Effect of increased blood fluidity through hemodilution on coronary circulation at rest and during exercise in dogs

Mechanism of exercise intolerance in heart diseases predicted by a computer model of myocardial demand-supply feedback system

BACKGROUND AND OBJECTIVE

The myocardial demand-supply feedback system plays an important role in augmenting blood supply in response to exercise-induced increased myocardial demand. During this feedback process, the myocardium and coronary blood flow interact bidirectionally at many different levels.

METHODS

To investigate these interactions, a novel computational framework that considers the closed myocardial demand-supply feedback system was developed. In the framework coupling the systemic circulation of the left ventricle and coronary perfusion with regulation, myocardial work affects coronary perfusion via flow regulation mechanisms (e.g., metabolic regulation) and myocardial-vessel interactions, whereas coronary perfusion affects myocardial contractility in a closed feedback system. The framework was calibrated based on the measurements from healthy subjects under graded exercise conditions, and then was applied to simulate the effects of graded exercise on myocardial demand-supply under different physiological and pathological conditions.

RESULTS

We found that the framework can recapitulate key features found during exercise in clinical and animal studies. We showed that myocardial blood flow is increased but maximum hyperemia is reduced during exercise, which led to a reduction in coronary flow reserve. For coronary stenosis and myocardial inefficiency, the model predicts that an increase in heart rate is necessary to maintain the baseline cardiac output. Correspondingly, the resting coronary flow reserve is exhausted and the range of heart rate before exhaustion of coronary flow reserve is reduced. In the presence of metabolic regulation dysfunction, the model predicts that the metabolic vasodilator signal is higher at rest, saturates faster during exercise, and as a result, causes quicker exhaustion of coronary flow reserve.

CONCLUSIONS

Model predictions showed that the coronary flow reserve deteriorates faster during graded exercise, which in turn, suggests a decrease in exercise tolerance for patients with stenosis, myocardial inefficiency and metabolic flow regulation dysfunction. The findings in this study may have clinical implications in diagnosing cardiovascular diseases.

Disentangling the Gordian knot of local metabolic control of coronary blood flow

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o₂) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po₂ has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po₂ relative to MV̇o₂ are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Right Ventricular Perfusion

Regulation of blood flow to the right ventricle differs significantly from that to the left ventricle. The right ventricle develops a lower systolic pressure than the left ventricle, resulting in reduced extravascular compressive forces and myocardial oxygen demand. Right ventricular perfusion has eight major characteristics that distinguish it from left ventricular perfusion: (1) appreciable perfusion throughout the entire cardiac cycle; (2) reduced myocardial oxygen uptake, blood flow, and oxygen extraction; (3) an oxygen extraction reserve that can be recruited to at least partially offset a reduction in coronary blood flow; (4) less effective pressure–flow autoregulation; (5) the ability to downregulate its metabolic demand during coronary hypoperfusion and thereby maintain contractile function and energy stores; (6) a transmurally uniform reduction in myocardial perfusion in the presence of a hemodynamically significant epicardial coronary stenosis; (7) extensive collateral connections from the left coronary circulation; and (8) possible retrograde perfusion from the right ventricular cavity through the Thebesian veins. These differences promote the maintenance of right ventricular oxygen supply–demand balance and provide relative resistance to ischemia-induced contractile dysfunction and infarction, but they may be compromised during acute or chronic increases in right ventricle afterload resulting from pulmonary arterial hypertension. Contractile function of the thin-walled right ventricle is exquisitely sensitive to afterload. Acute increases in pulmonary arterial pressure reduce right ventricular stroke volume and, if sufficiently large and prolonged, result in right ventricular failure. Right ventricular ischemia plays a prominent role in these effects. The risk of right ventricular ischemia is also heightened during chronic elevations in right ventricular afterload because microvascular growth fails to match myocyte hypertrophy and because microvascular dysfunction is present. The right coronary circulation is more sensitive than the left to α-adrenergic–mediated constriction, which may contribute to its greater propensity for coronary vasospasm. This characteristic of the right coronary circulation may increase its vulnerability to coronary vasoconstriction and impaired right ventricular perfusion during administration of α-adrenergic receptor agonists.

Regulation of myocardial oxygen delivery in response to graded reductions in hematocrit: role of K+ channels

