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Biomedical subjects

A P Harris

Publications and source records attributed to A P Harris.

17 recordsLinked to original sources
PubMed1992

Circulatory dynamics during periodic intracranial hypertension in fetal sheep.

The human fetal head is periodically compressed during labor. The resulting increase in intracranial pressure (ICP) may exceed the hydrostatic increase in mean arterial pressure (MAP), thereby decreasing cerebral perfusion pressure (CPP). We determined whether the cardiovascular system of near-term fetal sheep is capable of rapidly increasing MAP during periodic increases in ICP. In 12 chronically instrumented fetuses, we produced sinusoidal oscillations in ICP with a maximum of 52 +/- 1 mmHg (baseline MAP) and a minimum of 4 +/- 1 mmHg at a 3-min periodicity by ventricular fluid infusion and withdrawal. Phasic increases in MAP and decreases in electromagnetically determined renal blood flow tracked behind ICP by 0.3-0.5 min. By the sixth cycle, tonic peripheral vasoconstriction that occurred attenuated by the reduction in CPP during subsequent ICP oscillations. By the 10th cycle, plasma catecholamines and vasopressin increased 20-fold. To more closely simulate the pattern during labor, we produced an ICP triangular pulse train with 5-min periodicity and pulse duration of 1.5 min in six other fetuses. The MAP response was nearly out of phase with this more rapid rise of ICP. Thus the phasic component of the fetal pressor response is inadequate for maintaining CPP when ICP is increased to baseline MAP in less than 0.75 min. However, when the ICP pulse duration and frequency are sufficiently high, a tonic pressor response that may be humorally mediated acts to minimize transient cerebral ischemia.

Animals↗
PubMed1992

Anesthesia and analgesia for labor.

Epidural analgesia remains the mainstay for providing pain relief during labor. The search continues to find the ideal combination of analgesic agents and administration techniques that will provide excellent pain relief for the mother yet minimize side effects to the mother and fetus. This article reviews recent studies of epidural analgesia, including the increased use of epidural opioids, patient-controlled epidural analgesia, and the complications of epidural analgesia (including effects on gastric emptying, maternal temperature control, and hemodynamic changes to the mother and fetus). Intrathecal (spinal) analgesia, especially using opioids, is also discussed.

Analgesia, Epidural↗
PubMed1991

Efficacy of Cushing response during development in sheep.

Mean aortic pressure (MAP) increases (Cushing response) when intracranial pressure (ICP) approaches MAP. We elevated ICP to levels equivalent to normal baseline MAP with infusion of mock cerebrospinal fluid (CSF) into the lateral cerebral ventricles and contrasted responses in near-term fetal sheep, 1-wk-old lambs, and adult sheep anesthetized with pentobarbital sodium. With CSF infusion 1-wk-old lambs and adults produced sustained increases in MAP of 16 +/- 1 and 22 +/- 2 mmHg, respectively, over a 40-min period. However, cerebral blood flow fell 66 and 57%, and cerebral O2 uptake fell 34 and 37%, respectively. In the near-term fetus, MAP increased by 11 +/- 1 mmHg and cerebral blood fell 49% at 3 min of elevated ICP. However, by 15 min MAP had increased further (+17 +/- 2 mmHg) and cerebral blood flow was nearly restored. In contrast to postnatal sheep, cerebral O2 uptake was maintained throughout in the fetus. The mechanism of increased MAP differed among groups. In adults total peripheral resistance fell significantly, whereas in the fetus and lamb it remained constant. Cardiac output increased in each group, but, because of the fall in peripheral resistance, increased cardiac output was relatively more important to the rise in MAP in adults. In addition, marked vasoconstriction occurred in intestines and skin in the fetus. The Cushing response is well-developed in near-term fetal sheep. After birth it may lose its effectiveness in providing for the basal metabolic demand of the brain.

Aging↗
PubMed1989

Cerebral and peripheral circulatory responses to intracranial hypertension in fetal sheep.

