Low Perfusion States Research Articles

Confirmatory methods for endotracheal tube placement in out-of-hospital settings: A systematic review of the literature

BackgroundConfirming proper placement of an endotracheal tube (ETT) is important, as accidental misplacements may occur and lead to critical injuries, potentially leading to adverse outcomes. Multiple methods are available for determining the correct ETT placement in prehospital care. ObjectiveTo assess the accuracy and reliability of the different methods used to confirm endotracheal intubation in prehospital settings. MethodsA comprehensive literature search was performed in the MEDLINE, EMBASE, Scopus, and Web of Science databases for studies that were published between 1-June-1992 and 12-June-2022 using a combination of predetermined search terms. Studies that met the inclusion criteria were included and assessed for risk of bias using “Risk of Bias in Non-randomized Studies of Intervention” tool. ResultsOf the 1016 identified studies, nine met the inclusion criteria. Capnography and point-of-care ultrasound showed high sensitivity and specificity rates when applied to confirm ETT placement in prehospital care. Other methods including capnometry, colorimetric detectors, ODDs, and auscultation showed varied sensitivity and specificity. Patient comorbidities and device failure contributed to decreased accuracy rates in prehospital care. Capnography was less reliable in distinguishing between endotracheal intubation and right main stem intubation, which is known as a complication in out-of-hospital endotracheal intubation. Point-of-care ultrasound was more accurate and reliable in detecting oesophageal and endobronchial misplacements. ETCO2 monitors, i.e., capnometry and colorimetric detectors, were less reliable in patients with low perfusion states. ConclusionThis systematic review showed that there is no single method with 100% accuracy in confirming the correct ETT placement and detecting the occurrence of accidental oesophageal or endobronchial misplacements in prehospital care. Further studies with a larger sample size are needed to assess the accuracy of multiple confirmatory methods in prehospital settings.

Low Perfusion and Missed Diagnosis of Hypoxemia by Pulse Oximetry in Darkly Pigmented Skin: A Prospective Study.

Retrospective clinical trials of pulse oximeter accuracy report more frequent missed diagnoses of hypoxemia in hospitalized Black patients than White patients, differences that may contribute to racial disparities in health and health care. Retrospective studies have limitations including mistiming of blood samples and oximeter readings, inconsistent use of functional versus fractional saturation, and self-reported race used as a surrogate for skin color. Our objective was to prospectively measure the contributions of skin pigmentation, perfusion index (PI), sex, and age on pulse oximeter errors in a laboratory setting. We enrolled 146 healthy subjects, including 25 with light skin (Fitzpatrick class I and II), 78 with medium (class III and IV), and 43 with dark (class V and VI) skin. We studied 2 pulse oximeters (Nellcor N-595 and Masimo Radical 7) in prevalent clinical use. We analyzed 9763 matched pulse oximeter readings (pulse oximeter measured functional saturation [Sp o2 ]) and arterial oxygen saturation (hemoximetry arterial functional oxygen saturation [Sa o2 ]) during stable hypoxemia (Sa o2 68%-100%). PI was measured as percent infrared light modulation by the pulse detected by the pulse oximeter probe, with low perfusion categorized as PI < 1%. The primary analysis was to assess the relationship between pulse oximeter bias (difference between Sa o2 and Sp o2 ) by skin pigment category in a multivariable mixed-effects model incorporating repeated-measures and different levels of Sa o2 and perfusion. Skin pigment, PI, and degree of hypoxemia significantly contributed to errors (bias) in both pulse oximeters. For PI values of 1.0% to 1.5%, 0.5% to 1.0%, and <0.5%, the P value of the relationship to mean bias or median absolute bias was <.00001. In lightly pigmented subjects, only PI was associated with positive bias, whereas in medium and dark subjects bias increased with both low perfusion and degree of hypoxemia. Sex and age was not related to pulse oximeter bias. The combined frequency of missed diagnosis of hypoxemia (pulse oximeter readings 92%-96% when arterial oxygen saturation was <88%) in low perfusion conditions was 1.1% for light, 8.2% for medium, and 21.1% for dark skin. Low peripheral perfusion combined with darker skin pigmentation leads to clinically significant high-reading pulse oximeter errors and missed diagnoses of hypoxemia. Darkly pigmented skin and low perfusion states are likely the cause of racial differences in pulse oximeter performance in retrospective studies.

