Explore This IssueACEP News: Vol 31 – No 04 – April 2012
Ultrasound of the IVC
Ultrasound is a key component of a noninvasive approach to sepsis. The clinical ability to estimate volume status is limited at best. Dialysis literature has shown that measuring IVC collapse by ultrasound is a reliable method for estimating volume status. Ultrasound also reveals discordances between patients’ stated “dry weight,” vital signs, physical exam, and the patient’s true volume status. IVC ultrasound has been used successfully at many dialysis centers to guide therapy in volume loading or reducing patients.10
Studies have correlated increased IVC collapse with low CVP in septic patients.9 A correlation between low right atrial pressures and increased IVC collapse has also been described.6 Also, IVC collapsibility decreases in response to fluid therapy.11 Patients who were predicted to be volume responders (measured IVC collapse over 50%) showed statistically significant improvements in catheter-measured cardiac index, cardiac output, and mean arterial pressure after fluid resuscitation.12 A decrease in IVC collapsibility was found using visual qualitative and quantitative measures following multiple fluid boluses (20 mL/kg). This study also found a high degree of correlation between visual qualitative and M-mode quantitative measurements. This finding suggests that visual impression is as meaningful as objective measurement.13 These studies support the notion that IVC ultrasound can be used to track volume status in real time, giving emergency physicians data to support the aggressive volume resuscitation required in the early hours after sepsis has been identified.
Learning to estimate CVP is easy for the novice with limited or no ultrasound experience. Trained internal medicine residents with no formal ultrasound training were able to accurately estimate CVP, which was subsequently confirmed by atrial pressure via right heart catheterization.17 The easiest method is a visual, qualitative measurement. The diameter of the IVC is noted; any variation of this diameter with the cardiac cycle or obvious collapse suggests potential for volume responsiveness.18 The formal measurement of IVC collapse occurs as the patient “sniffs.” Qualitative visual IVC collapse of 50%-100% with this negative inspiratory pressure maneuver is highly suggestive of intravascular depletion that is volume responsive. Quantitative measurement involves placing the M-mode indicator 2-3 cm below the RA/IVC junction and measuring the diameter over time. As the M-mode traces the IVC diameter across the screen, ask the patient to “sniff.” If the comparison of caliper measurements of the IVC diameter before and after this maneuver shows collapse ranging from 50% to 100%, the patient is likely to be volume responsive.19
Following serum lactate is the other key component of the noninvasive approach to sepsis management. Lactate is an important marker used as a predictor for mortality in sepsis and shock.20 Lactate is easily obtained from any peripheral source; there is a high correlation between levels from arterial and venous sources.21 Further, lactate is a very reliable point-of-care test.22 A recent multicenter, randomized, controlled trial used lactate-guided therapy in septic patients, which led to patients receiving vasopressors and fluids earlier in therapy. It was also noted that a decreasein lactate levels within the first 8 hours of sepsis correlated with a decrease in mortality and morbidity.23 More specifically, a decrease in serum lactate of 10% over 2 hours in response to therapy corresponds with a significant decrease in mortality.24