Instantaneous blood flow responses to positive end-expiratory pressure with spontaneous ventilation.

Variable hemodynamic responses to positive end-expiratory pressure (PEEP) with spontaneous ventilation have been reported. To clarify these responses, 15 awake patients were studied using a catheter-tip velocity transducer to record phasic aortic root blood flow continuously before, during and after PEEP (10 cm H2O) applied with a face mask. Central blood volume and effective ventricular filling pressures were measured. Phasic pulmonary artery blood flow was also simultaneously recorded in three of these patients. PEEP produced an acute aortic blood flow reduction, detected within one respiratory cycle. Stroke volume decreased 12%, and since heart rate was unchanged, cardiac output also declined (p less than 0.05). Inspiratory-to-expiratory aortic flow changes were less during PEEP. In contrast, inspiratory-to-expiratory pulmonary artery flow alterations were exaggerated due to a marked flow decline during expiration. Central blood volume and effective left ventricular filling pressure decreased 9% and 19%, respectively (p less than 0.05 in all patients). The decrease in pulmonary artery flow was associated with a decrease in central blood volume in the three patients in whom pulmonary flow was measured. PEEP promptly reduces cardiac output during spontaneous ventilation, related to a decrease in pulmonary flow in expiration.     

Effects of dopexamine and positive end-expiratory pressure on intestinal blood flow and oxygenation: the perfusion pressure perspective

OBJECTIVE: To evaluate the net effects of the concomitant use of positive end-expiratory pressure (PEEP) and dopexamine on intestinal tissue perfusion and oxygenation during predefined artificial reductions in intestinal perfusion pressure (IPP). DESIGN: Prospective, self-controlled, experimental study. SETTING: University hospital research laboratory. SUBJECTS: Seven female pigs. MEASUREMENTS: In barbiturate-anesthetized pigs, we measured mesenteric blood flow (QMES) [by transit-time ultrasonic flowmetry], jejunal mucosal perfusion (by laser Doppler flowmetry), and tissue PO(2) (by microoximetry). Based on blood sampling, we calculated the intestinal net lactate production and oxygenation. INTERVENTIONS: These measurements and calculations were performed at three predefined and controlled IPP levels, which were obtained by an adjustable clamp around the superior mesenteric artery. At each IPP level, measurements were performed prior to and during PEEP (10 cm H(2)O), both with and without simultaneous dopexamine infusions (at 0.5 and 1.0 microg/kg/min). RESULTS: Within the IPP range of 77 to 33 mm Hg, intestinal perfusion and oxygenation were maintained irrespective of whether PEEP and/or dopexamine were applied or not. At IPP < 33 mm Hg, QMES and intestinal oxygenation deteriorated, resulting in regional net lactate production. At this IPP range, tissue oxygen perfusion was entirely pressure-dependent, and even small reductions in IPP led to prominent increases in intestinal net lactate production. Dopexamine did not modify this pattern. CONCLUSIONS: We describe maintained intestinal tissue oxygen perfusion within a wide perfusion pressure range. Within this perfusion pressure range, PEEP did not induce any adverse regional circulatory effects. Below the perfusion pressure range for effective autoregulation, intestinal tissue oxygen perfusion deteriorated, and regional ischemia occurred. In this situation, dopexamine was unable to counteract IPP-dependent decreases in intestinal tissue oxygen perfusion. The regional ischemic threshold can be defined either as an IPP of < 33 mm Hg or as an intestinal tissue PO(2) of < 45 mm Hg.     

Expiratory flow limitation and intrinsic positive end-expiratory pressure at zero positive end-expiratory pressure in patients with adult respiratory distress syndrome