Indocyanine Green Angiography Overpredicts Postoperative Necrosis Compared to Multispectral Reflectance Imaging.

Sufficient perfusion is foundational to successful reconstructive surgery. Various technologies have been developed to help determine whether tissue is adequately perfused, or whether it will be prone to necrosis postoperatively. Indocyanine green (ICG) angiography is one such method that uses fluorescence and analyzes tissue perfusion. Multispectral reflectance imaging (MSRI) is an alternative technology that analyzes optical properties of oxygenated and deoxygenated hemoglobin to determine tissue viability. Because tissue in low-perfusion states may still survive because of sufficient oxygenation, the authors hypothesized that compared to MSRI, ICG angiography overpredicts necrosis, potentially resulting in unnecessary resection of viable tissue. This study expands on preliminary work to investigate this hypothesis. This was a prospective cohort of patients undergoing prepectoral direct implant reconstruction at a single institution. Each patient was examined intraoperatively with both ICG angiography and MSRI. Decisions to resect tissue were made in conjunction with MSRI and ICG images collected purely for data analysis. Patients were followed postoperatively for at least 2 months for signs of postoperative necrosis. Fifty-three cases were included. ICG angiography accurately predicted viability in 40 of 40 patients (100%) and incorrectly predicted necrosis in 11 of 13 patients (84.6%). Simultaneously, MSRI predicted necrosis in zero patients and accurately predicted viability in 51 of 53 patients (96.2%). There was no statistically significant difference in demographic data among patients predicted to experience necrosis by means of ICG angiography versus those predicted to have entirely viable tissue. This study suggests that ICG angiography is prone to overpredicting postoperative necrosis in comparison to MSRI. This study suggests that multispectral reflectance imaging may benefit practicing plastic surgeons in determining the likelihood of postoperative necrosis. Diagnostic, II.

Pulse Oximeter Performance, Racial Inequity, and the Work Ahead.

It has long been known that many pulse oximeters function less accurately in patients with darker skin. Reasons for this observation are incompletely characterized and potentially enabled by limitations in existing regulatory oversight. Based on decades of experience and unpublished data, we believe it is feasible to fully characterize, in the public domain, the factors that contribute to missing clinically important hypoxemia in patients with darkly pigmented skin. Here we propose 5 priority areas of inquiry for the research community and actionable changes to current regulations that will help improve oximeter accuracy. We propose that leading regulatory agencies should immediately modify standards for measuring accuracy and precision of oximeter performance, analyzing and reporting performance outliers, diversifying study subject pools, thoughtfully defining skin pigmentation, reporting data transparently, and accounting for performance during low-perfusion states. These changes will help reduce bias in pulse oximeter performance and improve access to safe oximeters.

