Induced hypothermia in the management of refractory low cardiac output states following cardiac surgery in infants and children

Heart failure treatment in the intensive care unit in children

Although pediatric heart failure is generally a chronic, progressive disorder, recovery of ventricular function may occur with some forms of cardiomyopathy. Guidelines for the management of chronic heart failure in adults and children have recently been published by the International Society for Heart and Lung Transplantation the American College of Cardiology, and the American Heart Association. The primary aim of heart failure therapy is to reduce symptoms, preserve long-term ventricular performance, and prolong survival primarily through antagonism of the neurohormonal compensatory mechanisms. Because some medications may be detrimental during an acute decompensation, physicians who manage these patients as inpatients must be knowledgeable about the medications and therapeutic goals of chronic heart failure treatment. Understanding the mechanisms of chronic heart failure may foster improved understanding of the treatment of decompensated heart failure.

Induced hypothermia prior to left ventricular assist device

Induced hypothermia, a therapy that recently gained the attention of a broad spectrum of US and international medical authorities for its neuroprotective benefits in post-cardiac arrest patients, may represent an underexplored therapeutic option in patients with severe cardiac failure by optimizing hemodynamics and augmenting cardiac contractility. The authors present the first case report, to their knowledge, of a patient with severe congestive heart failure who underwent cooling prior to successful left ventricular assist device implantation.

Mechanisms of action, physiological effects, and complications of hypothermia

BACKGROUND

Mild to moderate hypothermia (32-35 degrees C) is the first treatment with proven efficacy for postischemic neurological injury. In recent years important insights have been gained into the mechanisms underlying hypothermia's protective effects; in addition, physiological and pathophysiological changes associated with cooling have become better understood.

OBJECTIVE

To discuss hypothermia's mechanisms of action, to review (patho)physiological changes associated with cooling, and to discuss potential side effects.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

A myriad of destructive processes unfold in injured tissue following ischemia-reperfusion. These include excitotoxicty, neuroinflammation, apoptosis, free radical production, seizure activity, blood-brain barrier disruption, blood vessel leakage, cerebral thermopooling, and numerous others. The severity of this destructive cascade determines whether injured cells will survive or die. Hypothermia can inhibit or mitigate all of these mechanisms, while stimulating protective systems such as early gene activation. Hypothermia is also effective in mitigating intracranial hypertension and reducing brain edema. Side effects include immunosuppression with increased infection risk, cold diuresis and hypovolemia, electrolyte disorders, insulin resistance, impaired drug clearance, and mild coagulopathy. Targeted interventions are required to effectively manage these side effects. Hypothermia does not decrease myocardial contractility or induce hypotension if hypovolemia is corrected, and preliminary evidence suggests that it can be safely used in patients with cardiac shock. Cardiac output will decrease due to hypothermia-induced bradycardia, but given that metabolic rate also decreases the balance between supply and demand, is usually maintained or improved. In contrast to deep hypothermia (<or=30 degrees C), moderate hypothermia does not induce arrhythmias; indeed, the evidence suggests that arrhythmias can be prevented and/or more easily treated under hypothermic conditions.

CONCLUSIONS

Therapeutic hypothermia is a highly promising treatment, but the potential side effects need to be properly managed particularly if prolonged treatment periods are required. Understanding the underlying mechanisms, awareness of physiological changes associated with cooling, and prevention of potential side effects are all key factors for its effective clinical usage.

Therapeutic hypothermia and controlled normothermia in the intensive care unit: practical considerations, side effects, and cooling methods

BACKGROUND

Hypothermia is being used with increasing frequency to prevent or mitigate various types of neurologic injury. In addition, symptomatic fever control is becoming an increasingly accepted goal of therapy in patients with neurocritical illness. However, effectively controlling fever and inducing hypothermia poses special challenges to the intensive care unit team and others involved in the care of critically ill patients.

OBJECTIVE

To discuss practical aspects and pitfalls of therapeutic temperature management in critically ill patients, and to review the currently available cooling methods.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

Cooling can be divided into three distinct phases: induction, maintenance, and rewarming. Each has its own risks and management problems. A number of cooling devices that have reached the market in recent years enable reliable maintenance and slow and controlled rewarming. In the induction phase, rapid cooling rates can be achieved by combining cold fluid infusion (1500-3000 mL 4 degrees C saline or Ringer's lactate) with an invasive or surface cooling device. Rapid induction decreases the risks and consequences of short-term side effects, such as shivering and metabolic disorders. Cardiovascular effects include bradycardia and a rise in blood pressure. Hypothermia's effect on myocardial contractility is variable (depending on heart rate and filling pressure); in most patients myocardial contractility will increase, although mild diastolic dysfunction can develop in some patients. A risk of clinically significant arrhythmias occurs only if core temperature decreases below 30 degrees C. The most important long-term side effects of hypothermia are infections (usually of the respiratory tract or wounds) and bedsores.