It has been suggested that in patients with adult respiratory distress syndrome (ARDS), intrinsic positive end-expiratory pressure (PEEPi) is generated by a disproportionate increase in expiratory flow resistance. Using the negative expiratory pressure (NEP) technique, we assessed whether expiratory flow limitation (EFL) and PEEPi were present at zero PEEP in 10 semirecumbent, mechanically ventilated ARDS patients. Because bronchodilators may decrease airway resistance, we also investigated the effect of nebulized salbutamol on EFL, PEEPi, and respiratory mechanics in these patients, and in seven patients we measured the latter variables in the supine position as well. In the semirecumbent position, eight of the 10 ARDS patients exhibited tidal EFL, ranging from 5 to 37% of the control tidal volume (VT), whereas PEEPi was present in all 10 subjects, ranging from 0.4 cm H(2)O to 7.7 cm H(2)O. The onset of EFL was heralded by a distinct inflection point on the expiratory flow-volume curve, which probably reflected small-airway closure. Administration of salbutamol had no statistically significant effect on PEEPi, EFL (as %VT), or respiratory mechanics. EFL (%VT) and PEEPi were significantly higher in the supine position than in the semirecumbent position, whereas the other respiratory variables did not change. Our results suggest that in the absence of externally applied PEEP, most ARDS patients exhibit EFL associated with small-airway closure and a concomitant PEEPi.     

Coronary flow limits right ventricular performance during positive end-expiratory pressure

The effect of positive end-expiratory pressure (PEEP) on right ventricular performance and myocardial blood flow was determined in 16 dogs before and after right coronary artery (RCA) occlusion. Right ventricular ejection fraction (RVEF), end-diastolic volume (EDV) and end-ejection volume were measured by thermodilution. Right ventricular end-ejection pressure-volume relations (RVEEPVR) were determined at baseline and at 20 cm H2O PEEP, both before and after RCA occlusion. In four of the dogs, RVEEPVR were also determined at 10 cm H2O PEEP after RCA occlusion. With intact RCA flow, RVEF declined with PEEP (37 +/- 5 to 19 +/- 6%) with no significant change in EDV (50 +/- 11 to 42 +/- 11 ml) or end-ejection volume (31 +/- 7 to 36 +/- 9 ml). RVEEPVR and right ventricular myocardial blood flow were also unchanged with PEEP. After RCA occlusion, RVEF declined with PEEP (27 +/- 4 to 15 +/- 5%) in association with a significant increase in end-ejection volume (39 +/- 8 to 49 +/- 10 ml), but no change in EDV (53 to 55 ml). In addition, RVEEPVR and myocardial blood flow declined with RCA occlusion, and declined further with 20 cm H2O, but not with 10 cm H2O PEEP, after RCA occlusion. Therefore, in this experimental model, right ventricular performance was adversely affected during PEEP when right coronary blood flow was limited.     

Vagal cooling and positive end-expiratory pressure reduce systemic to pulmonary bronchial blood flow in dogs

Positive end-expiratory pressure (PEEP) reduces systemic to pulmonary bronchial blood flow [Qbr(s-p)] presumably because it increases bronchial vascular resistance. Since PEEP increases lung volume and thus could stimulate pulmonary stretch receptors, we investigated the hypothesis that the PEEP-related decrease in bronchial blood flow was due to a reflex mediated by the vagus. In open-chest dogs the left lower lobe (LLL) was isolated, independently ventilated, perfused in situ with a closed pulmonary vascular circuit and weighed continuously. Qbr(s-p) was measured as LLL vascular circuit overflow and changes in LLL weight. When LLL PEEP was increased from 5 to 15 cm H2O in a group of 11 dogs Qbr(s-p) was reduced by half from 60.8 +/- 10.5 to 31.6 +/- 6.1 ml/min/100 g dry lobe weight. In another group of 7 dogs Qbr(s-p) was 46.5 +/- 6.9 with PEEP = 5 cm H2O; it decreased to 28.3 +/- 6.8 with bilateral cervical vagal cooling (0-1.5 degrees C) and did not decrease further after increasing PEEP to 15 cm H2O. We conclude that the effect of resting vagal tone is to increase Qbr(s-p) and that the effect of PEEP on Qbr(s-p) may be mediated at least partially by vagal influences.     

