Cardiac Output in Four-, Five-, and Six-Week-Old Broilers, and Hemodynamic Responses to Intravenous Injections of Epinephrine

Transcription

1 Cardiac Output in Four-, Five-, and Six-Week-Old Broilers, and Hemodynamic Responses to Intravenous Injections of Epinephrine ROBERT F. WIDEMAN, JR.1 Department of Poultry Science, University of Arkansas, Fayetteville, Arkansas ABSTRACT Female broilers were evaluated at 4, 5, and 6 wk of age (1.2, 1.8, and 2.3 kg BW, respectively) to assess changes in cardiac output and related hemodynamics associated with BW gain, and to evaluate cardiopulmonary hemodynamic adjustments occurring secondary to i.v. injections of epinephrine (0.1 mg/ kg BW). Cardiac output increased with BW (253, 348, and 434 ml/min at 4, 5, and 6 wk, respectively) due to increases in stroke volume (0.70, 1.03, and 1.33 ml/beat) that more than compensated for reductions in heart rate (362, 337, and 328 bpm). Normalization for BW eliminated the differences in cardiac output and stroke volume. Increases in cardiac output were not associated with age- or BW-related increases in mean systemic arterial pressure (101.5, 108.6, and mm Hg) due to corresponding reductions in total peripheral resistance (0.41, 0.32, and 0.26 relative resistance units). Epinephrine initially triggered immediate (within 90 s) threefold increases in total peripheral resistance and pulmonary vascular resistance, which, in turn, increased the systemic arterial pressure and pulmonary arterial pressure in spite of concurrent reductions in cardiac output that were associated with diminished venous return and dependent reductions in stroke volume and heart rate. Within 150 s after epinephrine injection, the systemic and pulmonary vascular resistances returned to preinjection control levels. By 300 s postinjection, stroke volume and heart rate increased, causing cardiac output to rise above preinjection control levels, which, in turn, elicited variable pulmonary arterial pressure responses apparently reflecting individual variability in the capacity for flow-dependent pulmonary vasodilation. These studies demonstrate that chronic (age- and BW-related) and acute (epinephrine-induced) changes in cardiac output in broilers reflect complex interactions among hemodynamic variables that include stroke volume, heart rate, and systemic and pulmonary vascular resistances. (Key words: broiler, pulmonary hypertension, hemodynamics, cardiac output, epinephrine) 1999 Poultry Science 78: INTRODUCTION Cardiac output is defined as the volume of blood pumped by a single ventricle over a given time, and is the product of the heart rate and stroke volume (Jones and Johansen, 1972; Sturkie, 1986a). The magnitude of the cardiac output directly reflects the whole body demand for oxygen, as reflected by the Fick convection equation V O2 = Cardiac Output (CaO 2 CvO 2 ) in which V O2 is the rate of oxygen consumption; cardiac output is the rate of blood flow to systemic tissues; and (CaO 2 CvO 2 ) is the difference in oxygen content between systemic arterial blood and mixed venous blood (Jones and Johansen, 1972; Grubb, 1983; Faraci, Received for publication June 25, Accepted for publication October 20, To whom correspondence should be addressed: rwideman@comp.uark.edu 1986). For example, cardiac output increases linearly along with oxygen consumption during exercise in domestic fowl, ducks, and pigeons (Grubb, 1982; Barnas et al., 1985). Cardiac output and oxygen consumption increase as domestic fowl acclimate to cool temperatures, whereas cardiac output and oxygen consumption decrease during acclimation to high temperatures (Vogel and Sturkie, 1963; Whittow et al., 1966; Sturkie, 1970, 1986a; Gleeson, 1986; May, 1989). Cardiac output increases when the arterial blood becomes moderately undersaturated with oxygen (hypoxemia) in various avian species (Butler, 1967, 1970; Jones and Johansen, 1972; Besch and Kadono, 1978; Black and Tenney, 1980a,b; Faraci, 1986; Sturkie, 1986a). Cardiac output and metabolic rates are reduced by feed restriction in domestic fowl (Harvey and Klandorf, 1983; Sturkie, 1986a; May, 1989). These changes in cardiac output are mediated by intrinsic (intracardiac) and extrinsic (neural Abbreviation Key: PHS = pulmonary hypertension syndrome; RV: TV = right:total ventricular weight ratio. 392