CONCLUSIONS

Temperature management and hypothermia induction are gaining importance in critical care medicine. Intensive care unit physicians, critical care nurses, and others (emergency physicians, neurologists, and cardiologists) should be familiar with the physiologic effects, current indications, techniques, complications and practical issues of temperature management, and induced hypothermia. In experienced hands the technique is safe and highly effective.

Induced hypothermia and fever control for prevention and treatment of neurological injuries

Increasing evidence suggests that induction of mild hypothermia (32-35 degrees C) in the first hours after an ischaemic event can prevent or mitigate permanent injuries. This effect has been shown most clearly for postanoxic brain injury, but could also apply to other organs such as the heart and kidneys. Hypothermia has also been used as a treatment for traumatic brain injury, stroke, hepatic encephalopathy, myocardial infarction, and other indications. Hypothermia is a highly promising treatment in neurocritical care; thus, physicians caring for patients with neurological injuries, both in and outside the intensive care unit, are likely to be confronted with questions about temperature management more frequently. This Review discusses the available evidence for use of controlled hypothermia, and also deals with fever control. Besides discussing the evidence, the aim is to provide information to help guide treatments more effectively with regard to timing, depth, duration, and effective management of side-effects. In particular, the rate of rewarming seems to be an important factor in establishing successful use of hypothermia in the treatment of neurological injuries.

Effect of inhaled hydrogen sulfide on metabolic responses in anesthetized, paralyzed, and mechanically ventilated piglets

OBJECTIVE

Induced hypometabolism may improve the balance between oxygen delivery and consumption and may help sustain tissue viability in critically ill patients with low cardiac output state. Inhaled hydrogen sulfide (H2S) has been shown to induce a suspended animation-like state in mice with a 90% decrease in oxygen consumption. We conducted a preclinical study to explore the potential effect of H2S on metabolic rate in large mammals.

DESIGN

Prospective study.

SETTING

Animal laboratory in a university hospital.

SUBJECTS

Eleven anesthetized, paralyzed, and mechanical ventilated piglets (5.8 +/- 0.7 kg).

INTERVENTIONS

The right carotid artery and superior vena cava were cannulated for arterial pressure monitoring and blood gas sampling. Seven piglets were sequentially exposed to 20, 40, 60, and 80 ppm of H2S over a period of 6 hrs (each level for 1.5 hrs) (H2S group), and additionally four piglets were exposed to air over the same period (control group).

MEASUREMENTS AND MAIN RESULTS

Ambient temperature was fixed at 22 degrees C throughout. Central body temperature, arterial pressure, and heart rate were continuously monitored. Oxygen consumption and carbon dioxide production were continuously measured using respiratory mass spectrometry. Cardiac output was calculated using the Fick principle. Central temperature and oxygen consumption significantly and linearly decreased over the H2S exposures (p < .0001 for both), the rates of which were significantly less compared with those in the control group (p < .01 for both). Mean arterial pressure increased significantly (p = .007), whereas heart rate (p = .14), cardiac output (p = .89), and lactate (p = .67) did not change significantly during H2S exposures in H2S group; all the variables decreased significantly in the control group (p < .01 for all), and p < .01 by comparison with H2S group except for lactate (p = .05).

CONCLUSIONS

H2S does not appear to have hypometabolic effects in ambiently cooled large mammals and conversely appears to act as a hemodynamic and metabolic stimulant.

The determinants of right ventricular function in patients with atrial septal defect

OBJECTIVE

The purpose of this study was to ascertain the determinants of right ventricular (RV) systolic and diastolic functions in patients with atrial septal defect.

METHODS

Thirty-three patients with atrial septal defect having left to right shunt were enrolled in this study. RV function parameters were assessed echocardiographically. RV systolic function was assessed using tricuspid tissue Doppler S velocity (St). With regard to RV diastolic function parameters, E/A ratio, deceleration time (DT), E/Et ratio (Et = tissue Doppler E velocity), RV isovolumetric relaxation time (RVIVRT) were assessed. RV myocardial performance index (MPI) was calculated as an index of both systolic and diastolic function. Pulmonary artery stiffness (PAS) was also calculated. After echocardiography, right and left heart catheterization was performed. Mean pulmonary artery pressure (MPAP), mean right atrial pressure (MRAP), systemic flow (Qs), pulmonary flow (Qp), systemic vascular resistance (SVR), and pulmonary vascular resistance (PVR) were obtained using the data of invasive measurements.

