Endotoxemia is a common complication of colic in adult horses and its outcome is mainly related to cardiovascular status. Endotoxic shock, like septic shock, involves hypovolemic, cardiogenic and distributive components leading to tissue hypoperfusion with a high risk for multiple organ failure and an elevated mortality rate in equine and human intensive care units. Via the release of pro-inflammatory mediators, endotoxins induce a fall in systemic vascular resistance (SVR) and a drop of cardiac output (CO) resulting both from reduction in venous return and myocardial depression. First, CO is preserved thanks to a compensatory vasoconstriction and an increase in heart rate (HR), myocardial contractility and venous return. However, a loss of vascular responsiveness to catecholamines and a myocardial depression finally result in a drop of CO and SVR with a progressive hypotension despite a persistent tachycardia. The time point during the course of septic shock when hemodynamic monitoring is used has been proven to influence outcome, allowing early goal-directed therapy in humans. Unfortunately, hemodynamic monitoring in conscious horses is often limited due to animal size, working conditions and lack of patient cooperation. Therefore, such as in human patients, echocardiography could provide useful non invasive information about stroke volume (SV) and CO, systolic and diastolic function as well as fluid status. In human septic shock patients, this technique has emphasised a myocardial depression with both diastolic and systolic dysfunction reversible in survivors. Previous studies in horses focused on quantitative evaluation of hemodynamic parameters after an experimental endotoxin challenge. However, LV function had not been reported in naturally-occurring endotoxemia in horses.
The purpose of the first prospective study was to assess the impact of clinically-occuring endotoxic shock on echocardiographic indices of LV systolic function in horses. Fifty horses admitted in clinic on emergency for colic were submitted to an echocardiographic exam on admission. A shock score from 1 to 4 was established for each horse based on clinical evaluation, non invasive systolic blood pressure and blood tests measurements. LV echocardiographic parameters were compared between the 4 groups according to the shock score (SS) (SS1: no or discrete signs of shock, n=11; SS2 : mild, n=17; SS3: moderate, n=12; SS4: severe, n=10) using a multivariable analysis. Significance was set at p < 0.05. Stroke volume, stroke index (SI), ejection time (ET), ejection time index corrected for HR (ETI), aortic velocity time integral (AoVTI), aortic flow acceleration time (TTP) and deceleration time (DTAo) were significantly lower, whereas aortic flow acceleration rate (dv/dtAo) was significantly higher in shocked horses than in SS1. CO was not significantly different between groups. Even if those results were difficult to interpret because of the shock-induced changes in loading conditions of the heart, they suggested that alterations in some indicators of systolic function can be quantified by Doppler echocardiography in horses with colic-induced endotoxemic shock.
The aim of the second study was to non invasively assess the impact of colic-related endotoxic shock on equine diastolic cardiac function. Sixty-eight horses admitted in clinic on emergency with colic were included in this study. A similar SS was established for each horse. LV echocardiographic parameters were compared between the 4 groups according to the SS (SS1 n=14; SS2 n=16; SS3 n=27; SS4 n=12) using a multivariable analysis. Significance was set at p < 0.05. Deceleration rate of peak early LV filling velocity, peak late LV filling velocity and E/Em (peak early LV filling velocity (E) to peak early myocardial velocity (Em) ratio) were significantly higher whereas time to Em onset was significantly lower in high shock score groups compared with horses with no or mild signs of shock. The results of this second study could suggest a diastolic dysfunction with a rise of LV filling pressure ocurring with endotoxic shock in horses. Since E/Em has shown to be a very reliable predictor of mortality in human with septic shock, the results of this second study warranted further investigations.
To assess the prognostic value of echocardiographic parameters of LV function in horses with endotoxic shock, 41 horses admitted for colic with clinical evidence of systemic inflammatory response syndrome associated to hyperlactatemia or hypotension underwent echocardiographic examination on admission. LV echocardiographic parameters were compared between nonsurvivors (n=29) and horses that survived to discharge (n=12). With comparable heart rate and LV preload estimate, LV SV, AoVTI, DTAo, ET and ETI of aortic flow and Em were lower in the nonsurviving than in the surviving horses while pre-ejection period to ejection time ratio (PEP/ET) of Doppler aortic flow and E/Em ratio were higher (p<0.05). A cut-off value of 0.26 for PEP/ET predicted mortality with 100% sensitivity and 42% specificity (area under the receiver operating characteristic curve (AUC): 0.71), whereas a cut-off value of 2.67 for E/Em predicted mortality with 100% sensitivity and 83% specificity (AUC: 0.89). The results of this third study suggested that echocardiography may provide prognostic information in colic horses with clinical evidence of endotoxic shock. Especially, PEP/ET and E/Em could be useful markers of systolic and diastolic dysfunction, respectively, to detect horses with a high risk of death requiring more intensive cardiovascular monitoring as it has been reported in human patients with septic shock.
Finally, to assess LV function after preload restoration in colic horses, 5 horses were followed echocardiographically from admission to at least 24 hours after colic surgery in a preliminary study. Systolic dysfunction was detected in 4/5 horses and diastolic dysfunction assumed in 2/5 horses and suspected in the 3 other horses during hospitalisation. Moreover, echocardiography provided useful diagnosis informations about hemodynamic and fluid status suggesting that ultrasonographic monitoring of cardiovascular function could be of interest in equine intensive care