Preload Assessment

Preload AssessmentToo much of a good thingIntravenous fluid boluses are used liberally in medicine to achieve a variety of goals.In critical care we use them most often to improve cardiovascular parameters and replace known lossesPrompt, appropriate resuscitation with fluid can prevent the need for vasoactive drugsToo much of a good thingExcessive or inappropriate use of fluid can cause problems both acutely with APO or worsening of ALI and over the longer term with potentially delayed extubation, persistent oedema predisposing to infections or potential surgical complications like anastomotic breakdowns. Preload assessmentSince it is harmful to give too much fluid as well as to give too little we need to have a good idea when its indicated. When treating a cardiovascular parameter it is important to treat the patient not the numbersE.g. Hypotension with no end organ dysfuncionAfter establishing that the cardiac output/Blood pressure needs to be improved the next important question is: Will this patient benefit from increased pre-load?What is pre-load?What is pre-load?Ventricular preload is the end-diastolic volume producing the initial passive stretch of the myocardium prior to active contraction.Volume not pressure.

What is pre-load?According to Starling ,One of 3 Determinants of Cardiac stroke volumePre-loadSympathetic ToneAfter-load

Why are we interested?Why are we interested?Increasing Pre-load increases Stroke volume.Stroke volume x Heart rate = Cardiac OutputCardiac output influencesSystemic Blood pressureEnd organ perfusionOxygenationNutrientsAcid and waste removalThe answer to life the universe and everythingFrank and StarlingCardiovascular PhysiologistsDeveloped animal models where RA was canulated and supplied with blood from a resovior. Height of resovior determined pressure (Preload)Aorta was attached to a variable resistor (Afterload) and fed back to resovior.The Vagus and Sympathetic nerves could be artificially Stimulated.

Image from P Cam Physiology for the AnaesthetistFrank and StarlingThe Ventricular volumes could then be measured.These graphs show how increased end diastolic volume increased the stroke volume to a point then output was decreased as overstretch occurred.Of note, these are volumes not pressures. The pressure curve has more of a plateau

Image from P Cam Physiology for the AnaesthetistFrank and StarlingThis shows the same part of the curve we are interested in using, but substitues pressure as preloadVolume is proportional to pressure

Frank and StarlingIs it really that simple?GuytonThe heart is a demand pump.It cant pump what it doesnt receive from venous return.In health the maximum Cardiac output achievable from solely increasing Preload is 10-15L/minAdding sympathetic stimulation this increases to 20-30L/minSo there is usually some reserve

Image from Current Opinion in Critical Care 2010,16:289296 Fluid status and Fluid ResponsivenessGuytonGuytons venous return and cardiac output curves can be plotted simultaneously to show their point of intersection. If you change the pre-load after a few cardiac cycles a new intersection point will be reached with a new cardiac output/venous return. There is a plateau to the CO curve an increase in pre-load will not always work.

Preload assessmentThe most important part about pre-load assessment is not what the pre-load currently is (CVP) but weather an increase will lead to a clinically significant effect.The assessment of Pre-load and prediction of fluid responsiveness can be divided into Static and Dynamic measurements.Each seeks to find a surrogate for the gold standard measurement of preload; the Left (or right) Ventricular End Diastolic volume (or pressure)Static vs dynamic predictorsStatic predictors seek to measure a single value in order to make a prediction on fluid responsivenessPressures CVP / PpaoVolumes; Global end diastolic volume

Dynamic predictors apply a controlled and reversible preload variation and measure the heamodynamic response.Variation with respiration: Stroke volume Variation / Pulse Pressure variationPlethysmographic variationIVC variationSystolic Pressure VariationPassive leg raise testValsalva manoeuvre

Ref: Crit Care Clin 26 (2010) Preload AssessmentStatic predictorsPressures CVP Ppao

Volumes Ventricular volumes (thermodilution)Left Ventricular End Diastolic Area (U/S)

Static predictorsCentral Venous PressureGives you a surrogate for Left Ventricular end diastolic pressure. Very unreliableNeeds to have a reproducible reference point to use absolute values (e.g. CVP of less than 8 needs fluid)Stopcock of transducer needs to be zeroed level with right atrium.Varies with Respiratory cycle, Varies (Inconsistently) given different PEEP valuesTotally unreliable if any right sided valvular disease.