RESULTS

In multivariate analysis, MPAP was found to be the parameter closest related to RVIVRT (r = 0.73, p < 0.001) and E/Et (r = 0.66, p < 0.001), while PAS was found to be the parameter closest related to MPI (r = 0.53, p = 0.002). In addition, St velocity was found the only parameter related to PVR (r = -0.39) in univariate analysis. There was no relationship between QP/QS and any of the RV function parameters.

CONCLUSION

The pulmonary vascular bed appears to be the predictor of the RV functions in patients with atrial left to right shunts, and the amount of the shunt seems to have no direct adverse influence on the RV functions.

The effects of hypothermia on human left ventricular contractile function during cardiac surgery

OBJECTIVES

We investigated the interaction of heart rate (HR), temperature and contractility using a validated load independent method.

BACKGROUND

Temperature manipulation is an integral part of cardiac surgery, and postoperative hypothermia is extremely common. Myocardial contraction is a series of enzymatic and physico-chemical reactions that may be differentially affected by temperature.

METHODS

Ten patients undergoing coronary artery bypass grafting were studied during moderately hypothermic cardiopulmonary bypass. After conduit procurement and heparinization but before grafting, the patient was placed on cardiopulmonary bypass and rewarmed to 37 degrees C, and the left ventricle (LV) was instrumented with a conductance catheter allowing continuous pressure and volume measurement. The LV pressure volume relationship was examined to assess the contractility at 37, 35, 33 and 31 degrees C, with fixed atrial pacing (100 beats/min) in five patients and at 80 and 120 beats/min, at 33 and 37 degrees C in five patients.

RESULTS

At a HR of 100 beats/min, lower temperature resulted in a highly significant decrease in maximal elastance (100% at 37 degrees C, 29 +/- 3.5% at 31 degrees C, p < 0.0001). At 37 degrees C, increasing HR increased contractility (80 beats/min 100%, 120 beats/min 205.9%, p = 0.0021); however, at 33 degrees C contractility fell with increasing HR (80 beats/min 100%, 120 beats/min, 53.7%, p = 0.0014).

CONCLUSIONS

At normothermia LV contractility has a direct relationship with HR. In hypothermic conditions this relationship inverses. Clinical strategies maintaining higher HRs at colder temperatures result in reduced contractility. These factors are important in the management of cardiac surgical patients.

Oxygen consumption after cardiopulmonary bypass surgery in children: determinants and implications

OBJECTIVE

We sought to assess oxygen consumption and its determinants in children shortly after undergoing cardiopulmonary bypass operations.

METHODS

Twenty children, aged 2 months to 15 years (median, 3.75 years), undergoing hypothermic cardiopulmonary bypass operations were studied during the first 4 hours after arrival in the intensive care unit. Central and peripheral temperatures were monitored. Oxygen consumption was continuously measured by using respiratory mass spectrometry. Oxygen delivery was calculated from oxygen consumption and arterial and mixed venous oxygen contents, which were sampled every 30 minutes. Oxygen extraction was derived by the ratio of oxygen consumption and oxygen delivery. Arterial blood lactate levels were measured every 30 minutes.

RESULTS

There was a correlation between oxygen consumption and age in patients older than 3 months (r = -0.76). Mean oxygen consumption increased by 14.7% during the study. The increase in oxygen consumption was correlated with the increase in central temperature (r = 0.73). Nine patients had an arterial lactate level above 2 mmol/L on arrival. There were no significant differences in oxygen consumption, oxygen delivery, and oxygen extraction between the group with lactate levels between 2 and 3 mmol/L and the groups with normal lactate levels both on arrival and at 2 hours. One patient with a peak lactate level of 6.8 mmol/L had initially low oxygen delivery (241.3 mL. min(-1). m(-2)).

CONCLUSIONS

During the early hours after a pediatric cardiac operation, the increase in oxygen consumption is mainly attributed to the increase in central temperature. Oxygen consumption is negatively related to age. Mild lactatemia is common and does not appear to reflect oxygen delivery or oxygen consumption or a more complicated recovery.

Induced hypothermia as salvage treatment for refractory cardiac failure following paediatric cardiac surgery

OBJECTIVE

Following corrective cardiac surgery in infants and children for congenital heart disease, a persistent low cardiac output refractory to conventional modes of treatment is associated with a mortality approaching 100%. We advocate the use of whole body hypothermia to reduce tissue oxygen demand and provide a degree of cellular protection against ischaemia allowing time for recovery. We describe our experience.