2 CARDIAC OUTPUT IN BROILERS 393 and hormonal) regulatory mechanisms capable of functioning independently or in concert to elicit chronotropic (frequency of contraction: heart rate) and inotropic (force of contraction: stroke volume) responses. Thus, increases in both heart rate and stroke volume contribute to the acute elevation in cardiac output during exercise, whereas a diminished venous return and reduction in stroke volume during hemorrhagic hypotension cause the cardiac output to decline in spite of increases in heart rate mediated by baroreceptor reflexes (Ploucha et al., 1981; Grubb, 1982; Barnas et al., 1985; Ploucha and Fink, 1986). In contrast, increases in cardiac output during acute episodes of mild hypoxia are attributable almost exclusively to corresponding increases in heart rate, whereas the cardiac output may remain elevated in spite of reductions in heart rate during severe or chronic hypoxia (Butler, 1967; Besch and Kadono, 1978; Faraci, 1986). The four-chambered heart of birds separates the high pressure systemic circulation from the normally low pressure pulmonary circulation, thereby reducing the risk of pulmonary edema and preventing intracardiac mixing of oxygenated and deoxygenated blood, while requiring both circulatory systems to accommodate the same cardiac output (Jones and Johansen, 1972; West et al., 1981). Substantial evidence supports the hypothesis that broilers develop pulmonary hypertension syndrome (PHS, ascites) when the right ventricle is forced to develop an elevated pulmonary arterial pressure to propel the requisite cardiac output through an anatomically inadequate or inappropriately constricted pulmonary vasculature (Wideman, 1988, 1997; Wideman and Kirby, 1995a,b, 1996; Wideman et al., 1996a,b, 1997, 1998a,b). According to this hypothesis, fast growth, cool temperatures, and hypoxia amplify the incidence of PHS by incurring an increase in cardiac output to match the metabolic demand for oxygen (Wideman, 1988, 1997; Wideman and Bottje, 1993). The implicit association between growth and cardiac output had not previously been demonstrated in broilers; however, the progressive decline in heart rate for domestic fowl between 1 d and 22 wk of age (Ringer et al., 1957; Sturkie and Chillseyzn, 1972; Sturkie, 1986b) indicates that a compensatory increase in stroke volume must occur for cardiac output to increase in parallel with body mass. Indeed, increases in heart size have been broadly correlated with corresponding increments in stroke volume and cardiac output for a wide range of avian species (Brush, 1966; Burton and Smith, 1967; May and Deaton, 1974; Sillau et al., 1980; Grubb, 1982, 1983; West et al., 1981; Faraci, 1986, 1991; Mirsalimi et al., 1993). One objective of the present study was to evaluate cardiac output and associated hemodynamic parameters in rapidly growing broilers at 4, 5, and 6 wk of age. A second objective was to evaluate the cardiopulmonary hemodynamic responses to epinephrine. 2Cobb-Vantress Inc., Siloam Springs, AR Epinephrine increases cardiac output through direct positive chronotropic and inotropic mechanisms, and increases the total peripheral resistance by triggering a generalized systemic vasoconstriction, thereby contributing to systemic hypertension by increasing both components on the right side of the equation Systemic Arterial Pressure = Cardiac Output Total Peripheral Resistance (Harvey et al., 1954; Bolton, 1967; Bolton and Bowman, 1969; Tummons and Sturkie, 1969; Akester, 1971; Jones and Johansen, 1972; Sturkie 1986a,b; Wilson and West, 1986; Kamimura et al., 1995). The analogous equation for the pulmonary circulation Pulmonary Arterial Pressure = Cardiac Output Pulmonary Vascular Resistance indicates the magnitude of the pulmonary hypertension induced by an elevated cardiac output can be amplified if epinephrine also causes pulmonary vasoconstriction. In this context, adrenergic and cholinergic nerve fibers penetrate the lungs of domestic fowl within the adventitia of the pulmonary arteries and veins, and putative noradrenergic and cholinergic nerve terminals have been observed within the adventitia of the interparabronchial arteries (Akester, 1971; Bennett, 1971; King et al., 1978). Applications of norepinephrine or epinephrine constrict the