1790 Black Esophagus or Acute Esophageal Necrosis: A Case Series and Single-Institution Experience

INTRODUCTION: Acute esophageal necrosis (AEN) or ‘black esophagus’ is a rare disease of the esophagus with an endoscopic prevalence of 0.02%. It generally presents with signs of upper GI bleeding including melena and hematemesis. Its pathophysiology is largely attributed to severe illness states that affect the perfusion of the esophagus. The distal esophagus being least vascular is most commonly affected. Upper endoscopy reveals circumferential black discoloration. Biopsies are taken to confirm necrosis, as well as rule out infection or pigmentation. Management is generally supportive, with endoscopic resolution seen in many cases with correction of the underlying insult. CASE DESCRIPTION/METHODS: Three patients with AEN were identified over the last 2 years at our institution. Two females aged 67 and 78 were admitted within the same month in 2019 with epigastric pain and coffee-ground-emesis. Both were found to be in diabetic ketoacidosis (DKA). One of them was diagnosed with CHF with reduced ejection fraction (40%). Both patients had documented T2DM and atherosclerotic disease. Laboratory findings showed elevated serum glucose and anion-gap metabolic acidosis with elevated beta-hydroxyburate. Both underwent EGD which was notable for a circumferential black esophagus that abruptly terminated at the gastro-esophageal junction (Figures 1 and 2). The third patient was a 35-year-old male with poorly-controlled T1DM (A1c 9.1%) who noticed melenic stools 2 days prior and presented with hypovolemic shock. EGD was consistent with black esophagus. Biopsy showed acute inflammatory exudates with necrotic cells and numerous fungal hyphae. He was started on micafungin for two weeks and twice daily PPI. EGD 8 weeks later showed complete resolution of initial findings (Figure 3). DISCUSSION: AEN is a rare cause of upper GI bleed, more commonly reported in men and in patients with underlying vasculopathies who present with severe illness. Peak incidence is in the sixth decade of life. Existing literature reports DKA as a relatively common precipitating factor. All of our patients presented with upper GI bleed secondary to AEN and were in DKA superimposed on low perfusion states due to CHF, atherosclerotic disease and sepsis respectively. Two out of three patients were females. Based on our case series, it appears that DKA, a critical illness in itself, further worsens existing low perfusion states. This can manifest as upper GI bleeding. DKA demands prompt correction, especially in patients with concomitant vasculopathy.

Effect of Intravenous Versus Intraosseous Access in Prehospital Cardiac Arrest

ObjectiveThe prevailing standard of care in prehospital emergency medical services (EMS) is that either intravenous (IV) or intraosseous (IO) access is an acceptable route for obtaining vascular access and delivery of resuscitation medications and volume expanders in cardiac arrest patients. The aim of this study was to evaluate the effectiveness of IV access versus IO access in terms of return of spontaneous circulation (ROSC) for patients suffering from cardiac arrest. MethodsA retrospective chart review examining cardiac arrest data with a single advanced life support EMS agency over a 4-year period was performed. Cardiac arrest patients were identified from a quality assurance database. Exclusion criteria included trauma arrest, pediatrics, pregnancy, and obvious signs of death. ResultsA total of 795 patients remained after applying the exclusion criteria. A total of 183 (45.1%) out of 406 cardiac arrest patients achieved ROSC who had an IV placed. A total of 389 cardiac arrest patients had an IO placed with ROSC in 100 (25.7%). ConclusionsHigher ROSC rates were achieved with IV access versus IO access. Limitations include the small sample size, a single EMS agency, and the retrospective nature of the study. Future studies should further evaluate the effectiveness of IO versus IV access in cardiac arrest and other low perfusion states.

Endotracheal tube placement confirmation: 100% sensitivity and specificity with sustained four-phase capnographic waveforms in a cadaveric experimental model

