a vasoconstrictor. Phenylephrine increases preload when the heart is preload-dependant (J Appl Physiol 2012;11:281). Giving fluid is not necessarily the best option for every patient who is fluid responsive.

I agree with Dr McCulloch, and what the spinal example does is demonstrate the absurdity of using a single observation of anything that purports to indicate blood volume or even whether the patient will respond to volume. You could also give a vaso and venoconstrictor - no more PPV - end of problem? Not necessarily, because we don't know what the problem was in the first place.

But is there a problem? What is broken that you're trying to fix? What is the patient's main presenting condition? What comorbidities are there? What's the procedure? Is the patient hypotensive? Is he/she infracting? Going into LV failure? Going into diabetic KA? What kind of anaesthetic have you given? which volatile agent? or are you using TIVA and which agent? In theatre I don't find any of these monitors of much value in isolation; there's just too much happening at once - varying surgical stimulation, bleeding, moving guts around, assistant leaning on chest or liver, periosteal stimulation, cervical dilatation. And with a thoracotomy - all bets are off.

I find these monitors are more useful in the ICU setting. And to endorse Dr McCulloch's view - just because a machine tells you that the patient is Volume responsive, it doesn't mean it's the best thing to use.

So, in answer to the original question: not a lot!

Dr Schapera: in the ICU setting I would agree with you. In theatre (OR) I don't think it is as valuable as putting a finger on the pulse and looking over the screen to see what's going on in the surgical field

But VPP and VVS helps us confirm our suspicions in the surgical field, as does BIS with TIVA. The main limitation is the high price of all this. But soon be resolved with a new simple and accurate mobile aplplication. Believe it.

Ouch! Contextualisation is the key. The question was: what do I think about the use of PPV and SVV to assess blood volume in anesthesia? Let's break down my answer:

1. The devices do not assess blood volume, they suggest whether or not the SV or PP variability is likely to be reduced by giving a volume bolus. This is generally assumed to be due to a reduced intravascular blood volume-to-container ratio (caveat emptor).

2. Depending upon the problem that made you first look at the device, infusing fluids might be one of several options open to you.

2.It is not always the most appropriate option, it often isn't.

3. Irrespective of its general utility, under the specific circumstances of use in theatre, where the abdomen or chest is open or you can't do a passive leg raise or any number of issues arise that increase noise, it is not a particularly useful tool. So I don't think a lot of it in this context.

Apropos the reference, this should be contextualised to the circumstances we have in South Africa where untrained generalists are expected to provide safe obstetric anaesthesia. There is some discussion in the paper to the progressive development of situational awareness in anaesthetic training that cannot be taught in the brief exposure generalists have. And yes, if it came to the issue of whether I sent these guys into the field armed either with a PPV analyser or their fingers and their wits, I would definitely choose the latter!

As a little experiment I would like to draw your attention to a couple of (outrageously decontextualized) examples such as the attached file

Or this one (a photoplethysographic trace - what the machines use for their ratios)

I agree. However, in a resource-limited setting we can't have everything. The utility (added value) of information has to be maximised per dollar spent. I have often noticed the lack of end-tidal CO2 monitoring in intensive care units. If an administrator were to offer a swap of a capnograph for a PPV analyser between your ICU and my theatre (or my ICU and your theatre), I suspect we would both say no thank you. If they were both being offered as free donations, we would both probably say yes please!!

I agree with most of the previous views, however, I will try to summarize things as follow:

These monitors are not routinely used in OR, however it can be used in major operations for critical patients.

These monitors are not for blood volume measurements, but for detecting the fluid responsive hypotensive patient to avoid overloading unsuitable patient.

like any other monitor. The monitor itself is not a treatment and its value depends on how can you interpret it in addition to other monitoring and clinical situation to achieve the best patient management

I think some of the fundamental reasons for not monitoring the dynamic variables as they should are both accessibility to devices that measure, and the price of these devices are used.

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> In this sense, the emergence of CAPSTESIA, a Smartphone App for advanced monitoring, I think it's a revolution.

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> Using a mobile photography allows you to scan any monitor screen and gives you values ​​as PPV, CO, dP / dt ... at the time. For any patient with invasive blood pressure. No wires, no added monitors ...

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> This really can bring this type of monitoring many places where it was impossible, and reasonably priced.

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> https://play.google.com/store/apps/details?id=com.simu.capstesia