This study was designed to identify mechanisms responsible for coronary vasodilation in response to progressive decreases in hematocrit. Isovolemic hemodilution was produced in open-chest, anesthetized swine via concurrent removal of 500 ml of arterial blood and the addition of 500 ml of 37 °C saline or synthetic plasma expander (Hespan, 6% hetastarch in 0.9% sodium chloride). Progressive hemodilution with Hespan resulted in an increase in coronary flow from 0.39 ± 0.05 to 1.63 ± 0.16 ml/min/g (P < 0.001) as hematocrit was reduced from 32 ± 1 to 10 ± 1% (P < 0.001). Overall, coronary flow corresponded with the level of myocardial oxygen consumption, was dependent on arterial pressures ≥ ~ 60 mmHg, and occurred with little/no change in coronary venous PO₂. Anemic coronary vasodilation was unaffected by the inhibition of nitric oxide synthase (L-NAME: 25 mg/kg iv; P = 0.92) or voltage-dependent K⁺ (K _V) channels (4-aminopyridine: 0.3 mg/kg iv; P = 0.52). However, administration of the K _ATP channel antagonist (glibenclamide: 3.6 mg/kg iv) resulted in an ~ 40% decrease in coronary blood flow (P < 0.001) as hematocrit was reduced to ~ 10%. These reductions in coronary blood flow corresponded with significant reductions in myocardial oxygen delivery at baseline and throughout isovolemic anemia (P < 0.001). These data indicate that vasodilator factors produced in response to isovolemic hemodilution converge on vascular smooth muscle glibenclamide-sensitive (K _ATP) channels to maintain myocardial oxygen delivery and that this response is not dependent on endothelial-derived nitric oxide production or pathways that mediate dilation via K _V channels.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Chronic Inflammatory Disease Is an Independent Risk Factor for Coronary Flow Velocity Reserve Impairment Unrelated to the Processes of Coronary Artery Calcium Deposition

BACKGROUND

Chronic inflammatory disease (CID) is a complex multisystem disease characterized by chronic inflammation, which can lead to coronary microvascular dysfunction (CMD) and can also predispose to coronary artery calcium deposition, even in the absence of obstructive coronary artery disease.

METHODS

Twenty-one patients with systemic lupus erythematosus (SLE; mean age, 60 ± 11 years), 21 patients with systemic sclerosis (SSc; mean age, 66 ± 11 years), 32 patients with rheumatoid arthritis (RA; mean age, 65 ± 9 years), and 23 control subjects with comparable traditional risk factors for coronary artery disease (mean age, 65 ± 10 years) were prospectively enrolled in the outpatient clinic. All study participants underwent transthoracic Doppler-derived echocardiography for coronary flow velocity reserve (CFVR) measurement in the left anterior descending coronary artery; CFVR < 2.5 defined CMD. Coronary artery calcium score in the left anterior descending coronary artery was also assessed by computed tomography.

RESULTS

None of study participants had obstructive coronary artery disease. The prevalence of CMD was 26% in the control group, 67% in the SLE group, 76% in the SSc group, and 63% in the RA group (P < .05, CID groups vs control group). CFVR was significantly lower in all three CID groups than in the control group (control group, 3.01 ± 0.72; SLE group, 2.23 ± 0.71; SSc group, 2.14 ± 0.54; RA group, 2.33 ± 0.62; P < .05, CID groups vs control group). In contrast, coronary artery calcium scores were similar in the four groups and had no relation to CMD. The odds ratios for CMD in patients with SLE, SSc, and RA were 16.70, 25.78, and 8.44 (P < .05) after adjusting for age, body mass index, the presence or absence of anemia, and hemoglobin level. Multiple linear regression analysis showed that only the presence of CID was independently associated with reduced CFVR among all study participants.

CONCLUSIONS

CID strongly contributes to CMD identified by qualitative evaluation of CFVR independently of traditional coronary risk factors of atherosclerosis but does not predispose to coronary artery calcification.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Anemia and perioperative red blood cell transfusion: a matter of tolerance

In the past, anemia in the perioperative period has been treated by red blood cell (RBC) transfusions relatively uncritically. RBC transfusions were believed to increase oxygen delivery by increasing hemoglobin concentration. Arbitrary transfusion triggers such as the "10/30 rule" (i.e., RBC transfusion indicated below a hemoglobin concentration of 10 g/dL or a hematocrit of 30%) were applied. However, there is now increasing evidence that RBC transfusions are associated with adverse outcomes and should be avoided whenever possible. Restraining from RBC transfusions and maintaining normovolemia in patients suffering from surgical blood loss results in acute anemia. Therefore, knowing the compensatory mechanisms during acute anemia is crucial. This review focuses on acute anemia tolerance, its limits, and physiologic transfusion triggers in the perioperative period.

Hyperoxic ventilation at the critical hematocrit: effects on myocardial perfusion and function

BACKGROUND

Hemodilution reduces hematocrit (Hct) and blood oxygen content. Tissue oxygenation is mainly preserved by increased cardiac output. As myocardial O2-demands increase, coronary vasodilatation becomes necessary to increase myocardial blood flow. Myocardial ischemia occurs at a critical Hct-value (Hctcrit), with accompanying exhaustion of coronary reserve. Hyperoxic ventilation is known to both reverse peripheral tissue hypoxia at Hctcrit and also to induce coronary vasoconstriction. This study aimed to determine whether hyperoxic ventilation at Hctcrit further exacerbates myocardial ischemia and dysfunction.