呼气末正压对COPD合并呼吸衰竭患者病情影响的研究

目的探讨不同水平的呼气末正压（PEEP）对COPD并呼吸衰竭患者病情的影响。方法选择2007年10月至2009年2月确诊为COPD并呼吸衰竭需机械通气治疗的患者40例,随机分为A组（20例,PEEP5cm H2O）,B组（20例,PEEP10cm H2O）。监测两组患者呼吸支持后动脉血气、气道压力、血流动力学的变化。结果（1）动脉血气：两组患者上机后0.5h、24h动脉血气指标分别比较无差异,P〉0.05。（2）气道压力：上机后0.5h、24hB组气道峰压、气道平台压较A组明显升高,P〈0.05.（3）血流动力学：上机后0.5h、24hA、B两组患者的心率、平均动脉压、外周循环阻力指数变化无差异,P〉0.05;上机后0.5hA、B两组患者心排血量指数、左心做功指数、胸液水平变化无差异;但上机24h后B组上述3个指标较A组下降,P〈0.05。结论COPD并呼吸衰竭患者机械通气治疗时低水平PEEP是有益且安全的;避免高水平的PEEP给患者带来负面效应。     

Effects of positive end-expiratory pressure and body position on pulmonary blood flow redistribution in mechanically ventilated normal pigs

STUDY OBJECTIVES: To assess the respective effects of position and positive end-expiratory pressure (PEEP) on the distribution of regional pulmonary blood flow (PBF). DESIGN: Prospective randomized animal study. SETTING: Animal research facility in a university hospital. PARTICIPANTS: Normal pigs that were tracheostomized, anesthetized, and mechanically ventilated. INTERVENTIONS: PBF was measured in seven pigs in the supine position (SP) and the prone position (PP) at both zero end-expiratory pressure (ZEEP) and 10 cm H(2)O of PEEP. The regional PBF was assessed by the radioactive microsphere method. The lungs from each pig were sliced into 90 samples. The heterogeneity of PBF was estimated from its coefficient of variation. MEASUREMENTS AND RESULTS: The lung samples had a mean (+/- SD) weight of 1.60 +/- 0.39 g. Changing position from SP to PP at ZEEP redistributed PBF toward the anterior, superior, and peripheral regions and did not significantly reduce the coefficient of variation for regional PBF (reduction, 44.7 +/- 7% to 42.2 +/- 8%). Changing from the SP to PP position at PEEP induced a similar, but more marked, redistribution of PBF and a significant reduction in the coefficient of variation from 53 +/- 13% to 30.4 +/- 7% (p < 0.001). In the SP, PEEP redistributed PBF toward the posterior, inferior, and central regions without changing the heterogeneity of PBF. In the PP, PEEP had little effect on the PBF redistribution but significantly reduced the coefficient of variation of PBF from 42.2 +/- 8% to 30.4 +/- 7% (p < 0.05). CONCLUSIONS: Pigs in the PP had altered gravitational dependence of PBF compared to that observed when pigs were in the SP. This effect was enhanced by using a PEEP of 10 cm H(2)O.     

呼气末正压的临床应用

呼气末正压是指在机械通气时,人为的使呼气末气道及肺泡内压力高于大气压.临床常用于治疗急性呼吸窘迫综合征、慢性阻塞性肺疾病、急性左心衰竭等,通过设置不同水平的呼气末正压,减轻肺渗出,改善氧合;改善患者与呼吸机的协调性以降低呼吸功;固定胸壁等.但针对不同疾病呼气末正压的作用机制有所不同,本文将逐一介绍.     

Redistribution of pulmonary blood flow induced by positive end-expiratory pressure and dopamine infusion in acute respiratory failure

The mechanism by which mechanical ventilation (MV) with positive end-expiratory pressure (PEEP) improves hypoxemia in patients with acute respiratory failure (ARF) is unclear, and may be attributed in part to a decrease in cardiac output inducing by itself a reduction of the shunt. Using the multiple inert gas elimination technique we evaluated the effects of PEEP on ventilation-perfusion (VA/Q) distribution in 8 patients while cardiac output was maintained at control value by means of a dopamine infusion. In each patient, evaluation was performed during MV without PEEP (control) then with PEEP (17 +/- 2 cm H2O) and dopamine. After application of PEEP, PaO2, PvO2, and oxygen transport (TO2) increased significantly, whereas venous admixture decreased from 37.5 +/- 5 to 17 +/- 2% (p less than 0.01). Comparison of VA/Q distribution during PEEP and zero end-expiratory pressure documented a redistribution of pulmonary blood flow; the shunt decreased markedly from 30 +/- 4 to 13 +/- 2% (p less than 0.001), whereas the fraction of cardiac output distributed to "normal" VA/Q ratio units (0.1 to 10) increased from 62 to 78.5% (p less than 0.001). Dead space increased slightly with PEEP, from 44 to 49% (p less than 0.01) of total ventilation. The pattern of ventilation distribution was essentially unaltered; specifically, no additional high VA/Q mode was observed during PEEP. It is concluded that cardiac output maintenance with dopamine infusion during PEEP does not suppress the beneficial effects of PEEP on gas exchange, but induces a redistribution of pulmonary blood toward the main VA/Q ratio.     