Fetal head compression during normal labor can increase intracranial pressure (ICP). We studied the cerebral and peripheral blood flow responses to ICP elevation in utero in chronically catheterized fetal sheep using the radiolabeled microsphere technique. ICP was elevated, stepwise, in increments of 6 +/- 1 mm Hg by infusion of artificial cerebrospinal fluid into a lateral ventricle. When ICP was raised to within 28 mm Hg of baseline mean arterial blood pressure (i.e., ICP above 22 mm Hg), arterial pressure began to increase. Above this ICP level, up to 41 mm Hg, mean cerebral perfusion pressure was maintained by equivalent increases in arterial pressure. Cerebral blood flow and O2 uptake at the highest ICP levels were not different from baseline values. Changes in peripheral organ blood flow were graded according to the level of ICP. At the highest level (ICP = 41 mm Hg), renal, gastrointestinal, and skin blood flow decreased by 68%, 69%, and 65%, respectively. Myocardial and adrenal blood flow doubled, whereas heart rate and cardiac output were unchanged. Placental blood flow increased in proportion to arterial pressure. Arterial plasma epinephrine, norepinephrine and arginine vasopressin increased by nearly two orders of magnitude. Therefore, as ICP approaches baseline mean arterial pressure, fetal lambs are capable of sustaining cerebral perfusion by initiating profound visceral vasoconstriction without curtailing placental blood flow. Since cerebral O2 uptake was maintained, there is no evidence that stimulation of the peripheral response requires pronounced cerebral ischemia. This highly developed Cushing response may be important for ensuring cerebral viability when the fetal head is compressed during parturition.

Animals↗
PubMed1988

Absorption characteristics of human fetal hemoglobin at wavelengths used in pulse oximetry.

The absorption characteristics of fetal and adult human hemoglobin samples were determined for the range of 600 to 1,050 nm. Over this range, fetal hemoglobin absorption is nearly identical to that of adult hemoglobin. Since currently available two-wavelength pulse oximeters base their calculations of arterial oxyhemoglobin saturation on absorption at the wavelengths of 660 and 920 nm, we conclude that the accuracy of two-wavelength pulse oximetry previously demonstrated in adults can be extrapolated to infants with high concentrations of fetal hemoglobin.

Fetal Hemoglobin↗
PubMed1988

Accuracy of pulse oximetry during arterial oxyhemoglobin desaturation in dogs.

An animal model was developed to evaluate the accuracy of pulse oximetry over a wide range of oxyhemoglobin desaturation. The fractional inspired oxygen concentration was varied from 0.03-1.0 in five anesthetized dogs. One hundred and twelve simultaneous pulse oximeter oxygen saturation measurements (SpO2) and IL 282 CO-Oximeter arterial oxygen saturation (SaO2) measurements were made. Variance of SpO2 was increased for SaO2 less than 22%. Linear regression analysis of the data for SaO2 greater than 22% produced the equation y = 0.93x + 9.8 (r2 = 0.97). The mean difference between SpO2 and SaO2 was +5.5% +/- 4.2% (SD) over the range of 22-100%. Spectral analyses of oxygenated (O2Hb) and reduced (RHb) canine and human hemoglobins were performed. The absorption spectra of canine O2Hb and RHb were nearly identical to those of human O2Hb and RHb. Therefore, 1) SpO2 measurements in dogs at SaO2 greater than 22% are relatively accurate, and 2) hemoglobin absorption characteristics support the contention that such canine pulse oximeter studies can be extrapolated to humans.

Adult↗
PubMed1987

Monitoring the resuscitation of preterm infants in the delivery room using pulse oximetry.

The first few minutes after birth are a critical time of adaptation of the newborn infant to extrauterine life. The adequacy of that adaptation has been evaluated by means of the summed Apgar score. In preterm infants, Apgar score may correlate less with adequacy of cardiopulmonary function because of developmental immaturity. Measurement of arterial oxygen saturation by means of pulse oximetry offers a physiologic, real time method of monitoring the progress of cardiopulmonary adaptation by which the clinician can evaluate the need for and success of resuscitative efforts. Four preterm infants are reported in whom pulse oximetry was useful in assessing the changes in oxygen saturation during resuscitation.

Blood Gas Monitoring, Transcutaneous↗
PubMed1986

Organ blood flow during high-frequency ventilation at low and high airway pressure in dogs.