METHODS

Nine anesthetized pigs ventilated on room air were hemodiluted by 1:1 exchange of blood with pentastarch (6%HES) to Hctcrit, defined as onset of myocardial ischemia (ECG changes). At Hctcrit, hyperoxic ventilation was started. Measurements were performed at baseline, at Hctcrit, and after 15 min of hyperoxic ventilation. We determined myocardial blood flow (microsphere method), arterial O2-content, subendocardial O2-delivery and myocardial function (left ventricular pressure increase).

RESULTS

At Hctcrit 7 (6;8)%, O2-content was reduced [3.7 (3.1;3.9) ml dl(-1)]. Despite a compensatory increase of myocardial blood flow [531 (449;573), ml min(-1)100 g(-1)], all pigs displayed myocardial ischemia and compromised myocardial function (P < 0.05). Hyperoxic ventilation produced increased coronary vascular resistance secondary to vasoconstriction, and reduced myocardial blood flow [426 (404;464), ml min(-1)100 g(-1); P < 0.05]. Myocardial oxygenation was found to be maintained by increased O2-content [4.4 (4.2;4.8), ml dl(-1); P < 0.05], the contribution of dissolved O2 to subendocardial O2-delivery increased (32 vs. 8%; P < 0.05), which preserved myocardial function.

CONCLUSION

Hyperoxic ventilation at Hctcrit is followed by coronary vasoconstriction and reduction of coronary blood flow. However, myocardial oxygenation and function is maintained, as increased O2-content (in particular dissolved O2) preserves myocardial oxygenation.

Cerebral and peripheral oxygen saturation during red cell transfusion

BACKGROUND

Changes in regional hemoglobin oxygen saturation occur in response to blood transfusion and can be measured by near infrared spectroscopy.

PATIENTS AND METHODS

Cerebral (CsO2) and peripheral (PsO2) oxygen saturation were monitored with an INVOS 4100 near infrared spectroscopy oximeter in 29 patients undergoing 84 intraoperative blood transfusions during aortic or spinal surgery. Hemoglobin concentration was measured before and after transfusion. Mean arterial pressure, end tidal carbon dioxide tension, and arterial oxygen saturation were also monitored.

RESULTS

Mean arterial pressure, arterial oxygen saturation and end tidal carbon dioxide tension remained stable during transfusion, while CsO2 rose by a mean (95% CI) of 4.2 (3.2-5.2%; P = 0.001) and PsO2 rose by a mean (95% CI) of 1.6 (0.3-2.8%; P = 0.016). The rise in CsO2 correlated well with the rise in hemoglobin (r = 0.59, P < 0.001) and with the volume transfused (r = 0.58, P < 0.001). PsO2 correlated with the volume transfused (r = 0.35, P = 0.019) but not with hemoglobin concentration (r = 0.08, P = 0.47).

CONCLUSIONS

Near infrared spectroscopy detected significant rises in tissue oxygenation in response to blood transfusion, particularly in the cerebral cortex. CsO2 may be developed into a blood loss monitor if further research confirms our findings.

Heart rate increases linearly in response to acute isovolemic anemia

BACKGROUND

The cardiovascular response to acute isovolemic anemia in humans is thought to differ from that of other species. Studies of anesthetized humans have found either no change or a decreased heart rate. A previous study showed that in 32 healthy unmedicated humans, heart rate increased during acute isovolemic anemia. The hypothesis that heart rate in humans increases in response to acute isovolemic anemia and that the increase is affected by gender was tested.

STUDY DESIGN AND METHODS

Acute isovolemic anemia to a Hb concentration of approximately 5 g per dL in 95 unmedicated healthy humans was produced by simultaneous withdrawal of blood and IV replacement with 5-percent HSA and autologous platelet-rich plasma. The relationship between heart rate and Hb concentration was examined using a mixed-effects linear regression model that allowed each person to have a fitted line with its own slope and intercept. Cubic and quadratic terms were added to determine if these improved the goodness of fit. The effect of gender was tested by including it and its interactions with Hb in the mixed model.

RESULTS

The relationship between heart rate and Hb concentration was linear (p < 0.001) and consistent among the population studied: heart rate = 116.0-4.0 [Hb] (slope 95% CI: -4.2 to -3.8 beats/min/g Hb). Adding a cubic or quadratic term did not significantly improve the goodness of fit of the mathematical expression to the data, confirming the linear nature of the relationship between heart rate and Hb concentration. For women, the slope of the heart rate response was significantly greater than it was for males (difference +/- SE: 0.70 +/- 0.23, p < 0.005).

CONCLUSION

In 95 unmedicated, healthy humans, heart rate was a linear function of Hb during acute isovolemic anemia. Females had a significantly greater slope of increase in heart rate with decreasing Hb concentration than did males. The relationship is consistent among individuals, is similar to that reported for conscious dogs, and differs from that found previously in anesthetized humans.