BackgroundWaveform capnography is considered the gold standard for verification of proper endotracheal tube placement, but current guidelines caution that it is unreliable in low-perfusion states such as cardiac arrest. Recent case reports found that long-deceased cadavers can produce capnographic waveforms. The purpose of this study was to determine the predictive value of waveform capnography for endotracheal tube placement verification and detection of misplacement using a cadaveric experimental model. MethodsWe conducted a controlled experiment with two intubated cadavers. Tubes were placed within the trachea, esophagus, and hypopharynx utilizing video laryngoscopy. We recorded observations of capnographic waveforms and quantitative end-tidal carbon dioxide (ETCO2) values during tracheal versus extratracheal (i.e., esophageal and hypopharyngeal) ventilations. Results106 and 89 tracheal ventilations delivered to cadavers one and two, respectively (n=195) all produced characteristic alveolar waveforms (positive) with ETCO2 values ranging 2–113mmHg. 42 esophageal ventilations (36 to cadaver one and 6 to cadaver two), and 6 hypopharyngeal ventilations (4 to cadaver one and 2 to cadaver two) all resulted in non-alveolar waveforms (negative) with ETCO2 values of 0mmHg. Esophageal and hypopharyngeal measurements were categorized as extratracheal (n=48). A binary classification test showed no false negatives or false positives, indicating 100% sensitivity (NPV 1.0, 95%CI 0.98–1.00) and 100% specificity (PPV 1.0, 95%CI 0.93–1.00). ConclusionThough current guidelines question the reliability of waveform capnography for verifying endotracheal tube location during low-perfusion states such as cardiac arrest, our findings suggest that it is highly sensitive and specific.

Role of acidic pH of intravenous fluids in subsequent development of metabolic acidosis- may not be what it seems.

Sir, We extend our thanks to Singh et al. for drawing attention toward the controversies surrounding the use of acidic intravenous fluids (IVFs) infusion.[1] They have rightly mentioned the propensity of 5% dextrose (D5) and 0.9% saline (NS) to cause thrombophlebitis and other deleterious effects.[1] But unlike mentioned by the authors, current literature discusses the known and putative reasons for the pH of 5.5 of NS.[2] Dissolved CO2 from atmosphere, the ionic composition of the solution and inherent properties of the packaging material in which the solutions are supplied, are thought to determine it.[2] We believe that the same set of reasons may also explain the acidic pH (4.2) of D5. Free acid activity is indicated by pH but the nondissociated acid molecules in a solution may not be characterized by the pH value.[2,3] Thus solutions with high titratable acid have reservoir of hydrogen ion (H+) irrespective of pH.[3] Citing the work of Gaudry et al. the authors attribute the acidity of IVFs to their method of sterilization, Gaudry et al. in their manuscript mentioned that the titratable acidity of majority of solutions under study (e.g. D5, NS) varied between 0.063 to 9.480 meq acid/l, depending on the method used for preparation and sterilization.[1,4] Gaudry et al. also mention that though many have measured the pH of common IVFs, not much data are available regarding their titratable acidity.[4] The work of Gaudry et al. dates back to 1972 and a PubMed search by us with the term ‘titratable acidity fluid/titratable acidity solution/titratable acidity infusion’ revealed only two studies relevant to this discussion. These studies date back to 1973 and 2005 and only the abstracts could be retrieved by us.[3,5] Though the in vitro pH of D5 and NS is acidic, their effect on H+ balance is probably negligible.[2,4] It may be because of their low titratable acid content.[4] But metabolic acidosis accompanies infusion of these solutions.[1,2,4,6,7] This acidosis was explained by dilution of bicarbonate content in plasma.[2,6,7] Subsequently Stewart's physiochemical approach was used to describe the in vivo effect on acid base status by IVFs.[6,7] According to the later concept, the acid-base effect of an IVF is determined by the strong ion deference (SID) and the total weak acid concentration (ATOT) of the fluid.[6,7] pH falls with decreasing SID, increasing ATOT or CO2TOT(total CO2 content) / PCO2.[6,7] As the IVFs equilibrate with ECF, they alter SID of ECF, as SID of these IVFs defers appreciably from that of plasma.[6,7] SID of D5 and NS is 0, they decrease the SID of plasma and exert an acidifying effect.[6,7] These crystalloids are devoid of any weak acids and decrease ATOT of plasma due to its diluting effect causing alkalosis.[6,7] Whereas some colloids (e.g. albumin, gelatin) itself may contribute toward ATOT of plasma, thus, acidifying it.[6,7] Although CO2TOT/PCO2 also independently influences the pH of a biological system, its contribution in determining in vivo acid base activity of an IVF is generally regarded to be low as the concentration of dissolved CO2 is itself low (about 0.012 mmol/l).[2,6,7] But it should be kept in mind that IVFs with high CO2TOT have impact on intracellular acid base status, more during rapid administration or low perfusion states.[6,7] Contrary to the prediction by Singh et al. the pH of hypertonic saline (3%) is 5.8 and it causes more acidosis than NS because of its higher osmolarity that draws more water from other body fluid compartments.[6,7] Although the pH of Hartmann`s solution is 5, its in vivo SID is 27 meq/l, which is close to the ideal SID of 24 meq/l.[2,6,7] And despite its pH of 5, it is known to prevent infusion-related metabolic acidosis.[2,6,7] In-depth discussion on application of the concept of quantitative physical chemistry on the in vivo effect of acid base homeostasis is available.[6,7] The effect on plasma pH by solutions with same pH may be different and differing pH can have similar acid-base effects.[2,6] Thus, the effect of an IVF on plasma/ECF SID, ATOT and CO2TOT depending on the amount infused should be considered while predicting its possible in vivo effect on acid base status, not merely its in vitro pH.[1,2,3,4,5,6,7]