pulmonary arteries of domestic fowl and ducks; however, injections of epinephrine sufficient to elevate the systemic arterial pressure by > 40 mm Hg had a minimal effect on pulmonary arterial pressure in turkeys (Somlyo and Woo, 1967; Akester, 1971; West et al., 1981; Sturkie, 1986a). Acclimation to cool temperatures increases adrenal catecholamine synthesis and plasma levels of epinephrine in domestic fowl (Lin and Sturkie, 1968), hypoxia increases adrenal blood flow and catecholamine secretion in ducks and geese (Faraci et al., 1985; Faraci, 1991), and toxic doses of epinephrine (10 mg/kg BW) or the elimination of cholinergic (vagal) restraint can cause massive pulmonary congestion, edema, and ascites in domestic fowl (Harvey et al., 1954; Fedde and Burger, 1963; Burger and Fedde, 1964; Wideman et al., 1995). Increases in circulating epinephrine concentrations may contribute to the pathophysiology of pulmonary hypertension leading to ascites in broilers. MATERIALS AND METHODS Female by-product chicks of a Cobb2 male breeder line were reared on fresh wood shavings litter in an environmental chamber (8 m2 floor space). They were brooded at 32 and 30 C during Weeks 1 and 2, respectively, and thereafter the temperature was maintained at 24 C. They received 24 h of light during Days 1 to 5, and thereafter the photoperiod was 23 h light:1 h

3 394 dark. Throughout the experiment they were fed a cornsoybean meal-based broiler ration formulated to meet or exceed the minimum NRC (1984) standards for all ingredients, including 22.7% CP, 3,059 kcal/kg ME, 1.5% arginine, and 1.43% lysine. Feed and water were provided for ad libitum consumption. The surgical procedure has been described in detail (Wideman and Kirby, 1995b; Wideman et al., 1996a,b, 1998a,b). Experiments were conducted when the birds reached 4 wk (Days 24 to 28; n = 14), 5 wk (Days 34 to 36; n = 12), and 6 wk (Days 40 to 43; n = 12) of age. At each interval, the largest clinically healthy birds were anesthetized to a light surgical plane with allobarbital (5,5-diallyl-barbituric acid3 25 mg kg/bw), and were fastened in dorsal recumbency on a surgical board thermostatically regulated to maintain a surface temperature of 30 C. A 2% (wt/vol) Lidocaine4 solution was infiltrated intracutaneously as a supplemental local anesthetic along the midline of the thoracic inlet, the thoracic inlet was opened, and a Transonic 2.5SB or 3SB ultrasonic flowprobe5 was positioned on the left pulmonary artery. The probe and adjacent pulmonary artery were surrounded with accoustical gel, the probe was connected to a Transonic T206 blood flow meter5 to confirm signal acquisition, then the skin of the thoracic inlet was sealed with surgical wound clips. The right cutaneous ulnar vein and left brachial artery were cannulated with polyethylene tubing filled with heparinized saline. A solution of 2.5% mannitol (25 g mannitol/l of water) was infused through the venous cannula at a constant rate (0.1 ml/min kg BW) to hydrate the birds and to serve as a route for i.v. epinephrine administration. The arterial cannula was advanced to a position near the descending aorta and the distal end of the cannula was attached to a BLPR pressure transducer6 interfaced through a Transbridge preamplifier6 to a Biopac MP 100 data acquisition system using AcqKnowledge software7 for continuous monitoring of systemic arterial pressure. As reported previously, broilers were large enough at 6 wk of age to consistently permit Silastic Tubing8 filled with heparinized saline to be advanced through the left cutaneous ulnar vein into the right pulmonary artery (Owen et al., 1995b). The distal end of this cannula was attached to a BLPR blood pressure transducer for continuous monitoring of pulmonary arterial pressure. Pulmonary artery cannulations were not attempted in 4- or 5-wk-old broilers. After surgical preparations were complete, control data were collected for 30 min, then epinephrine9 (0.1 3Sigma Chemical Co., St. Louis, MO Interstate Drug Exchange, Inc., Amityville, NY Transonic Systems Inc., Ithaca, NY World Precision Instruments, Sarasota, FL Biopac Systems, Inc., Goleta, CA Dow Corning Corp., Midland, MI Anpro Pharmaceutical, Arcadia, CA WIDEMAN mg/kg BW as 1 mg/ml wt/vol) was injected i.v., followed by continued i.v. infusion of 2.5% mannitol