Static predictorsCentral Venous PressureMarik and Rodrigo did an update meta-analysis published in July this year By the Society of Critical Care MedicineThis reinforced that there was little to no correlation between CVP values and fluid responsiveness. Making a plea stating this app to fluid resuscitation should be abandoned.Ref: Does the Central Venous Pressure Predict Fluid Responsiveness? An Updated Meta-Analysis and a Plea for Some Common Sense* Crit CareMed 2013; 41:17741781Static predictorsPulmonary arterial occlusion pressureSwan-Ganz catheter is passed into a branch of the pulmonary artery, a baloon inflated and the pressure beyond the balloon measured.This pressure is thought to be proportional to Left atrial pressure and an idea of Left Ventricular preload gleaned much simmilar to using the static measurement of CVP.It is an Invasive and potentially dangerous procedure and whilst a swan ganz is still the gold standard for measureing Cardiac output, performing Ppao has gone out of vouge

Ref: Static Measures of Preload Assessment Crit Care Clin 26 (2010) 295305Static predictorsPulmonary arterial occlusion pressureA 2002 review by Michard and Teboul of 12 studies found that Ppao could not reliably predict fluid responsiveness in any of the studies.

In general the best use for CVP or Ppao are in their negative predictive value.A high CVP (e.g. >10 mm Hg in one study) is less likely to respond to a fluid bolusA high Ppao (e.g. >11mm Hg in 2007 Osman Etal) is less likely to respond to a fluid bolus.

Ref: Static Measures of Preload Assessment Crit Care Clin 26 (2010) 295305Static predictorsPressures CVP Ppao

Volumes Ventricular volumes (thermodilution)Left Ventricular End Diastolic Area (U/S)

Static predictorsVolumes Right Ventricular End diastolic volume(thermodilution)A Pulmonary artery catheter with a rapidly responsive thermister in the tip and a means of warming or cooling a known quantity and temperature of blood in the right atrium allows measurement of the SV, CO and RVEF. The RVEDV can then be derived from the SV and the RVEF.Diebel etal. In 1994 compared RVEDVI in trauma patients to fluid responsiveness (20% increase in CI) and found that cut-offs of 140 mL/m^2 had predictive values of 64%, 27% and 0% respectively.

Static predictorsVolumes Global End Diastolic Volume (and index)PiCCO, uses a Central line and a specially designed Femoral Arterial line to calculate, by thermo-dilution the GEDVI Numerous studies exist showing good correlations between increases in GEDVI and SVI and one even compared these to CVPs which did not correlate to changes in SVIStatic predictorsIn summary;Pressures (CVP and Ppao) are only really useful to predict lack of response to fluids in the ICU and even that is subject to debate.Volumes measured by a swan ganz catheter (RVEDV) or PiCCO (GEDVI) have a better correlation with fluid responsiveness than static Pressures but, by nature require specialised invasive monitoring.

Dynamic PredictorsDynamic predictors apply a controlled and reversible preload variation and measure the heamodynamic response.Variation with respiration: Stroke volume Variation / Pulse Pressure variationPlethysmographic variationIVC variationSystolic Pressure VariationPassive leg raise testValsalva manoeuvre

Frank and StarlingFor a constant Afterload and Sypathetic stimulation, An Increase in Pre-load will produce an increase in Stroke volume as described by this curve.

Frank and StarlingSmall variations in pre-load, like those due to respiration can cause changes in the stroke volume or pulse pressure.These small dynamic changes can be used to obtain an idea of the position on their Cardiac output curve and the potential for response to a fluid bolus.

Frank and StarlingDuring positive pressure ventilation inspiration right ventricular Preload is decreased due to IVC and RA compression leading to decreased venous return.In turn this decreases RV output, Pulm Blood flow, LV Filling and LV output.

Frank and StarlingThis also shows how static measures of pressure will be missleading in the setting of a failing ventricle. The upper line represents a normal ventricle whilst the lower is failing, the same Pra representing dramatically different curve and potential for fluid responsiveness.

Dynamic PredictorsVariation with respiration: Stroke volume Variation Measured with a PiCCO or LiDCO via pulse contour analysis.Shows promise as a predictor of fluid responsiveness but studies have given inconsistent resultsSystolic Presure VariationIn its simplest for its a swing in the art line trace of a ventilated patient.Preisman S et al showed that a variation of 8mmHg of systolic pressure correlates to fluid responsiveness with 82% sensitivity and 86% specificity

Preisman S et al Br J Anaesth 2005;95:74655

Dynamic PredictorsVariation with respiration: Pulse Pressure VariationExpressed as a percentage; PPV=100x(Ppmax-Ppmin)/[(Ppmax + PPmin/2]More sensitive and specific than SPV. Difference of 13% PPV would lead to at least 15% increase in Cardiac output with 500ml colloid (HES) (sens 94%, Spec 96%)

Dynamic PredictorsVariation with respiration: Plethysmography assessed for swing similar to art line during respiration. Numerical value calculated usually giving a %change Currently unreliable according to studiesAuto-gain in many systems dampens changes in the traceSignificant intra- and inter-patient variability.No accepted standard or signal processing between manufacturers to facilitate reproduction between different institutions.