Uncompensated blood loss is not tolerated during acute normovolemic hemodilution in anesthetized pigs

Clinically, hemodilution to a hematocrit of 9% has been studied, but the effects of hypovolemia during this degree of hemodilution have not been elucidated. We studied the response to blood loss during extreme hemodilution and evaluated indicators of hypovolemia. Systemic and myocardial hemodynamics, oxygen transport, and blood lactate concentrations were measured in 12 anesthetized pigs exposed to a graded blood loss of 10, 20, 30, and 40 mL/kg. Six animals were hemodiluted (hematocrit 10.8% +/- 1.4%, mean +/- SD), and six animals served as controls (hematocrit 34.6% +/- 1.5%). Hemodilution decreased systemic oxygen delivery to 9.5 +/- 0.6 mL x kg(-1) x min(-1) (controls 21.7 +/- 3.9 mL x kg(-1) x min(-1)) (P < 0.01) despite a 31% increase in cardiac output. Systemic oxygen uptake was unchanged. Arterial lactate increased to 3.3 +/- 1.1 mM/L (controls 1.6 +/- 0.6 mM/L) (P < 0.05), and mixed venous oxygen saturation (SvO2) decreased to 38.2% + 4.8% (controls 68.6% +/- 2.9%) (P < 0.01). At a blood loss of 10 mL/kg, cardiac output continued to be greater in the hemodiluted animals (P < 0.01). Arterial blood pressure decreased to 61 +/- 8 mmHg (controls 84 +/- 18 mm Hg) (P < 0.05), whereas heart rate was unchanged. Systemic oxygen delivery decreased to 8.8 +/- 1.2 mL x kg(-1) x min(-1) (controls 14.1 +/- 2.5 mL x kg(-1) x min(-1)) (P < 0.01). Systemic oxygen uptake was maintained by a further increase in oxygen extraction, and SvO2 decreased to 29.7% +/- 7.3%, compared with 55.3% +/- 9.0% in controls (P < 0.01). Arterial lactate increased to 4.9 +/- 1.4 mM/L (controls 1.8 +/- 0.8 mM/L) (P < 0.01). Myocardial oxygen delivery and lactate uptake were unchanged. When the blood loss equaled 30 mL/kg, myocardial lactate production occurred, and two hemodiluted animals died of circulatory failure. Central venous and capillary wedge pressures changed minimally during the blood loss and did not differ between groups. We conclude that a decrease in arterial blood pressure and SvO2 were early signs of hypovolemia during hemodilution, whereas central venous pressure and pulmonary capillary wedge pressure were insensitive indicators.

IMPLICATIONS

Anesthetized pigs with extremely low hemoglobin levels (one third of normal) showed poor tolerance to blood loss >10 mL/kg. A decreasing arterial blood pressure, a decreasing oxygen saturation in the venous blood, and an increase in arterial blood lactate concentration were useful indicators of blood loss.

Nitrous oxide reduces inspired oxygen fraction but does not compromise circulation and oxygenation during hemodilution in pigs

BACKGROUND

The use of nitrous oxide (N2O) during hemodilution has been questioned. Nitrous oxide reduces the inspired oxygen fraction (F1O2), depresses myocardial function and may reduce cardiac output (CO) and systemic oxygen delivery (DO2SY). The aim of this study was to evaluate the importance of the effects of nitrous oxide on systemic and myocardial circulation and oxygenation during extreme, acute, normovolemic hemodilution.

METHODS

Ten midazolam-fentanyl-pancuronium anesthetized pigs were exposed to 65% N2O before and after extreme isovolemic hemodilution (hematocrit 33 +/- 1% and 10 +/- 1%, respectively). Systemic and myocardial hemodynamics, oxygen delivery and consumption and blood lactate were measured before (at F1O2 1.0 and 0.35) and during N2O exposure.

RESULTS

Hemodilution caused an increase in CO from 137 +/- 43 to 229 +/- 32 ml.kg-1.min-1 (P < 0.01), a decrease in systemic vascular resistance (from 42 +/- 14 to 20 +/- 4 mmHg.L-1.min-1, P < 0.05), a decrease in mean arterial blood pressure (from 119 +/- 19 to 100 +/- 26 mmHg, P < 0.05) and a decrease in DO2SY from 21.1 +/- 6.9 to 13.7 +/- 2.1 ml.kg-1.min-1 (P < 0.01). Cardiac venous blood flow increased by 135% (P < 0.01) and cardiac venous saturation from 25 +/- 6 to 41 +/- 5% (P < 0.05). After hemodilution, changing F1O2 from 1.0 to 0.35 reduced arterial blood oxygen content from 59.4 +/- 3.7 to 52.3 +/- 5.1 ml.L-1 (P < 0.01), mixed venous saturation (SvO2) from 75 +/- 9 to 47 +/- 7% (P < 0.05) and DO2SY from 13.7 +/- 2.1 to 11.9 +/- 2.3 ml.kg-1.min-1 (P < 0.05). Dissolved oxygen at F1O2 = 1.0 and F1O2 = 0.35 constituted 25.4 +/- 3.1% and 10.1 +/- 1.5%, respectively, of systemic oxygen delivery after hemodilution, compared with 10.7 +/- 1.2% and 3.9 +/- 0.5% before hemodilution (P < 0.01). Left ventricular oxygen delivery and consumption were unchanged. Exposure to N2O did not affect mean arterial blood pressure or systemic vascular resistance before or after hemodilution. After hemodilution during N2O-exposure, CO and DO2SY decreased by 9% (P < 0.01 and P < 0.05, respectively), but no changes in SvO2, systemic oxygen uptake or arterial lactate were observed. The effect of N2O on myocardial oxygenation was similar before and after hemodilution; cardiac venous blood flow, left ventricular oxygen delivery and uptake decreased, but no animals showed left ventricular lactate production.