for an additional 10 min. This pharmacological dose of epinephrine was selected because in pilot studies it reliably induced skin blanching indicative of profound cutaneous vasoconstriction, indicating that any pulmonary vasoconstrictive efficacy of epinephrine also should be revealed by injecting the same dosage in appropriately instrumented broilers. Previous studies conducted under similar experimental conditions demonstrated that 1.5-mL bolus i.v. injections of the 2.5% mannitol carrier vehicle had no influence on any of the physiological variables evaluated in the present study (Wideman et al., 1998b,c). At the end of each experiment, birds were killed with a 10 ml i.v. injection of 0.1 M KCl, and were dissected to obtain lung weights as well as heart weights for calculating the right:total ventricular weight (RV:TV) ratio as an index of pulmonary hypertension (Burton et al., 1968; Cueva et al., 1974; Sillau et al., 1980; Huchzermeyer et al., 1988; Peacock et al., 1989). The Biopac MP 100 data acquisition system recorded two (4- and 5-wk-old birds) or three (6-wk-old birds) primary data channels throughout each experiment, including systemic arterial pressure (millimeters of Hg), blood flow through the left pulmonary artery (milliliters per minute), and pulmonary arterial pressure (millimeters of Hg). Data were measured electronically from the Biopac recordings as described previously (Wideman et al., 1996a,b, 1998a,b). Based on the assumption that cardiac output (milliliters per minute) normally is divided approximately equally between two lungs of equal size, cardiac output was calculated as 2 blood flow. Heart rate (beats per minute) was obtained by counting systolic peaks over time in the systemic arterial pressure recording. Stroke volume (milliliters per beat) was calculated as cardiac output/heart rate. Assuming atrial pressure remains close to 0 mm Hg and has little impact on the vasoconstrictive responsiveness of pulmonary arterioles, then the pressure gradients across the pulmonary and systemic circulations are essentially equal to pulmonary arterial pressure and systemic arterial pressure, respectively (Wideman and Bottje, 1993). Pulmonary vascular resistance was calculated in relative resistance units as pulmonary arterial pressure cardiac output), and total peripheral resistance was calculated in relative resistance units as mean systemic arterial pressure cardiac output (Besch and Kadono, 1978; Sturkie, 1986a; Wideman et al., 1996a,b, 1998a,b). Data were analyzed across sample intervals within an age group using the SigmaStat (Jandel Scientific, 1994) repeated measures Analysis of Variance procedure, and means were differentiated by the Student-Newman- Keuls method. Responses to epinephrine were considered significant (P 0.05) when they differed from all preinjection control values. Within a single sample interval across age groups, the SigmaStat one-way Analysis of Variance procedure and Student-Newman- Keuls method for means separation were used to assess

4 CARDIAC OUTPUT IN BROILERS 395 TABLE 1. Body weight, heart values, and lung weights for 4-, 5-, and 6-wk-old broilers 1 Age 4wk 5wk 6wk Variable (n = 14) (n = 12) (n = 12) Body weight,kg 1.22 ± 0.03 c 1.81 ± 0.03 b 2.33 ± 0.04 a Right ventricle (RV) weight, g 1.23 ± 0.09 b 1.49 ± 0.08 b 2.10 ± 0.23 a Left ventricle + septum (LVS) weight, g 3.79 ± 0.12 c 4.92 ± 0.10 b 6.59 ± 0.23 a Total ventricle (TV) weight, g 5.02 ± 0.16 c 6.41 ± 0.16 b 8.69 ± 0.41 a RV:TV ratio 0.24 ± ± ± 0.01 Right lung weight, g 4.07 ± 0.25 c 5.86 ± 0.17 b 6.87 ± 0.29 a Left lung weight, g 4.24 ± 0.20 c 5.93 ± 0.20 b 6.61 ± 0.36 a a cmeans within a variable with no common superscript differ significantly (P 0.05). 