Dynamic PredictorsVariation with respiration: IVC variationThe IVC pressure as it passes thru the diaphragm has nearly the same pressure as the Right Atrium. It also receives nearly all of the added pressure of Positive pressure ventilation.During positive pressure inspiration the abdominal IVC expands as the abdominal pressure (usually) is significantly lower. This distension (and collapse) of the IVC only occurs in a fluid responsive patient. If it is full (i.e. Right heart failing) than ventilation will have little effect on IVC distension.Using an Echo and M-mode the distension can be measured

Dynamic PredictorsVariation with respiration: IVC variationBarbier et al showed that if you calculate the dispensability as a percentage (Dmax-Dmin)/Dmin% if it was greater than 18% than it predicted fluid responsiveness with 90% sens and 90% spec.

Dynamic PredictorsVariation with respiration Caveats : Must be Positive pressure ventilation, NOT triggering/PSV modes (I.e. Heavily sedated/narcotised, paralysed or just hyperventilated beyond pts resp drive)Sinus Rhyth.Limited by same things as Art Line (bubbles, kinks, excessive length etc)Evidence of Fluid responsiveness does not equate to fluid Deficiency.Limited data on extremes of ventilation E.g. Smaller tidal volumes with higher resp rates

Dynamic PredictorsPassive Leg Raising Auto-transfusion from leg ~150-750 mLs depending on pt and technique. Usable in Spontaneously ventilated patients.Usable in Non-Sinus Rhythm.Produces a change in CO within 30sec. Which resolves spontaneously (regardless of leg position due to blood re-distribution)Most sensitive when pt starts semi-recumbent at 45deg then is laid flat and legs elevated. Requires some means to rapidly assess changes in Cardiac output (usually TOE or TTE)Needs more studies to make more easily clinically applicable.

Dynamic PredictorsValsalva meneuverAssessing primarily blood pressure trace as pt progresses thru the 4 phases of the Valsalva meneuver.The presence of the variations in blood pressure, usually quantified using Pulse pressure Variation methods described earlier, with PPV of 52% or greater predicted fluid responsiveness.Still inadequate studies, i.e. Difficult to justify using a Vigelo or PiCCO with such a simple bedside test.

Dynamic Predictors

Dynamic Predictors

Dynamic PredictorsDynamic predictors apply a controlled and reversible preload variation and measure the heamodynamic response.Variation with respiration: Stroke volume Variation / Pulse Pressure variationPlethysmographic variationIVC variationSystolic Pressure VariationPassive leg raise testValsalva manoeuvre

In SummaryThe Frank-Starling principle of ventricular stretch producing increased shortening/Stroke volume is key to assessing if a patient will respond to a fluid bolusHealthy individuals will all respond to a fluid bolus! Just because a patient may respond doesnt mean they need one. Use your clinical judgement on the adequacy of their cardiac output prior to assessing for responsiveness.CVP only has some use as a predictor of NON-responsivenessDynamic measures ranging from Leg raise and Systolic pressure variation up to SVV and IVC distension are all potentially helpful tools in determining preload responsiveness. Each having their pros and cons.Some potentially good research project ideas there!ReferencePower and Kam, Principles of Physiology for the Anaesthetits 2e 2008Dynamic Indicies of Preload T. Miko Enomoto, Louise Harder, Crit Care Clin 26 (2010) 307321Static Measures of Preload Assessment Richard A. Nahouraii, Susan E. Rowell, Crit Care Clin 26 (2010) 295305 Optimal volaemic status and predicting fluid responsiveness Lorna Eyre, Andrew Breen Continuing Education in Anaesthesia, Critical Care & Pain | Volume 10 Number 2 2010Fluid status and fluid responsiveness Sheldon Magder Curr Opin Crit Care 16:289296Does the Central Venous Pressure Predict Fluid Responsiveness? An Updated Meta-Analysis and a Plea for Some Common Sense Paul E. Marik, Rodrigo Cavallazzi, Crit Care Med 2013; 41:17741781