Tidal Volumes during General Anesthesia

Tidal Volumes during General Anesthesia

Bladder Mucosal CO2Compared with Gastric Mucosal CO2as a Marker for Low Perfusion States in Septic Shock

Recent reports indicate the possible role of bladder CO2 as a marker of low perfusion states. To test this hypothesis, shock was induced in six beagle dogs with 1 mg/kg of E. coli lipopolysaccharide, gastric CO2 (CO2-G) was measured with a continuous monitor, and a pulmonary catheter was inserted in the bladder to measure CO2 (CO2-B). Levels of CO2-B were found to be lower than those of CO2-G, with a mean difference of 36.8 mmHg (P < 0.001), and correlation between both measurements was poor (r 2 = 0.16). Even when the correlation between CO2-G and ΔCO2-G was narrow (r 2 = 0.86), this was not the case for the relationship between CO2-B and ΔCO2-B (r 2 = 0.29). Finally, the correlation between CO2-G and base deficit was good (r 2 = 0.45), which was not the case with the CO2-B correlation (r 2 = 0.03). In our experience, bladder CO2 does not correlate to hemodynamic parameters and does not substitute gastric CO2 for detection of low perfusion states.

Heartlight - Acquisition Times for a Novel Forehead Heart Rate Sensor in Delivery Room Resuscitation of Preterm Infants

Background: Approximately 10% of newborn infants require some resuscitation at birth and heart rate (HR) is considered the best indicator of effective resuscitation. Current HR assessment, using a stethoscope, is non-continuous, can interrupt resuscitation and is estimated incorrectly in 20-30% of cases. Pulse oximetry (PO) is not specifically designed for newborn HR monitoring and can be unreliable in low perfusion states. A simple forehead HR sensor would be advantageous allowing quick placement, continuous monitoring and improved reliability in low perfusion states. Aims: Further development of a user friendly, quick and reliable forehead HR sensor for use during resuscitation of preterm infants in the delivery room. Methods: Our novel forehead HR sensor (HeartLight), utilises patented reflectance photoplethysmography (PPG), to rapidly detect HR and can be sited in ∽5 seconds. We investigated acquisition time of HeartLight versus PO at birth in preterm infants in the delivery room (gestation 32+6weeks ±20days). Time to acquire a reliable signal was measured from the time each sensor was applied. Results: Median time to obtain 1 and 2 seconds of PPG signal were calculated (table). Conclusion: HeartLight rapidly detects a pulsatile signal in newborn preterm infants, more quickly than PO. HeartLight may offer a rapid, real time monitoring solution for HR during delivery room resuscitation and is undergoing further clinical trials. Funding: MRC DPFS portfolio award