1Data are means ± SEM. significant (P 0.05) differences among means. The SigmaStat linear regression procedure was used to evaluate relationships between cardiac output and other cardiopulmonary variables. RESULTS As shown in Table 1, age-related increases in BW were accompanied by increases in the right ventricle, left ventricle plus septum, and total ventricular weights. The right ventricle and left ventricle plus septum weights increased in direct proportion to one another, resulting in a constant RV:TV ratio. No differences were detected between the weights of right and left lungs within an age group, and the weights of both lungs increased equally at weekly intervals. Hemodynamic values for 4-, 5-, and 6-wk-old female broilers during control sample intervals are summarized in Table 2. Cardiac output and stroke volume increased with age, whereas heart rate decreased with age. Normalization for BW eliminated the age-related differences in cardiac output and stroke volume. The increases in cardiac output were not associated with age- or BW-related increases in mean systemic arterial pressure due to corresponding reductions in total peripheral resistance that were eliminated (P = 0.09) when normalized for differences in BW (Table 2). Linear regression analysis was used to evaluate the relationship between cardiac output during control sample intervals and other cardiopulmonary variables across all three broiler ages (Table 3). Cardiac output was positively correlated with age-related increases in BW as well as with age-related increases in the right ventricle, left ventricle plus septum, and total ventricular weights. A strong positive correlation was demonstrated between cardiac output and age-related increases in stroke volume, whereas a weakly negative correlation existed between cardiac output and age-related decreases in heart rate. The hemodynamic responses of a 6-wk-old broiler to two consecutive i.v. injections of epinephrine are shown in Figure 1. Most broilers received only one i.v. injection. Epinephrine triggered an immediate increase in mean systemic arterial pressure (Peak MAP) and a decrease in cardiac output, followed by a delayed increase in cardiac output (Peak CO) as the mean systemic arterial pressure subsided toward the preinjection control level. The pulmonary arterial pressure increased transiently to 35 TABLE 2. Control hemodynamic values for 4-, 5-, and 6-wk-old broilers 1 Age 4wk 5wk 6wk Variable (n = 14) (n = 12) (n = 12) Cardiac output, CO, ml/min 253 ± 12 c 348 ± 18 b 434 ± 24 a CO/kg BW, ml/min kg 208 ± ± ± 11 Stroke volume, 2 SV, ml/beat 0.70 ± 0.03 c 1.03 ± 0.05 b 1.33 ± 0.08 a SV/kg BW, ml/beat kg 0.58 ± ± ± 0.04 Heart rate, HR, beats/min 362 ± 4 a 337 ± 6 b 328 ± 8 b Mean systemic arterial pressure, MAP, mm Hg ± ± ± 2.8 Total peripheral resistance, 3 TPR, relative units 0.41 ± 0.02 a 0.32 ± 0.02 b 0.26 ± 0.02 c TPR/kg BW, 4 relative units/kg 0.50 ± ± ± 0.04 a cmeans within a variable with no common superscript differ significantly (P 0.05). 1Data are means + SEM of four control sample intervals per bird averaged over 30 min. 2SV = CO/HR. 3TPR = MAP/CO. 4TPR/kg BW = MAP/(CO/kg BW).

5 396 WIDEMAN TABLE 3. Linear regression equations, Pearson correlation coefficients (r), coefficients of determination (r 2 ), and probability (P) values for relationships between the average cardiac output (CO) during control sample intervals vs BW, heart values, or other cardiopulmonary variables during control intervals for the pooled data of 4-, 5- and 6-wk-old broilers CO vs variables Equation n r r 2 P CO vs BW CO = BW CO vs right ventricle (RV) weight CO = RV CO vs left ventricle + septum (LVS) weight CO = 57.5 LVS CO vs total ventricle (TV) weight CO = 43.0 TV CO vs stroke volume (SV) CO = SV CO vs heart rate (HR) CO = 1.27 HR mm Hg (Peak PAP) and returned toward the preinjection control value immediately after epinephrine injection, then the pulmonary arterial pressure exhibited a modest secondary increase contemporaneous with the delayed increase in cardiac output (Figure 1). Individual birds differed in the time required to attain the peak mean arterial pressure and cardiac output responses following epinephrine injection, and it is these peak responses that are most characteristic of the observed hemodynamic alterations of interest. Measurements FIGURE 1. Physiograph recording of mean systemic arterial pressure (MAP), pulmonary arterial pressure (PAP), and cardiac output (CO) in a 6-wk-old broiler during two i.v. injections of epinephrine at 0.1 mg/kg BW. The Peak MAP value occurred within the first 60 s after epinephrine injection, and the Peak CO value occurred within 120 s after epinephrine injection. The CO data represent a direct tracing from a flow probe on the left pulmonary artery, with the Y-axis values doubled in this figure to represent the CO flowing through both pulmonary arteries.