CONCLUSION

Nitrous oxide did not compromise systemic and myocardial circulation and oxygenation during acute normovolemic hemodilution in pigs. Possible adverse effects from the use of nitrous oxide during hemodilution seem to be related to a reduced F1O2, reducing the safety margin for systemic oxygen delivery.

Hemodilution significantly decreases tolerance to isoflurane-induced cardiovascular depression

BACKGROUND

Hemodilution is used to reduce the need for allogenic blood transfusion. The aim of this study was to evaluate to what extent acute extreme normovolemic hemodilution affects the circulatory response to isoflurane.

METHODS

Ten midazolam-fentanyl-pancuronium anesthetized pigs were exposed to isoflurane at end-tidal concentrations of 0, 0.5, 1.0, 1.5 and 2%, before and after extreme normovolemic hemodilution (hematocrit 33 +/- 3% and 11 +/- 1%, respectively). Systemic and myocardial hemodynamics and oxygen delivery and consumption were measured.

RESULTS

At zero end-tidal isoflurane concentration, hemodilution caused an increase in cardiac output (from 157 +/- 12 to 227 +/- 39 ml kg min-1, P < 0.01) a decrease in systemic vascular resistance (from 39 +/- 7 to 18 +/- 5 mmHg.L-1.min-1, P < 0.01) a decrease in mean arterial blood pressure (MAP) (from 130 +/- 13 to 91 +/- 13 mmHg, P < 0.01) and a decrease in systemic oxygen delivery (from 23.1 +/- 2.7 to 11.8 +/- 1.7 ml.kg-1.min-1, P < 0.01). When the end-tidal isoflurane concentration was increased from 0 to 2% after hemodilution, cardiac output decreased by 86 +/- 37 ml.kg-1.min-1, as compared with 36 +/- 20 ml.kg-1.min-1 (P < 0.01) before hemodilution. Likewise, systemic vascular resistance decreased with increasing isoflurane concentrations; at 2%, the decrease was 7 +/- 4 mmHg.L-1.min-1 after hemodilution and 18 +/- 5 mmHg.L-1.min-1 before hemodilution (P < 0.01). At an end-tidal isoflurane concentration of 2%, MAP had decreased to 43 +/- 6 mmHg after hemodilution, and to 61 +/- 15 mmHg before hemodilution (P < 0.01). After hemodilution, isoflurane concentrations above 1% decreased systemic oxygen delivery enough to cause delivery-dependent oxygen consumption and hyperlactemia; and at 2% isoflurane, myocardial blood flow became insufficient, as indicated by myocardial lactate production.

CONCLUSIONS

isoflurane-induced cardiovascular depression had adverse effects on cardiac output and oxygen delivery during extreme hemodilution because: 1) The vasodilatory effect of isoflurane was insufficient to compensate for the myocardial depression, and also contributed to a critically low arterial blood pressure; 2) A decrease in cardiac output produced delivery-dependent oxygen consumption and hyperlactemia; and 3) A decrease in myocardial blood flow caused myocardial ischemia which may have exacerbated the myocardial depression.

Tissue oxygenation with graded dissolved oxygen delivery during cardiopulmonary bypass

BACKGROUND

Intravascular perfluorochemical emulsions together with a high oxygen tension may increase the delivery of dissolved oxygen to useful levels. The hypothesis of this study is that increasing the dissolved oxygen content of blood with incremental doses of a perfluorochemical emulsion improves tissue oxygenation during cardiopulmonary bypass in a dose-related fashion.