A structural model of perfusion and oxygenation in low-flow states

Background Recent investigations underscore the critical importance of ventilation strategies on resuscitation outcomes. In low perfusion states, such as cardiac arrest and traumatic shock, the rise in intrathoracic pressure that accompanies positive-pressure ventilation can significantly impede venous return and lead to a decrease in cardiac output. The optimal ventilation strategy in these “low-flow” states remains unclear. Objective To create a mathematical model of perfusion and oxygenation to predict the effects of PPV with both normotension and hypotension. Methods The lung pressure–volume relationship was modeled using a novel formula allowing manipulation of various lung characteristics. A separate formula was then derived to predict mean intrathoracic pressure (MITP) for specific minute ventilation values using the pressure–volume formula. The addition of positive end-expiratory pressure was also modeled. Finally, a formula was derived to model oxygen absorbance as a function of alveolar surface area and flow based on ventilation rate and MITP. Results Mathematical models of the lung pressure–volume relationship, MITP, and absorbance were successfully derived. Manipulation of total lung capacity, compliance, upper and lower inflection points, positive end-expiratory pressure, and minute ventilation allowed prediction of optimal ventilation rate and tidal volume for a normal lung and with various abnormal characteristics to simulate particular disease states, such as acute respiratory distress syndrome (ARDS). For a normal lung, ventilation rates of 4–6 breaths/min with higher tidal volumes (15–20 mL/kg) resulted in the lowest predicted MITP values (5 cm H 2O) and the highest absorbance. The input of lung parameters that would simulate ARDS resulted in optimal ventilation rates of 10–12 breaths/min with lower tidal volumes (8–10 mL/kg) and higher predicted MITP values (10–15 cm H 2O). Conclusions A mathematical model of ventilation was successfully derived allowing manipulation of multiple pulmonary physiological variables to predict MITP and potentially identify optimal ventilation strategies. This model suggests the use of lower ventilation rates and larger tidal volumes to minimize the hemodynamic effects of positive pressure ventilation in patients with hypoperfusion but normal lung characteristics.

Year in review 2010: Interstitial lung diseases, acute lung injury, sleep, physiology, imaging, bronchoscopic intervention and lung cancer

Year in review 2010: Interstitial lung diseases, acute lung injury, sleep, physiology, imaging, bronchoscopic intervention and lung cancer

Abstract 45: A Mathematical Model of Ventilation, Perfusion, and Oxygenation in Low-Flow States

Background: Recent investigations underscore the critical importance of ventilation strategies on resuscitation outcomes. In low perfusion states, such as cardiac arrest and traumatic shock, the rise in intrathoracic pressure that accompanies positive-pressure ventilation (PPV) can significantly impede venous return and lead to a decrease in cardiac output. The optimal ventilation strategy in these “low-flow” states remains unclear. Objectives: To create a mathematical model of perfusion and oxygenation to predict the effects of PPV with both normotension and hypotension. Methods: The lung pressure-volume relationship was modeled using a novel formula allowing manipulation of various lung characteristics, including vital capacity, compliance, and the upper and lower inflection points. A separate formula was then derived to predict mean intrathoracic pressure for a given minute ventilation using the pressure-volume formula. The addition of positive end-expiratory pressure was also modeled. Finally, a formula was derived to model oxygen absorbance as a function of alveolar surface area and flow based on ventilation rate and mean intrathoracic pressure. Results: Mathematical models of the lung pressure-volume relationship, mean intrathoracic pressure, and absorbance were successfully derived. Manipulation of vital capacity, compliance, upper and lower inflection points, positive end-expiratory pressure, and minute ventilation allowed prediction of optimal ventilation rate and tidal volume for a normal and an ARDS lung. For a normal lung, optimal values for both mean intrathoracic pressure and absorption were achieved with a ventilation rate of 4 breaths/min. A decrease in the upper inflection point or increase in minute ventilation resulted in faster optimal ventilation rates, although none exceeded 14 breaths/min. Conclusions: A mathematical model of ventilation was successfully created allowing manipulation of multiple variables related to lung compliance and ventilation strategy. This model suggests the use of lower ventilation rates with larger tidal volumes to minimize the hemodynamic effects of PPV and maximize oxygen absorbance.