6 based on precise time increments following epinephrine injection would have failed to capture the actual peak values for most birds. Consequently, each peak within a specified time range was identified first, then precisely contemporaneous data were extracted from the remaining physiograph channels. Data were compiled for the following intervals, as shown in Figures 2 to 7: within 3 min after the start of data recording (Start); 10 and 15 min after the start of control data recording (sample intervals A, B); 2 min prior to the injection of epinephrine (sample interval C); within 90 s after epinephrine injection when the systemic and pulmonary arterial pressures had reached their maximum values (peak mean arterial pressure); midway between the peak mean arterial pressure and cardiac output values (sample interval D); within 300 s after epinephrine injection when the peak value for cardiac output had been attained; and, 10 min after epinephrine injection CARDIAC OUTPUT IN BROILERS 397 FIGURE 3. Stroke volume (mean ± SEM) in 4-, 5-, and 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10-s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10 s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Within each group, single asterisks designate increases and double asterisks designate decreases (P 0.05) compared with all four of the control samples preceding the epinephrine injection. Different letters designate differences (P 0.05) among group values within a single sample interval. FIGURE 2. Cardiac output (mean ± SEM) in 4-, 5-, and 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10-s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10-s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Within each group, single asterisks designate increases and double asterisks designate decreases (P 0.05) compared with all four of the control samples preceding the epinephrine injection. Different letters designate differences (P 0.05) among group values within a single sample interval. when control values had been restored (sample interval E). In broilers of all three ages, epinephrine caused an immediate but transient reduction in cardiac output (value at peak mean arterial pressure), followed by a delayed increase in cardiac output (value at peak cardiac output) that subsided to preinjection control levels within 10 min (Figure 2). As shown in Figures 3 and 4, the immediate reduction in cardiac output coincided with reductions in both stroke volume and heart rate (values at peak mean arterial pressure), whereas the subsequent delayed increase in cardiac output coincided with the restoration of heart rate to preinjection control levels, coupled with increases in stroke volume above the respective preinjection control values for each age group (values at peak cardiac output). Epinephrine triggered immediate increases in total peripheral resistance and mean systemic arterial pressure that subsided to control levels within 300 s in all three age

7 398 WIDEMAN along with stroke volume, ventricular weight, and BW, providing direct evidence that the right ventricle was forced to propel a rapidly increasing cardiac output through the pulmonary vasculature (Wideman and Bottje, 1993; Wideman, 1997). The contemporaneous decline in heart rate demonstrates conclusively that the growth-associated increments in cardiac output are attributable wholly to increases in stroke volume. Positive chronotropic mechanisms assist in transiently elevating the cardiac output during acute adaptive responses in birds, but chronically sustained increments in cardiac output generally are effected through increases in stroke volume rather than in heart rate (Jones and Johansen, 1972; Grubb, 1983; Sturkie 1986a,b; Faraci, 1991). Accordingly, modest reductions in heart rate during the onset of PHS in rapidly growing broilers (Roush et al., 1996, 1997; Kirby et al., 1997; Olkowski and Classen, 1997; Wideman et al., 1998d), particularly when accompanied by generalized ventricular enlargement (Julian et al., 1987), cannot be construed as evidence of a static or declining cardiac output unless comprehensive FIGURE 4. Heart rate (mean ± SEM) in 4-, 5-, and 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10 s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10-s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Within each group, single asterisks designate increases and double asterisks designate decreases (P 0.05) compared with all four of the control samples preceding the epinephrine injection. Different letters designate differences (P 0.05) among group values within a single sample interval. groups (Figures 5 and 6). Epinephrine also caused an immediate increase in pulmonary vascular resistance (value at peak mean arterial pressure) that within 300 s had subsided to, and then remained at, control levels (Figure 7). The pulmonary arterial pressure increased along with pulmonary vascular resistance during the immediate response to epinephrine injection (value at peak mean arterial pressure); however, pulmonary arterial pressure was not elevated above control values during the later increase in cardiac output (value at peak cardiac output). DISCUSSION Heart size has been positively correlated with the cardiac output required to meet the oxygen demand incurred by growth, behavior, and environment for a broad spectrum of avian species (see Introduction). In the present study, the cardiac output increased linearly FIGURE 5. Total peripheral resistance (mean ± SEM) in 4-, 5-, and 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/ kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10-s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10-s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Within each group, single asterisks designate increases (P 0.05) compared with all four of the control samples preceding the epinephrine injection. Different letters designate differences (P 0.05) among group values within a single sample interval.

