Karl Hultquist

Effect of hypothermia on ventilation in anesthetized, spontaneously breathing rats: theoretical implications for mechanical ventilation

Objective: To test if hypothermia, induced by a sustained pentobarbital anesthesia, in rats can reduce ventilatory demands without compromising pulmonary gas-exchange efficiency. Design: Prospective study. Setting: Research laboratory in a hospital. Subjects: One group of 11 female Sprague Dawley rats. Interventions: The rats were anesthetized with 45 mg/kg pentobarbital, tracheostomized and intubated; their femoral veins and arteries were cannulated. After surgery, anesthesia and fluid balance were maintained (10 mg/kg per h pentobarbital, and 5 ml/kg per h saline, i. v.). Rectal temperature, mean arterial blood pressure (MAP), and heart rate (HR) were continuously monitored. The respiratory variables and gas-exchange profiles were determined at 38 °C (normothermia), and during stepwise hypothermia at 37, 35, 33, 31 and 29 °C. The arterial pressure of carbon dioxide (PaCO2), pH and arterial pressure of oxygen (PaO2) during hypothermia were corrected at body temperature. Measurements and results: Graded systemic hypothermia, with maintained anesthesia, produced a strong correlation between reduction in the respiratory frequency and rectal temperature (r2 = 0.55; p < 0.0001; n = 66). The minute volume was significantly reduced, starting at 35 °C, without significant changes in the tidal volume (repeated measures of analyses of variance followed by Dunnett multiple comparisons test). No significant changes occurred in the PaCO2, pH, PaO2, hemoglobin oxygen saturation, the calculated arterial oxygen content and estimated alveolar-arterial oxygen difference during mild hypothermia (37–33 °C). The PaO2, however, was significantly reduced below 31 °C. The MAP remained stable at different levels of hypothermia, whereas HR was significantly reduced below 33 °C. Conclusions: Mild hypothermia in rats, induced by a sustained pentobarbital anesthesia, reduces ventilation without compromising arterial oxygenation or acid-base balance, as measured at body temperature. Theoretically, our observations in spontaneously breathing rats imply that a combination of mild hypothermia with anesthesia could be safely utilized to maintain adequate ventilation, using relatively low minute ventilation. We speculate that such a maneuver, if applied during mechanical ventilation, may prevent secondary pulmonary damage by allowing the use of lower ventilator volume-pressure settings.

Oxygen-carrying capacity during 10 hours of hypercapnia in ventilated dogs

Objective To test if a relatively long-term exogenous hypercapnia, equivalent to those maintained during permissive hypercapnia, can persistently increase oxygen-carrying capacity in ventilated dogs. Design Prospective study. Setting Research laboratory in a hospital. Subjects Six mongrel dogs (3 males; 3 females). Interventions The dogs were anesthetized (30 mg/kg pentobarbital, iv), intubated, and cannulated in one femoral artery, one femoral vein, and the right jugular vein. The mean arterial blood pressure, heart rate, and mean pulmonary artery pressure were continuously recorded. Anesthesia, fluid balance, and normothermia were maintained. Arterial hypercapnia was generated by the addition of 60 torr dry CO2 (8 kPa) to the inspired air for 10 hrs, continuously. All subjects were paralyzed (vecuronium bromide) and ventilated with room air, while the ventilator settings were kept constant. Measurements and Main Results Arterial and venous gas exchange profiles, hemoglobin concentration, oxygen saturation, oxygen content, cardiac output, and oxygen consumption were determined, before, during, and after 10 hrs of hypercapnia, periodically. Both hemoglobin concentration and oxygen content were gradually increased during hypercapnia and reached significant levels at 8 and 10 hrs of hypercapnia, respectively. These increases continued up to 2 hrs after termination of hypercapnia. The Pao2/Fio2, as an index of arterial oxygenation, was significantly increased during the first 3 hrs of hypercapnia and then remained at the normoxic level up to 10 hrs of hypercapnia. No significant changes occurred in the mean arterial blood pressure and oxygen consumption. The heart rate and cardiac output were significantly reduced at 4 and 8 hrs of hypercapnia, respectively. The mean pulmonary artery pressure was increased throughout the hypercapnic trial. Conclusions A relatively long-term exogenous hypercapnia can significantly increase oxygen-carrying capacity in normal ventilated dogs. Whether this effect can occur during permissive hypercapnia because of controlled ventilation in patients warrants investigation.

