Increased Variability in Mitral (>25%) Inflow Velocity
Increased Variability in Tricuspid (>40%) Inflow Velocity
Evaluation of Volume Status in Septic Shock (see Sepsis, [[Sepsis]])
Physiology
Mechanical Ventilation in the Passive Patient
Inspiration -> Increases Intrathoracic Pressure and RA Pressure, Resulting in IVC Distention
Expiration -> Decreases Intrathoracic Pressure and RA Pressure, Resulting in IVC Collapse
Rationale
A Fluid-Responsive Circulation Will Demonstrate Significant Cyclic Respiratory Variation in IVC Volume and Left Ventricular Stroke Volume
In Contrast, if Circulation is Not Fluid-Responsive, Only Small Respirophasic Changes Will Be Seen in the IVC or Left Ventricular Stroke Volume
Caveats
Lung Distention Increases the Pressure Around Pulmonary Capillaries, Increasing RV Afterload
Normally, this Doesn’t Have Significant Consequence for the Circulation
However, in the Setting of RV Failure, this will Result in Fluid-Unresponsiveness Despite Significant Respiratory Variation in the Left Ventricular Stroke Volume
Technique of IVC Diameter Measurement
IVC is Imaged in a Subxiphoid, Long-Axis View (Either off the Frozen Image with Caliper Function or with M-Mode Imaging)
IVC Diameter is Measured 2-3 cm Below the Right Atrium or Just Caudad to the Inlet of the Hepatic Veins: allows an estimation of right atrial pressure
IVC Diameter Should Be Measured at End-Expiration
Clinical Efficacy
Minimal/Maximal IVC Diameter as a Guide to Fluid Responsiveness in Sedated, Mechanically-Ventilated Patients (Intensive Care Med, 2004) [MEDLINE]
Correlations: r = 0.58 (minimal IVC diameter) and r = 0.44 (maximal IVC diameter)
Variation in IVC Diameter = Max Diameter-Min Diameter/Mean Diameter
Respiratory Variation in IVC Diameter was Greater in Fluid Responders than in Fluid Non-Responders
Threshold Variation in IVC Diameter of 12% (Max Diameter-Min Diameter/Mean Diameter) or 18% (Max Diameter-Min Diameter/Min Diameter) Separated Fluid Responders (Positive Predictive Value: 93%) from Fluid Non-Responders (Positive Predictive Value: 92%)
In Spontaneously Breathing Patient, A Dilated IVC (>2 cm) without a >50% Decrease in IVC Diameter with Gentle Sniffing Usually Indicates an Elevated Right Atrial Pressure (Chest, 2005) [MEDLINE]
However, this is Less Specific in Mechanically-Ventilated Patients, Since there is a High Prevalence of IVC Dilation in These Patients
General Features of Echocardiogram Which Predict Fluid Responsiveness (Chest, 2012) [MEDLINE]
Assumptions: patient is either on mechanical ventilation with respiratory efforts or is breathing spontaneously
If the Left Ventricle is Hyperdynamic with End-Systolic Effacement, There is a High Probability of Fluid Responsiveness
If the IVC is <1 cm in Diameter, There is a High Probability of Fluid Responsiveness
If the IVC is Between 1-2.5 cm, There is an Indeterminate Probability of Fluid Responsiveness
If the IVC is >2.5 cm in Diameter, There is a Low Probability of Fluid Responsiveness
Technique
Background-Normal Echocardiogram
Assessment of RV Size
RV is normally 60% of the LV size at end-diastole (best seen in apical 4-chamber view)
End-Diastolic RV Cavity Size >60% of the End-Diastolic LV Cavity Size Indicates Moderate-Severe RV Enlargement
Assessment of RV Wall Thickness
RV wall is normally <4 mm thick
RV Wall Thickness >5 mm is Abnormal
Assessment of Septal Kinetics
Rationale: the septum normally moves toward the LV during ventricular systole
Interpretation
Pressure overload of the right ventricle causes a straightening of the interventricular septum during systole: results in a D-shape of the LV during LV during systole
Volume overload of the RV causes a straightening of the septum during diastole: results in a D-shape of the LV during LV during diastole
Assessment of RV Systolic Function
Rationale: the RV has more longitudinal rather than transverse motion during systole, so the degree of longitudinal systolic movement of the tricuspid annulus correlates with overall RV systolic function
Technique
Tricuspid annular plane systolic excursion (TAPSE): M-mode interrogation in apical 4-chamber view using a line through the tricuspid annulus
Interpretation
Normal TAPSE: >17 mm (values below this indicate decreased RV systolic function)
Estimation of Pulmonary Artery Systolic Pressure
Technique: pulmonary artery systolic pressure assessment requires the presence of a tricuspid regurgitation (TR) jet
Continuous wave Doppler interrogation line is placed along the main axis of the TR jet to measure the blood flow velocity of the regurgitant jet
With use of the modified Bernoulli equation, the pressure gradient across the valve is calculated
Once the pressure gradient is known, adding the RA pressure to this gradient yields an estimate of pulmonary artery systolic pressure
The RA pressure may be measured directly if a central venous catheter is in place or estimated from the size and respiratory variation of the inferior vena cava (IVC
Assessment of Preload Sensitivity
Clinical Efficacy
Change in Stroke Volume After Passive Leg Raising Identifies Preload Sensitivity (Intensive Care Med, 2007) [MEDLINE]
References
The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med. 2004;30(9):1834-1837 [MEDLINE]
Bedside ultrasonography in the ICU: part 1. Chest. 2005 Aug;128(2):881-95 [MEDLINE]
Passive leg raising predicts fluid responsiveness in the critically ill. Crit Care Med. 2006;34(5):1402-1407 [MEDLINE]
Diagnosis of central hypovolemia by using passive leg raising. Intensive Care Med. 2007;33(7):1133-1138 [MEDLINE]
Shock: Ultrasound to guide diagnosis and therapy. Chest 2012; 142(4):1042-1048. Doi:10.1378/chest.12-1297 [MEDLINE]
Focused critical care echocardiography. Crit Care Med. 2013;41:2618–2626 [MEDLINE]
Advanced echocardiography for the critical care physician: Part 1. Chest. 2014;145:129–134 [MEDLINE]
Advanced echocardiography for the critical care physician: Part 2. Chest. 2014;145:135–142 [MEDLINE]
Bedside ultrasonography for the intensivist. Crit Care Clin. 2015 Jan;31(1):43-66. doi: 10.1016/j.ccc.2014.08.003. Epub 2014 Oct 3 [MEDLINE]
Addressing the Controversy of Estimating Pulmonary Arterial Pressure by Echocardiography. J Am Soc Echocardiogr. 2016 Feb;29(2):93-102. Epub 2015 Dec 11 [MEDLINE]
