Introduction {#Sec1}
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Dynamic arterial elastance (Ea~dyn~), the relationship between pulse pressure variation (PPV) and stroke volume variation (SVV), has been suggested as a functional assessment of arterial load for predicting the arterial pressure response after volume expansion (VE)^1^. Although changes in Ea~dyn~ have been related with variations in arterial load^2^, the effect of acute arterial tone changes on Ea~dyn~ and the impact on its performance for predicting the arterial pressure response after VE has not yet been determined.

Objective {#Sec2}
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To evaluate the effect of acute arterial tone changes on Ea~dyn~ and the influence on its performance for predicting arterial changes after fluid administration.

Methods {#Sec3}
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12 anesthetized and mechanically ventilated rabbits. Arterial tone changes were induced by phenylephrine (PHENY) infusion on 6 animals (HighMAP group) and by sodium nitroprusside (SNP) on the other 6 animals (LowMAP group), until reach a 50% of change on mean arterial pressure (MAP) from its baseline value. A volume challenge (10 mL/Kg) was then performed on all animals. Animals were monitored with an indwelling femoral arterial catheter and an esophageal Doppler (CardioQ-Combi). Arterial load was assess by the systemic vascular resistance, net arterial compliance and effective arterial elastance. Ea~dyn~ was calculated as the simultaneous ratio between PPV and SVV obtained from the Doppler monitor.

Results {#Sec4}
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At baseline, Ea~dyn~ and other arterial load parameters were similar on both groups. In the LowMAP group, SNP significantly decreased arterial load, reduced MAP by 44%, and consistently increased Eadyn by 75% (Figure [1](#Fig1){ref-type="fig"}). In the HighMAP group, PHENY increased arterial load, raised MAP by 58%, and significantly reduced Ea~dyn~ by 41% (Fig. [1](#Fig1){ref-type="fig"} and [2](#Fig2){ref-type="fig"}). Overall, VE increased cardiac output by 10%, stroke volume by 21% and MAP by 15%, and decreased Ea~dyn~ from 1.08 ± 0.67 to 0.88 ± 0.45 (Fig.[1](#Fig1){ref-type="fig"}). There was a significant relationship between Ea~dyn~ after arterial tone changes and increases in all components of arterial pressure after VE: systolic (R^2^=0.89), diastolic (R^2^=0.41), mean arterial (R^2^=0.61) and pulse pressure (R^2^=0.67), respectively. Animals with a MAP increase ≥ 10% after VE had a higher preinfusion Ea~dyn~ value (1.54 ± 0.49 vs. 0.46 ± 0.15; P \< 0.001).Figure 1SVV and PPV example on HighMAP group.Figure 2SVV and PPV example on LowMAP group.

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Comparison of arterial load parameters during different experimental conditions in HighMAP abnd LowMAP groups (n=6 on both experimental arms).

                               Baseline      After change in arterial pressure   Postinfusion       P value^a^
  ---------------------------- ------------- ----------------------------------- ------------------ ------------
  **EA** ~**dyn**~                                                                                  
  HighMAP                      0.92 ± 0.12   0.52 ± 0.23\*                       0.49 ±.0.18        \<0.001
  LowMAP                       0.91 ± 0.16   1.64 ± 0.44\*                       1.26 ± 0.22^†^\*   
  **Ea, cmHg/mL**                                                                                   
  HighMAP                      3.99 ± 0.93   5.42 ± 1.46\*                       4.45 ± 1.11^†^     \<0.001
  LowMAP                       4.50 ± 1.44   1.80 ± 0.63\*                       1.52 ± 0.50^†\*^   
  **C, mL/cmHg**                                                                                    
  HighMAP                      0.66 ± 0.13   0.41 ± 0.10\*                       0.48 ± 0.08^†\*^   \<0.001
  LowMAP                       0.53 ± 0.16   1.92 ± 0.86\*                       1.83 ± 0.58\*      
  **TVSR, MPa\'s/m** ^**3**^                                                                        
  HighMAP                      1514 ± 453    3157 ± 1299\*                       3372 ± 1306\*      \<0.001
  LowMAP                       1814 ± 585    709 ± 215\*                         720 ± 218\*        

C: new arterial compliance; Ea: effective arterial elastance; Eadyn: dynamic arterial elastance; TSVR: total systemic vascular resistance. Data are presented as mean ± standard deviation. Data normally distributed according to Kolomogorov-Smirnov test. Note that pressure units are expressed as cmHg for convenience. \*p\<0.05 vs baseline. ^†^p\<0.05 vs. change in arterial pressure. \*p value refers to ANOVA test for time and group interaction

Conclusions {#Sec5}
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In this experimental settings acute modifications on arterial tone induced significant changes on Ea~dyn~: arterial vasodilation increased Ea~dyn~, whereas vasoconstriction decreased it. Nevertheless, preinfusion Ea~dyn~ still determined the arterial pressure response after volume administration.