METHODS AND RESULTS

Oxygen utilization was studied in a profoundly anemic canine model of hypothermic cardiopulmonary bypass. Forty-two dogs underwent normovolemic hemodilution to a hematocrit of 15.8% +/- 0.6% (mean +/- standard error of the mean). Cardiopulmonary bypass was begun and resulted in a hematocrit of 9.4% +/- 0.6%. A standard priming solution was used in the control group (n = 12), and the test groups received 1.35 gm perfluorochemical.kg-1 (n = 10 dogs), 2.7 gm perfluorochemical.kg-1 (n = 10 dogs), or 5.4 gm perfluorochemical.kg-1 (n = 10 dogs) through the venous return cannula. Each animal underwent a series of randomized pump flows (0.25, 0.5, 1.0, 1.5, 2.0, and 3.0 L.min-1.m-2) at 32 degrees C. After the randomized flows were completed at 32 degrees C, the temperature was raised to 38 degrees C and cardiopulmonary bypass was discontinued. Mortality from cardiac failure on separation from cardiopulmonary bypass was 42% in the control group and 20% in perfluorochemical-treated groups. The mean perfluorochemical dose was higher in survivors than in nonsurvivors (2.9 +/- 0.4 versus 1.3 +/- 0.5 gm perfluorochemical.kg-1; p < 0.05). No differences in oxygen consumption or transbody lactate gradient were found between groups during cardiopulmonary bypass. Analysis of mixed venous oxygen tension (a surrogate measure for tissue oxygenation) as a function of cardiopulmonary bypass flow normalized to body surface area showed that the control group had significantly lower mixed venous oxygen tension (p < 0.05) than the perfluorochemical emulsion-treated groups. Furthermore, the differences were related to the perfluorochemical emulsion dose. These differences in mixed venous oxygen tension continued after termination of cardiopulmonary bypass. The coronary sinus oxygen tension and cardiac arterial-venous oxygen content differences during and after cardiopulmonary bypass were similar among the control and perfluorochemical emulsion-treated animals. Dissolved oxygen consumption during and after cardiopulmonary bypass was calculated. Dissolved oxygen consumption increased in the perfluorochemical-treated animals in a perfluorochemical dose-related manner and was significantly higher in perfluorochemical-treated animals than in the control animals (p < 0.05).

CONCLUSIONS

Graded increases in mixed venous oxygen tension during cardiopulmonary bypass were observed in response to graded increases in the dissolved oxygen delivery. These data suggest that enhancing oxygenation with perfluorochemical-dissolved oxygen is an effective temporary substitute for the use of hemoglobin-bound oxygen during cardiopulmonary bypass. Perfluorochemical-dissolved oxygen may be particularly beneficial in the setting of multiple hypoxic stresses.

Use of current generation perfluorocarbon emulsions in cardiac surgery

The development of novel perfluorocarbon emulsions that contain higher concentrations of perfluorochemicals than previous emulsions has renewed interest in the use of this class of erythrocyte substitute in cardiopulmonary bypass (CPB). Perfluorocarbons have the potential to increase the oxygen content of the perfusate and thus increase the capacity of the heart-lung machine to deliver oxygen to the body during CPB. Increasing the capacity of the heart-lung machine to deliver oxygen to the body has important implications for the conduct of cardiac operations. For example, adding perfluorocarbons to the pump prime solution may allow larger volumes of blood to be withdrawn from the patient immediately prior to bypass for transfusion after bypass. Lowering the acceptable hematocrit during CPB with the use of perfluorocarbons may also decrease the need for homologous transfusions of erythrocytes in neonates or anemic adults who undergo CPB.

Effects of acute isovolemic hemodilution and anesthesia on regional function in left ventricular myocardium with compromised coronary blood flow

The effects of progressive, isovolemic hemodilution using Dextran 70 and the effect of halothane (0.7, 0.9, 1.1, and 1.3% end-tidal, administered randomly at each level of hemodilution) on global cardiovascular and regional LV contractile functions were investigated in 24 dogs with induced critical constriction of the left anterior descending coronary artery (LAD). Two additional groups of six dogs each (with and without LAD stenosis) not undergoing hemodilution served as time controls. Regional LV contractile function was assessed by sonomicrometry in the flow-compromised apical LAD territory, as well as in three non-compromised LV areas supplied by the left circumflex coronary artery. Regional myocardial function was found to be stable throughout the study period of 4-5 h in both time control groups. Mean arterial and coronary perfusion pressures as well as LV dP/dtmin decreased (P < 0.01) during hemodilution. LV dP/dtmax remained unchanged, and heart rate and LVEDP increased slightly (P < 0.05). Systolic shortening (SS) in the LAD territory was unchanged at a hematocrit (HCT) of 33.5 +/- 0.3% (mean +/- s.e. mean), and decreased marginally at an HCT of 24.2 +/- 0.1% (SS of 17.4 +/- 1.0% as compared to 20.2 +/- 1.6% at critical constriction (CC), P < 0.05). No increase in post-systolic shortening (PSS) occurred in the compromised area. Severe LAD dysfunction was observed in the LAD territory at an HCT of 14.9 +/- 0.1%, as systolic shortening decreased (11.8 +/- 1.1%, P < 0.01 vs CC) and PSS increased (31.2 +/- 3.4%, P < 0.01 vs CC). The effects of hemodilution on global cardiovascular and regional myocardial functions were unaffected by halothane.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of myocardial oxygen delivery