8 CARDIAC OUTPUT IN BROILERS 399 al., 1994; Jones, 1994); however, resting rates of oxygen consumption did not differ when clinically healthy and obviously preascitic individuals were compared within the same genetic line (Fedde et al., 1998). The cardiac output and oxygen consumption rate theoretically could be similar or even higher in resistant than in susceptible broilers, as long as resistant broilers also maintained a lower pulmonary vascular resistance to preclude the onset of pulmonary hypertension. For example, Jungle Fowl are highly resistant to PHS and exhibit an inherent capacity for flow-dependent pulmonary vasodilation, enabling their right ventricle to propel a higher relative cardiac output through a lower relative pulmonary vascular resistance at a similar pulmonary arterial pressure when compared with broilers (Wideman et al., 1998b). Thus, it is not necessarily the magnitude of cardiac output or rate of oxygen consumption per se, but FIGURE 6. Mean arterial pressure (mean ± SEM) in 4-, 5-, and 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/ kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10-s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10-s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Within each group, single asterisks designate increases (P 0.05) compared with all four of the control samples preceding the epinephrine injection. hemodynamic measurements have been obtained (Olkowski and Classen, 1997, 1998; Olkowski et al., 1998). When absolute values for cardiac output are normalized for differences in BW, the resulting relative cardiac outputs reported for male and female domestic fowl evaluated under widely differing experimental conditions range between 120 and 270 ml/min kg BW (Jones and Johansen, 1972; Sturkie, 1986a). The average relative cardiac output for all female broilers used in the present study, 194 ml/min kg BW, falls within the intermediate range reported for domestic fowl in general, but is higher than the average relative cardiac output, 135 ml/min kg BW, consistently recorded under similar experimental conditions for male broilers from a different genetic line (Wideman et al., 1996a, 1998a,b). These observations suggest that important differences may be discovered in future direct comparisons of cardiac output among genetic lines. Differences in oxygen consumption have been implicated in the comparative susceptibilities of separate broiler lines to PHS (Scheele et al., 1992; Buyse et al., 1994; Decuypere et FIGURE 7. Pulmonary vascular resistance (upper panel; mean ± SEM) and pulmonary arterial pressure (lower panel) in 6-wk-old broilers during 30 min of control data collection (Start, A, B, C), immediately following a single i.v. injection of 0.1 mg/kg BW epinephrine (Peak MAP, D, Peak CO), and 10 min later when control values had been restored (E). The value at Peak MAP was measured within 90 s after epinephrine injection during a 10-s interval bracketing the maximum increase in mean systemic arterial pressure. The value at Peak CO was measured within 300 s after epinephrine injection during a 10-s interval bracketing the maximum increase in cardiac output. The value at sample interval D was measured approximately midway between Peak MAP and Peak CO. Single asterisks designate increases (P 0.05) compared with all four of the control samples preceding the epinephrine injection.

9 400 rather the potential for a mismatch between cardiac output and pulmonary vascular resistance that theoretically can predispose susceptible broilers to the onset of pulmonary hypertension leading to PHS (Wideman and Bottje, 1993; Wideman et al., 1996a,b 1998a,b; Wideman, 1997). To the extent that resistance to blood flow is related to the anatomical cross sectional area of the pulmonary vascular bed, then assessments of gross lung morphometric criteria provide evidence for an agerelated diminution in the relative pulmonary volume available for vascular development. This assessment can be inferred in the present study from relative total lung weights averaging 6.8 ± 0.5, 6.5 ± 0.5, and 5.8 ± 0.6 g/kg BW at 4, 5, and 6 wk of age, and from previously reported progressive reductions in relative lung weight, lung length, and lung volume for 1- to 6-wk-old broilers (Julian, 1989; Owen et al., 1995a; Silversides et al., 1997). Because the space available to noninflating avian lungs is dictated primarily by the dimensions of the ribs between which they are inserted, and in view of the need to keep frame (skeletal) size relatively constant to maintain compatibility with existing equipment in processing plants, genetic selection for ever increasing rates of BW gain in broilers potentially can create a mismatch between pulmonary capacity, oxygen demand, and cardiac output (Wideman and Bottje, 1993; Wideman, 1997). Age-related reductions in total peripheral resistance allowed contemporaneous increases in cardiac output to be propelled through the systemic vasculature of growing broilers by an essentially constant mean systemic arterial pressure (Table 2). In effect, as the systemic vascular capacity expanded to supply oxygen and nutritional blood flow to growing tissues, the cumulative cross-sectional area of the precapillary arterioles offered a lower resistance to blood outflow from the arterial pressure reservoir. Intrinsic and extrinsic positive inotropic responses, triggered by increasing venous return to the heart and baroreceptor reflexes, respectively, presumably mediated the observed compensatory increases in stroke volume and cardiac output to keep the arterial reservoir filled at a constant pressure (Jones and Johansen, 1972; West et al., 1981; Sturkie, 