Cerebral tissue oxygenation during hypoxia and hyperoxia using artificial placentation in lamb.

Aiming at a better understanding of the pathophysiologic basis of perinatal encephalopathy, we evaluated patterns of tissue oxygenation during hypoxia and hyperoxia. We utilized both laserspectroscopy and invasive tissue-Po2 microneed measurements synchronously in five newborn lambs (141-143 days of gestation). The model of artificial placentation provided defined changes of the blood gases, using a extracorporeal circuit with interposition of membrane lung. During hyperoxia, the Po2 at the blood outlet port of the lung was raised to > 300 mmHg for five minutes. During hypoxia, Po2 was diminished as oxygen at the gas phasis was replaced by nitrogen. After the induction of hyperoxia, a rise of tissue-Po2 was observed. The synchronously recorded data of the laserspectroscopy showed adequately rising HbO2 values in concordance (r = 0.97, p < 0.001). As a constant finding we did not observe Cyt-aa3 changes during induced hyperoxia with tissue-Po2 values of < 40 mmHg. Furthermore, no changes in blood volume occurred in this case. A different pattern of the laserspectroscopic parameters was found when the tissue-Po2 rose above a value of > 40 mmHg and Cyt-aa3 rose after a lag-time occurred. During induced hypoxia an immediate fall of tissue-Po2 corresponding with a fall of HbO2 in the spectroscopic tracing occurred (r = 0.87, p < 0.001). A fall of the Cyt-aa3 level was seen with a lag-time when the tissue-Po2 had reached values of below 10 mmHg. In addition, a rise of blood volume was recorded in all cases of induced hypoxia. In conclusion, the results indicated that cellular redoxe state remains stable over a large range of oxygen partial pressure changes.

Nitric oxide and Nitrogen dioxide Concentrations During In Vitro High Frequency Oscillatory Ventilation † 1970

Nitric oxide and Nitrogen dioxide Concentrations During In Vitro High Frequency Oscillatory Ventilation † 1970

Open-bridge Extracorporeal Membrane Oxygenation: a New Method of Weaning from ECMO 235

Introduction: The standard ECMO circuit has a bridge between the venous drainage and infusion tubing. This allows temporary dissociation of the patient from the ECLS circuit. Traditionally, while weaning from ECMO, pump and patient flows are not reduced below 100 ml/min for newborn and kept much higher in older children to prevent clotting in the circuit and the membrane oxygenator. These low flows during weaning may represent more than a third of the patients cardiac output. Trialing off of the patients from ECMO at these flows may fail. We have used an open-bridge technique to maintain high pump flows through the circuit and were able to reduce the patient flows to much lower rates and achieved a more stable and successful rate of decannulation. We are reporting the successful clinical application of partially opened bridge ECMO for weaning.

In Vitro Evaluation of Variability in Systemic Arterial Pressure Changes During Closed vs Open-bridge Extracorporeal Life Support 234

Introduction: Extracorporeal life support (ECLS) is used for the support of cardiorespiratory failure. The standard ECLS circuit has a bridge between venous drainage and infusion tubing. This allows temporary dissociation of the patient from the ECLS circuit. The bridge is opened intermittently to prevent clotting (called flashing). Changes in systemic arterial pressure (SAP) due to changes in systemic vascular resistance (SVR) have been reported during flashing of the bridge which can be avoided if the bridge is kept partially open. An in vitro method is developed to study variations in SAP during closed and open-bridge ECLS.

Carinal and Proximal Pressures During Intratracheal Pulmonary Ventilation in an Artificial Lung and in Rabbits |[dagger]| 183

Introduction: Accurate measurement of airway pressures (Paw) are crucial during mechanical ventilation. Intratracheal pulmonary ventilation(ITPV) utilizes a reverse thruster device to deliver a continuous flow of gas at the level of the carina, thereby generating a different carinal Paw than conventional ventilation (CV). This study compares the proximal and carinal Paw during CV and ITPV, in both an artificial lung an in a rabbit model.