The effect of diazoxide on coronary blood flow

Hemodynamic measurements were made on anesthetized and unanesthetized dogs to observe the effects of injection of diazoxide. The measurements included the electrocardiogram, blood pressures, cardiac output and coronary blood flow. Calculated values included coronary and total peripheral resistance, left ventricular work and efficiency.

Coronary flow increased markedly and with larger doses the increase persisted for three to four hours. Calculated coronary arteriolar resistance decreased more in response to the diazoxide than did total peripheral resistance. Direct intracoronary injection of diazoxide in the conscious dog showed that the primary cardiac effect of this drug is to dilate the coronary arteriole.     

The effects of ventilation with positive end-expiratory pressure on the bronchial circulation

We studied the effects of ventilation with 10 cm H2O PEEP for 2 h in dogs with temporary unilateral pulmonary arterial occlusion (TUPAO) on bronchial blood flow to the occluded lung using the microsphere dispersion technique. We found that blood flow to the occluded left lung in dogs was 9.9 ml/min (0.122 ml X min-1 X g-1). Within 30 min following the addition of 10 cm H2O PEEP blood flow fell by 70-80% (to 2.3 ml/min) caused both by a 3-fold decrease in vascular conductance and a 25% fall in systemic blood pressure. The reduction in left bronchial blood flow persisted for at least 2 h. We conclude from these data that ventilation with PEEP in the presence of pulmonary artery occlusion has a severe, persistent adverse effect on bronchial blood flow. This reduction in bronchial blood flow is beyond what can be explained by the changes in airway pressure. The additional increase in bronchial vascular resistance may be caused by the increase in lung volume, by reflex bronchial vasoconstriction, or by release of mediators locally.     

Reflex responses to positive end-expiratory pressure

When positive end-expiratory pressure (PEEP) is applied to rabbits there is a large depression of cardiac output. The authors investigated the baroreceptor and lung stretch reflex responses to PEEP in rabbits to determine if reflex responses are important in causing the depression of cardiac output which occurs with PEEP. With PEEP there was a depression of cardiac output and blood pressure, no change in heart rate, and an increase in systemic vascular resistance. The responses to PEEP were similar after vagotomy and after a large dose of atropine, but, after treatment with phenoxybenzamine, blood pressure was lower during PEEP and systemic vascular resistance did not increase. The data suggests that lung stretch response is not an important factor in the cardiovascular response to PEEP in rabbits.     

经导管急性微血管栓塞对冠状动脉血流和冠状动脉血流储备的影响

目的　建立经导管的急性微血管栓塞动物模型 ,研究不同微栓塞水平冠状动脉血流(CBF)和冠状动脉血流储备 (CFR)的变化。方法　 10头小型猪 (2 1～ 2 5kg) ,不开胸经导管由前降支重复注入微栓塞球 (4 5 μm) ,使用多普勒导丝、压力导丝和腔内超声导管测量前降支中段平均流速、压力和腔内面积 ,CBF =平均流速×面积。冠状动脉内注射腺苷 18μg达到最大充血状态 ,CFR =充血CBF 基础CBF。在微栓塞前、注入 5万、10万、12万、14万和 15万微栓塞球时分别测量基础状态和充血状态的血流动力学指标。结果　注入 5万微球时基础CBF(rCBF)升高 ,较微栓塞前差异有显著性[(0 30± 0 0 9)ml·s- 1 vs (0 2 6± 0 10 )ml·s- 1 ,P <0 0 5 ],随着微球量的增加rCBF降低。CFR在微栓塞后降低 ,不同微栓塞水平的CFR与微栓塞前比较差异均有显著性。结论　经导管急性微血管栓塞中 ,随着微栓塞球量增加 ,rCBF呈现先增加后下降的双期改变 ;微栓塞后CFR下降 ,CFR在一定微栓塞程度时又有增加。     