Successful use of pharyngeal pulse oximetry with the oropharyngeal airway in severely shocked patients

We describe the successful use of pharyngeal oximetry with the oropharyngeal airway in two patients with severe shock in whom finger pulse oximetry failed. One patient was a 50-year-old man with septic shock and the other a 32-year-old woman with haemorrhagic shock. In both patients, an oropharyngeal airway with a paediatric pulse oximeter probe was inserted adjacent to the tracheal tube. A good waveform was obtained and oxygen saturation was 0-2% lower than arterial samples whereas finger pulse oximetry saturation was unobtainable or much lower than arterial oxygen saturation. Pharyngeal oxygen saturation with the oropharyngeal airway is feasible and more accurate than finger oximetry in low perfusion states.

Management of Acute Decompensated Heart Failure

Heart failure, a debilitating complex clinical syndrome, affects nearly 5 million people in the United States and presents a heavy socioeconomic burden. Neurohormonal abnormalities contribute to the pathophysiology of heart failure. Acute decompensated heart failure (ADHF) has emerged as a major health problem associated with poor prognosis, increased costs related to care, reduced quality of life, and frequent readmissions. Symptoms of ADHF are primarily related to congestion and/or low perfusion states. The use of biomakers such as B-natriuretic peptides is useful in distinguishing between cardiac and noncardiac causes of symptoms. Treatment for ADHF begins with identification and treatment of precipitating factors for acute decompensation. Initial goal of therapy is focused on symptom management followed by interventions that delay disease progression, reduce readmission, and prolong survival.

In Vivo Perfusion Estimation Using Subharmonic Contrast Microbubble Signals

The purpose of this study was to quantify perfusion in vivo using contrast-enhanced subharmonic imaging (SHI). A modified LOGIQ 9 scanner (GE Healthcare, Milwaukee, WI) operating in gray scale SHI mode was used to measure SHI time-intensity curves in vivo. Four dogs received intravenous contrast bolus injections (dose, 0.1 mL/kg), and renal SHI was performed. After 3 contrast agent injections, a microvascular staining technique based on stable (nonradioactive) isotope-labeled microspheres (BioPhysics Assay Laboratory Inc, Worcester, MA) was used to quantify the degree of perfusion in 8 sections of each kidney. Low perfusion states were induced by ligating surgically exposed segmental renal arteries followed by contrast agent injections and microvascular staining. Digital clips were transferred to a personal computer, and SHI time-intensity curves were acquired in each section using Image-Pro Plus software (Media Cybernetics, Silver Spring, MD). Subharmonic fractional blood volumes were calculated, and the perfusion was estimated from the initial slope of the fractional blood volume uptake averaged over 3 injections. Subharmonic perfusion data were compared with the gold standard (ie, the microspheres) using linear regression analysis. In vivo gray scale SHI clearly showed flow and, thus, perfusion in the kidneys with almost complete suppression of tissue signals. In total, 270 SHI time-intensity curves were acquired, which reduced to 94 perfusion estimates after averaging. Subharmonic perfusion estimates correlated significantly with microsphere results (r = 0.57; P < .0001). The best SHI perfusion estimates occurred for high perfusion states in the anterior of the kidneys (r = 0.73; P = .0001). The corresponding root mean square error was 2.4%. Subharmonic perfusion estimates have been obtained in vivo. The perfusion estimates were in reasonable to good agreement with a microvascular staining technique.

Pulse oximetry in supportive and palliative care.