In most organs, oxygen consumption is maintained at relatively constant levels as oxygen delivery decreases, until a critical level is reached. This biphasic action is not observed in the heart. Myocardial oxygen consumption is supply dependent at all levels of myocardial oxygen delivery, because changes in myocardial oxygen delivery modify ventricular loading conditions and hence myocardial oxygen consumption. Since the oxygen content of coronary sinus blood is very low, only limited increases in oxygen extraction are possible. Therefore, coronary dilation is the primary mechanism for increasing myocardial oxygen delivery. Four- to sixfold increases in coronary blood flow can occur in several animal species and in human beings. Apart from metabolic control mechanisms, the regulation of myocardial oxygen delivery is multifaceted; major factors include extravascular compressive forces, autoregulation, neural controls, and humoral factors. In situations of decreased myocardial oxygen delivery, coronary vessels dilate to increase flow, and as coronary flow reserve falls to zero, flow becomes exquisitely dependent on perfusion pressure. With onset of supply dependency, contractility falls in an effort to maintain cardiac output at a given myocardial oxygen consumption.

Cardiovascular responses, hemodynamics and oxygen transport to tissue during moderate isovolemic hemodilution in pigs

In conclusion, in contrast to many reports obtained from dogs, in pigs the rise in CO during moderate isovolemic hemodilution is, besides the increased venous return, more induced by increased work performance of the heart and less by a decreased SVR. The rise in CO did not compensate for the decrease in oxygen transport capacity. Our results confirm most of the reported findings in humans at the same stage of hemodilution. Besides changes in hemodynamics, in our study a gradual decreased oxygen affinity of hemoglobin could be observed. That the pig animal model for studying hemodilution and oxygen transport to the tissue is more appropriate than the dog model is open to discussion.

Capillary growth in anemia-induced ventricular wall remodeling in the rat heart

To determine whether anemia-induced cardiac hypertrophy affects ventricular size and shape and the component structures of the capillary network of the left and right ventricles, young male rats were fed an iron- and copper-deficient diet for 7 weeks. By that time, blood hemoglobin content fell to 5 +/- 1 g/dl, and packed cell volume fell to 18 +/- 3%. To further characterize the implications of anemia, red blood cell number, hemoglobin corpuscular content, systemic arterial pressure, heart rate, and blood viscosity were measured. Moreover, the changes in ventricular weights were analyzed in terms of the alterations in ventricular wall area and ventricular wall thickness to establish the impact of the elevation in load associated with a high cardiac output state on ventricular remodeling. The quantitative properties of the capillary circulation were also examined biventricularly by low power electron microscopic morphometry to evaluate the adaptive growth potential of the coronary microcirculation in this form of cardiac hypertrophy. Anemia was found to interfere with the production of red blood cells and their mean corpuscular hemoglobin content and resulted in a 40% reduction in blood viscosity and a 12% and 27% decrease in systolic and diastolic blood pressure, respectively. The changes in heart rate were not statistically significant. In comparison with control animals, heart weight increased by 50%, but the enlargement in right ventricular mass (65%) was greater than that of the left ventricle (47%). Ventricular hypertrophy occurred with increases in wall area and wall thickness although the former increased consistently more than the latter in either ventricle. Tissue growth was accompanied by a 60% lengthening of the capillary network, which in combination with an increase in capillary diameter resulted in a 65% and 34% expansion in capillary luminal volume and 56% and 20% larger luminal surface density in the left and right sides of the heart, respectively. In conclusion, hypochromic microcytic anemia leads to eccentric ventricular hypertrophy with a significant amount of capillary proliferation that may tend to protect the myocardium from the increased potential for ischemic injury.

Coronary vasodilator reserve after human orthotopic cardiac transplantation

Cardiac transplantation is frequently associated with accelerated coronary atherosclerosis and immune-mediated microvascular injury. To determine if orthotopic cardiac transplantation impairs the capacity of the coronary vasculature to vasodilate and conduct hyperemic blood flow, maximal coronary vasodilator reserve was measured in 25 cardiac allograft recipients with no evidence of rejection 6-57 months after transplantation and in 20 normal subjects. Left ventricular wall thickness was assessed echocardiographically, and epicardial coronary anatomy was evaluated by quantitative coronary angiography. Coronary vasodilator reserve (CVDR) was measured in all patients with a coronary Doppler catheter and a maximally vasodilating dose of intracoronary papaverine. CVDR measured in the transplant recipients with normal coronary arteries, left ventricular function, and wall thickness (5.0 +/- 0.3 [mean +/- SEM] peak/resting velocity; range, 3.8-7.3; n = 16) was not different from that of normal subjects (4.8 +/- 0.2; range, 3.7-8.3). CVDR in the five cardiac allograft recipients with diffuse coronary atherosclerosis producing 30 +/- 5% narrowing (range, 25-38%) of epicardial vessel diameter also was normal (5.1 +/- 0.3; range, 4.3-6.2; n = 5). The CVDR was reduced, however, in two of the four cardiac allograft recipients with left ventricular hypertrophy. In the only transplant recipient in whom a regional wall motion abnormality was present, CVDR was abnormal in the vascular distribution of the hypokinetic wall segment (1.8) but was normal in the artery that supplied normally functioning myocardium (4.0). These findings demonstrate that in the absence of allograft rejection, acquired left ventricular hypertrophy, and regional wall motion abnormalities, coronary vasodilator reserve is normal after orthotopic human cardiac transplantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Transluminal, subselective measurement of coronary artery blood flow velocity and vasodilator reserve in man