The effect of continuous positive airway pressure on total cerebral blood flow in healthy awake volunteers

Purpose

Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. However, the physiologic impact of CPAP on cerebral blood flow (CBF) is not well established. Ultrasound can be used to estimate CBF, but there is no widespread accepted protocol. We studied the physiologic influence of CPAP on CBF using a method integrating arterial diameter and flow velocity (FV) measurements obtained for each vessel supplying blood to the brain.

Methods

FV and lumen diameter of the left and right internal carotid, vertebral, and middle cerebral arteries were measured using duplex Doppler ultrasound with and without CPAP at 15 cm H₂O, applied in a random order. Transcutaneous carbon dioxide (PtcCO₂), heart rate (HR), blood pressure (BP), and oxygen saturation were monitored. Results were compared with a theoretical prediction of CBF change based on the effect of partial pressure of carbon dioxide on CBF.

Results

Data were obtained from 23 healthy volunteers (mean?±?SD; 12 male, age 25.1?±?2.6 years, body mass index 21.8?±?2.0 kg/m²). The mean experimental and theoretical CBF decrease under CPAP was 12.5 % (p?<?0.001) and 11.9 % (p?<?0.001), respectively. The difference between experimental and theoretical CBF reduction was not statistically significant (3.84?±?79 ml/min, p?=?0.40). There was a significant reduction in PtcCO₂ with CPAP (p?=?<0.001) and a significant increase in mean BP (p?=?0.0017). No significant change was observed in SaO₂ (p?=?0.21) and HR (p?=?0.62).

Conclusion

Duplex Doppler ultrasound measurements of arterial diameter and FV allow for a noninvasive bedside estimation of CBF. CPAP at 15 cm H₂O significantly decreased CBF in healthy awake volunteers. This effect appeared to be mediated predominately through the hypocapnic vasoconstriction coinciding with PCO₂ level reduction. The results suggest that CPAP should be used cautiously in patients with unstable cerebral hemodynamics.     

The effect of rapid decreases of blood pressure by different mechanisms on coronary flow and flow reserve in normal coronary arteries

BACKGROUND: Changes in mean blood pressure (MBP) alter coronary blood flow (CBF). We evaluated the acute effects of three hypotensive medications on CBF parameters in angiographically normal coronary arteries. METHODS: We performed CBF measurements using the Doppler wire at rest and during hyperemia produced by intracoronary adenosine (18 microg) as follows: 1) in the normal left circumflex coronary artery in 20 patients with coronary artery disease (measurements were performed without drugs, and after intravenous infusion of nitroprusside [0.5 to 2 microg/kg/min] and nitroglycerin [10 to 90 microg/min]; drugs were titrated to decrease MBP 20% to 25% below the control values, and heart rate was held constant using right atrial pacing); and 2) in the normal left anterior descending coronary artery in 19 patients without coronary artery disease (measurements were performed before and after intravenous clonidine infusion [150 microg in 5 min]; time-averaged peak velocity [APV], CBF, and coronary flow reserve [CFR] were measured). RESULTS: Similar decreases in MBP were obtained in the two patient groups. Lumen diameter at the site of Doppler measurements increased after all medications (P <.005), whereas CBF did not change significantly. The CFR decreased after nitroprusside (1.79 +/- 0.48 v 2.54 +/- 0.45, P=.000), did not change significantly after nitroglycerin (2.74 +/- 0.43 v 2.54 +/- 0.45, P =.097), and increased after clonidine (3.12 +/- 0.70 v 2.76 +/- 0.75, P =.006). CONCLUSIONS: In normal coronary arteries the infusion of three hypotensive medications to produce the same decreases in MBP is associated with different effects on CFR (increase with clonidine, decrease with nitroprusside, and no change with nitroglycerin).