METHODS

Between July 1986 and December 1995, 1885 infants and children underwent surgery (operative mortality, 6%), 1302 requiring cardiopulmonary bypass. Fifty-seven patients had a persistent low cardiac output, impaired respiratory function, decreased urine output and acidosis despite maximal intensive care treatment. Cooling to 32-33 degrees C was therefore started using a thermostatically controlled water filled cooling blanket.

RESULTS

Following cooling, there was a fall in heart rate (P<0.001), a rise in mean arterial pressure (P<0.001) and a fall in mean atrial pressure (P<0.001). Significant (P<0.001) increases in pH and urine output were also recorded. Thirty-one (54%) of the 57 patients treated with cooling survived to leave hospital. No long-term sequelae have been noted in these patients.

CONCLUSION

Induced hypothermia is a useful salvage treatment, in children following corrective cardiac surgery when all conventional treatment has been tried and failed.

Induced hypothermia in the postoperative management of refractory cardiac failure following paediatric cardiac surgery

Postoperative low cardiac output states are a major cause of postoperative mortality in infants and children following corrective cardiac surgery for congenital heart defects. In this unit, whole body hypothermia has been used since 1979 in the management of these low output states when they are refractory to conventional modes of therapy. Twenty cases treated in this way between July 1986 and June 1990 were reviewed in 1992. The current report reviews the 50 further cases treated with moderate hypothermia between July 1990 and December 1995. The median (range) age of patients was 8 months (0 days-16 years) with a median weight of 4.1 kg (2.5-33 kg). Following cooling, there was a decrease in heart rate (p < 0.001), an increase in mean arterial pressure (p < 0.001) and a decrease in mean atrial pressure (p < 0.001). Significant increases in pH and urine output were also noticed, the increase in urine output being greater in the surviving group (p = 0.02). A decrease in platelet count occurred (p < 0.001) but white blood cell count remained unchanged (p = 0.18). Twenty-five of the 50 patients survived to leave hospital. Induced hypothermia does not appear to be associated with any complications and after the failure of all conventional treatment, it seems likely that the technique may have been beneficial to outcome in some patients.

Right ventricular function in patients treated with inhaled nitric oxide after cardiac surgery for congenital heart disease in newborns and children

Measurement of right ventricular (RV) function is essential for complete assessment of the effects of inhaled nitric oxide in the postoperative cardiac patient; nitric oxide therapy can result in a decrease in pulmonary vascular resistance and improved echocardiographic RV ejection fraction without necessarily inducing a significant change in pulmonary artery pressure.

Influence of hypothermia on the cardiac effects of propranolol observed in isolated rat atria

1. The purpose was to determine if hypothermia influences cardiac responses to propranolol. 2. Rat atria were used and 11 test groups were created; 3 control groups were maintained at 35, 28 or 20 degrees C. Two additional groups, at each temperature, were exposed to 1.2 or 40 mumol/l propranolol. Developed force and effective refractory period (ERP) were measured. 3. At 35 degrees C, propranolol decreased developed force and lengthened ERP. At 28 degrees C, propranolol did not affect developed force, but ERP was lengthened. At 20 degrees C, 1.2 microM propranolol neither affected developed force or ERP, but 40 microM reduced developed force and lengthened ERP.

CONCLUSION

hypothermia reduced propranolol's usual negative inotropic effect.

Alteration of the cardiac effects of isoproterenol and propranolol by hypothermia in isolated rat atrium

1. Hypothermia alters the myocardial response to some inotropic maneuvers. By measuring developed force and effective refractory period in isolated left atrial preparations, we determined whether hypothermia affected the cardiac response to isoproterenol and propranolol. 2. Twelve experimental groups were formed, each consisting of 6 atrial preparations. Three groups maintained at either 35, 28 or 20 degrees C served to determine the effects of hypothermia alone. 3. At each temperature, 3 additional groups were exposed to 1.0 microM isoproterenol alone or in combination with either 0.3 or 10.0 microM propranolol. At 35 degrees C, isoproterenol produced an increase in developed force and decreased effective refractory period. Propranolol reversed these isoproterenol-induced effects in a concentration-dependent manner. 4. Decreasing temperature to either 28 or 20 degrees C significantly increased developed force and effective refractory period. At 28 degrees C, isoproterenol no longer produced a significant increase in developed force, although effective refractory period was still decreased. At 20 degrees C, isoproterenol significantly reduced both developed force and effective refractory period. These effects of isoproterenol were reversed by the addition of propranolol, so that the effective refractory period was increased and developed force was not different from that observed at 20 degrees C in the absence of isoproterenol. 5. These effects of isoproterenol might be explained by effects on Na+/Ca(2+)-exchange.