Assessment of coronary blood flow and the vasodilator reserve capacity of individual coronary arteries in the catheterization laboratory has been hampered by methodologic limitations. We have developed and validated a small Doppler catheter that can subselectively measure phasic coronary blood flow velocity (CBFV). In seven anesthetized calves, CBFV was varied from 0.1 to 5.7 times control CBFV. Changes in mean CBFV measured intraluminally by catheter in the left anterior descending and left circumflex arteries were similar to those measured simultaneously with an epicardial Doppler probe on the surface of the same vessel (n = 85, r = .95, slope = 1.04) and to changes in coronary sinus flow (n = 69, r = .97, slope = 1.06) measured with timed venous collections. Identical maximal coronary reactive hyperemic responses with the catheter present and absent in the artery being studied demonstrated that coronary obstruction by the catheter was minimal. Safety studies in six additional calves demonstrated that the catheter caused small changes in coronary endothelial permeability. Histologic studies revealed no endothelial denudation or thrombus formation. Stable phasic recordings of coronary blood flow velocity have been obtained in 58 of 70 patients studied. One of the 70 patients studied had abrupt coronary occlusion probably related to catheter-induced vasospasm. In 10 normal patients, intracoronary meglumine diatrizoate increased CBFV to 3.5 times that at rest (range 2.8 to 5.0). CBFV rose 5.0-fold after an intravenous infusion of dipyridamole (range 3.8 to 7.0). In each patient, dipyridamole produced greater vasodilation than meglumine diatrizoate. The time- and dose-response characteristics to dipyridamole infusion were heterogeneous, underscoring the advantage of continuous on-line measurement of CBFV in the measurement of vasodilator reserve. This method of measuring CBFV and assessing vasodilator reserve in the catheterization laboratory should facilitate studies of the coronary circulation in man.

Does visual interpretation of the coronary arteriogram predict the physiologic importance of a coronary stenosis?

To assess visual interpretation of the coronary arteriogram as a means of predicting the physiologic effects of coronary obstructions in human beings, we compared caliper measurements of the degree of coronary stenosis with the reactive hyperemic response of coronary flow velocity studied with a Doppler technique at operation, after 20 seconds of coronary arterial occlusion. In 39 patients (44 vessels) with isolated, discrete coronary lesions varying in severity from 10 to 95 per cent stenosis, measurement of the percentage of stenosis from coronary angiograms was not significantly correlated (r = -0.25) with the reactive hyperemic response. Results were the same for obstructions in the left anterior descending, diagonal, and right coronary arteries. Underestimation of lesion severity occurred in 95 per cent of vessels with greater than 60 per cent stenosis of the diameter by arteriography. Both overestimation and underestimation of lesions with less than 60 per cent stenosis were common. These results, together with the high interobserver and intraobserver variability of standard visual analysis of angiograms, suggest that the physiologic effects of the majority of coronary obstructions cannot be determined accurately by conventional angiographic approaches. The need for improved analytical methods for the physiologic assessment of angiographically detected coronary obstructions is apparent.

The effects of hemodilution during pulmonary edema in dogs

Because of their multiple medical problems, patients with the adult respiratory distress syndrome (ARDS) often develop anemia. In order to determine the effects of a low hemoglobin concentration on gas exchange in such patients, the authors studied the effects of isovolemic hemodilution in the dog oleic acid model of ARDS. Twelve splenectomized dogs with oleic acid-induced pulmonary edema and a consequent venous admixture of 31% +/- 5% (mean +/- SEM) (FIO2 = 0.21) underwent two-stage isovolemic hemodilution with Hetastarch followed by retransfusion of the withdrawn red cells. This resulted in hemoglobin levels at each stage of 12.7 +/- 0.7 g/100 ml, 9.1 +/- 0.6 g/100 ml, 6.5 +/- 0.5 g/100 ml, and 10.1 +/- 0.5 g/100 ml (mean +/- SEM). Oxygen transport fell from 363 +/- 25 ml/kg/min to 219 +/- 17 ml/kg/min (p less than 0.001) at maximum hemodilution during air ventilation and from 383 +/- 79 ml/kg/min to 292 +/- 91 ml/kg/min (p less than 0.001) during oxygen ventilation. Since oxygen consumption remained constant throughout the hemoglobin range studied, decreased hemoglobin resulted in declines in P-VO2. Hemodilution with Hetastarch did not affect intrapulmonary shunt or venous admixture despite the significant increase in cardiac output associated with hemodilution.