Effects of extremity trauma on physiological responses to hemorrhage in conscious rats

Although physiological responses to hemorrhage are well-studied, hemorrhage is often accompanied by trauma, and it remains unclear how injury affects these responses. This study examined effects of extremity trauma on cardiorespiratory responses and survival to moderate (37%; H-37) or severe (50%; H-50) hemorrhage in rats. Transmitter and carotid catheter implantation and extremity trauma (fibular fracture and muscle injury) were conducted 2 wk, 24 h, and 90 min, respectively, before conscious hemorrhage. Mean arterial pressure (MAP) and heart rate (HR; via telemetry), and respiration rate (RR), minute volume (MV), and tidal volume (TV; via plethysmography) were measured throughout the 25 min hemorrhage and remainder of the 4 h observation period. There were four groups: 1) H-37, no trauma (NT; n = 17); 2) H-37, extremity trauma (T, n = 18); 3) H-50, NT (n = 20); and 4) H-50, T (n = 20). For H-37, during and after hemorrhage, T increased HR (P ≤ 0.031) and MV (P ≤ 0.048) compared with NT rats. During H-50, T increased HR (0.041) and MV (P = 0.043). After hemorrhage, T increased MV (P = 0.008) but decreased HR (P = 0.007) and MAP (P = 0.039). All cardiorespiratory differences between T and NT groups were intermittent. Importantly, both survival time (159.8 ± 78.2 min vs. 211.9 ± 60.3 min; P = 0.022; mean ± SD) and percent survival (45% vs. 80%; P = 0.048) were less in T versus NT rats after H-50. Trauma interacts with physiological systems in a complex manner and no single cardiorespiratory measure was sufficiently altered to indicate that it alone could account for increased mortality after H-50.NEW & NOTEWORTHY In both civilian and military settings, severe hemorrhage rarely occurs in the absence of tissue trauma, yet many animal models for the study of hemorrhage do not include significant tissue trauma. This study using conscious unrestrained rats clearly demonstrates that extremity trauma worsens the probability of survival after a severe hemorrhage. Although no single cardiorespiratory factor accounted for the increased mortality, multiple modest time-related cardiorespiratory responses to the trauma were observed suggesting that their combined dysfunction may have contributed to the reduced survival.