Using the radiolabeled microsphere technique, the authors studied hemodynamic variables and regional blood flow to multiple peripheral organs during conventional positive-pressure ventilation (CV) and high-frequency ventilation (HFV) at low and high mean airway pressure (Paw). Twenty supine anesthetized, paralyzed dogs were ventilated using CV (14-16 breaths/min) and HFV (rate = 10 Hz) in random order. In the first group (low Paw, n = 10), Paw was maintained at 3 cmH2O during CV and HFV. In the second group (high Paw, n = 10), Paw was increased to 13 cmH2O during CV and HFV. Pulmonary capillary wedge pressure and right atrial pressure remained constant during low and high Paw trials. No differences in heart rate, systemic arterial pressure, intracranial pressure, or cardiac output were noted during CV and HFV within the low and high Paw groups. In addition, blood flow to multiple peripheral organs during CV and HFV remained constant within each Paw group, except for a small decrease in cerebellar blood flow during HFV at high Paw trials showed a significant decrease in hepatic arterial and outer kidney cortical flow at high Paw. Total cerebral blood flow was decreased at high Paw, as were regional flows to diencephalon, midbrain, pons, medulla, and cerebellum. However, these differences were not attributable to differences in cerebral perfusion pressure or intracranial pressure, and cerebral oxygen delivery was not different between high Paw and low Paw groups. It is concluded that under conditions of similar Paw in anesthetized dogs, HFV does not significantly alter hemodynamic patterns or regional circulation relative to CV.

Airway Resistance↗
PubMed1986

Role of O2-hemoglobin affinity in the regulation of cerebral blood flow in fetal sheep.

Cerebral blood flow (CBF) and cerebral O2 transport (CBF X arterial O2 content) in the fetal sheep are nearly twice that in the adult, despite similar rates of cerebral O2 utilization. We tested the hypothesis that the difference depends on the increased oxyhemoglobin affinity in the fetus, using P50 (PO2 at which hemoglobin is 50% saturated) as the index of oxyhemoglobin affinity. We studied 18 unanesthetized fetal sheep in utero. In six animals the P50 was raised from 16.6 +/- 1.2 (SD) mmHg to 31.7 +/- 4.7 mmHg by exchange transfusing the fetus with adult sheep red blood cells. We measured CBF (with radioactive microspheres) and the PO2, PCO2, pH, and O2 content in carotid artery and sagittal sinus blood twice at the original P50 and twice after exchange transfusion. Arterial O2 content fell significantly at the higher P50. Since the fall in O2 content was not accompanied by a corresponding rise in CBF, O2 transport fell by 45%. Cerebral O2 consumption (CMRO2) did not change and cannot be implicated in the fall of O2 transport. E (the ratio CMRO2/O2 transport) rose by 77%. Sham exchange transfusions in six fetuses showed that the exchange transfusion procedure itself was not responsible for this alteration. To determine whether the fall in O2 transport and the rise in E was reproducible over a range of arterial O2 contents, a third group of six fetuses was studied. Fetal arterial O2 content varied from 4 to 12 vol%, first at P50 = 17 +/- 1.8 mmHg and again after exchange transfusion at P50 = 29.6 +/- 3.9 mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1979

Renovascular hypertension: comparison of medical and surgical therapy.

1. Results of medical and surgical therapy were compared in 28 patients with hypertension and unilateral renovascular ischaemia. 2. Renal function remained normal in both groups throughout the study period. After 6 months of follow-up systolic and diastolic blood pressures were significantly lower in the surgically treated patients. After 12 months of follow-up there was no significant difference between the two groups in diastolic blood pressure and after 24 months of follow-up there was no significant difference between the two groups in either systolic or diastolic blood pressure. 3. Sixty per cent of surgically treated patients were cured 6 months after operation, but only 40% remained cured after 24 months of follow-up. 4. All patients cured at 6 months who subsequently required antihypertensive medication had arteriosclerotic renovascular disease.

Blood Pressure↗
PubMed1978

Attitudes toward elderly clients.

Many studies have found that social workers often have negative feelings about the elderly. The authors review research findings in this area, offer explanations for the negative feelings of professionals, and provide suggestions for insuring appropriate attitudes among those who work with elderly clients.

Aged↗