1986a,b). The ongoing growth-related increment in ventricular work performed to eject an increasing cardiac output against a constant systemic arterial pressure accounts for contemporaneous increases in ventricular weights and stroke volume, whereas the constant proportionality for RV:TV ratios indicates pulmonary arterial pressure must have remained essentially constant when the right ventricle propelled the same cardiac output through the pulmonary vasculature (Burton and Smith, 1967; Sillau et al., 1980). Consequently, the pulmonary vascular resistance apparently decreased in approximate proportion to the increase in lung size and cardiac output for the clinically healthy female broilers used in this study. Epinephrine increases the total peripheral resistance, has positive inotropic and chronotropic effects on the WIDEMAN heart, and, based on histologic and in vitro observations, has the potential to constrict the pulmonary vasculature in domestic fowl. In the present study, i.v. epinephrine injections elicited immediate three- to fourfold increases in total peripheral resistance and pulmonary vascular resistance. Within the systemic circulation, the vasoconstrictive response to epinephrine reduced blood outflow from the systemic arterial pressure reservoir more than it reduced blood inflow via left ventricular cardiac output, resulting in a 60 mm Hg increase in mean systemic arterial pressure. The immediate reduction in stroke volume following i.v. epinephrine injection most likely reflects a reduction in blood flow through the constricted pulmonary vasculature, which is consistent with previous observations that the relatively weak right ventricle of clinically healthy broilers cannot develop sufficient pressure to overcome large acute increases in pulmonary vascular resistance (Wideman et al., 1996a,b, 1998a,b). The reduction in heart rate immediately following epinephrine injection presumably reflects a baroreceptor-mediated negative chronotropic reflex, triggered by the rapid onset of systemic hypertension (West et al., 1981). Previously, when techniques were used that acutely increased the pulmonary vascular resistance without simultaneously increasing total peripheral resistance or mean systemic arterial pressure, changes in heart rate contributed minimally to the contemporaneous reductions in cardiac output (Wideman et al., 1996a,b, 1998a,b). The right ventricular hemodynamic responses immediately following epinephrine injection are not readily deduced. The high systemic precapillary arteriole resistance coupled with the decrease in left ventricular cardiac output should cause venous return and, thus, right ventricular stroke volume to decrease. A reduction in venous return and right atrial stretch would contribute to the reduction in heart rate by inhibiting the Bainbridge reflex in mammals, although the existence of an analogous reflex has not been investigated in birds (McCrady et al., 1966). However, if widespread constriction of the venous volume reservoirs also occurred, then central venous pressure would tend to increase and blood volume would be shifted transiently into the pulmonary circulation (Harvey et al., 1954; Fedde and Burger, 1963; Burger and Fedde, 1964). The initial increase in pulmonary arterial pressure immediately after epinephrine injection clearly reflects the transient elevation in pulmonary vascular resistance, regardless of the short-term changes in right ventricular cardiac output. The systemic and pulmonary vasoconstrictive responses to epinephrine dissipated rapidly, revealing more prolonged positive chronotropic (increase in heart rate) and inotropic (increase in stroke volume) cardiac responses that are typical for domestic fowl. The resulting increase in cardiac output coincided with the return of total peripheral resistance to control levels or lower, consequently mean systemic arterial pressure was not elevated by the peak increase in cardiac output.

10 CARDIAC OUTPUT IN BROILERS 401 Furthermore, the large pharmacological dose of epinephrine elicited only modest increases in cardiac output, suggesting that the broiler heart may be working at close to maximal capacity with little cardiac output reserve, as dictated by the demands of rapid growth and sedentary behavior. In most broilers, the peak increase in cardiac output evoked an increase in pulmonary arterial pressure; however, several individuals exhibited various degrees of flow-dependent pulmonary vasodilation. Consequently, the peak increase in cardiac output elicited by epinephrine in this study did not (P = 0.06) elevate pulmonary arterial pressure above the pre-injection control level. Previously, we demonstrated that most broilers cannot accomplish flow-dependent pulmonary vasodilation unless supplemental dietary L-arginine is provided, whereas the resistance of Jungle Fowl to pulmonary hypertension and PHS was at least partially attributed to their inherent capacity for rapidly accommodating large increases in cardiac output with minimal increases in pulmonary arterial pressure (Wideman et al., 1996a, 1998a). The differential responses of pulmonary arterial pressure to primary increases in cardiac output vs pulmonary vascular resistance presumably reflect Poiseuille s relationship, in which the vascular pressure gradient is linearly related to flow but exponentially related to the radius of the blood vessel raised to the fourth power, r4 (Sturkie, 1986a). The implication is that small increments in pulmonary vascular resistance can make more of a contribution to the onset of pulmonary hypertension than can small increases in cardiac output. 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