Pulse oximetry is a valuable, non-invasive method used for estimating oxyhaemoglobin saturation. It can give a bedside indication of the oxygenation and thus provide a valuable insight into the cause of breathlessness. Its use can help palliative care teams to determine the need to prescribe or to withhold oxygen therapy. The technology is well established and relatively inexpensive. Factors that influence readings include low perfusion states at the end of life. With a thorough understanding of its uses and limitations, pulse oximetry can assist multidisciplinary teams in providing better care to ill patients in the palliative care setting.

Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in adults

OBJECTIVE: The purposes of the study were to: (1) describe the aggregate strength of the relationship of arterial oxygen saturation as measured by pulse oximetry with the standard of arterial blood gas analysis as measured by co-oximetry, (2) examine how various factors affect this relationship, and (3) describe an aggregate estimate of the bias and precision between oxygen saturation as measured by pulse oximetry and the standard in vitro measures.DESIGN: A meta-analysis was conducted.SAMPLE: Seventy-four studies from 1976 to 1994 met the inclusion criteria of: (1) adult study population, (2) quantitative analysis of empirical data, and (3) bivariate correlations or bias and precision estimates between pulse oximeter and co-oximeter values.RESULTS: There were a total of 169 oximeter trials on 41 oximeter models from 25 different manufacturers. Studies were conducted in various settings with a variety of subjects, with most being healthy adult volunteers. The weighted mean r, based on 39 studies (62 oximeter trials) for which the r statistic and number of data points were available, was 0.895 (var[r] = 0.014). Based on 23 studies (82 oximeter trials) for which bias and precision estimates and number of data points were available, the mean absolute bias and precision were 1.999 and 0.233, respectively. Several factors were found to affect the accuracy of pulse oximetry.CONCLUSION: Pulse oximeters were found to be accurate within 2% (± 1 SD) or 5% (± 2 SD) of in vitro oximetry in the range of 70% to 100% SaO2. In comparing ear and finger probes, readings from finger probes were more accurate. Pulse oximeters may fail to record accurately the true SaO2 during severe or rapid desaturation, hypotension, hypothermia, dyshemoglobinemia, and low perfusion states.

The effects of ischemia and reperfusion on intestinal motility

The interdigestive migrating motor complex (MMC) has been demonstrated to be a reliable indicator of intestinal motility and function. The effects of low perfusion on the MMC have never been studied. Fourteen newborn Yorkshire piglets (5 to 18 days old, weighing 2.9 ± 0.4 kg) underwent celiotomy under general anesthesia with placement of four jejunal electrodes (50 cm apart) as well as a superior mesenteric artery (SMA) Doppler flow probe and a pericardial catheter. Group 1 (n = 5) had operation alone. Group 2 (n = 9) had nonocclusive mesenteric ischemia induced by reversible cardiac tamponade for 5 hours between postoperative days 6 to 12. All subjects had MMC phase III electrical activity, cycling time, and propagation velocity recorded daily. In group 2 MMCs were recorded prior to and during ischemia, and during reperfusion. Group 2 animals had 75% ± 4% decrease in SMA flow during the tamponade period. During the ischemic period, the MMC cycling time (CT) increased from 67 ± 10 (mean ± SEM) to 98 ± 12 minutes ( P < .05) and MMC propagation velocity (PV) decreased to 4.2 ± 2.2 from a baseline value of 10.5 ± 1.5 cm/min ( P < .05). During reperfusion CT and PV values were not significantly different from baseline. The validity of this model is confirmed by the comparable baseline recordings in groups 1 and 2, and by the return of MMC to baseline values within 4 to 7 hours of reperfusion, as seen in group 2. Phase III was similar in all subjects and it was not affected by ischemia, indicating that intrinsic electrical activity of the bowel was not disrupted. These findings in the young piglet—an excellent model of human small bowel motility—demonstrate a dysfunctional MMC pattern induced by nonocclusive mesenteric ischemia. This may represent a key factor for development of bacterial overgrowth and translocation from the gastrointestinal tract during low perfusion states.