The effects of alpha adrenergic blockade on central haemodynamics and regional blood flows during positive pressure ventilation

The effects of alpha receptor blockade on cardiac output distribution in pigs were studied. Recordings were made during spontaneous breathing (SB), during ventilator treatment with 8 Pa positive end-expiratory pressure (8 PEEP), after alpha blockade during SB (SB-alpha) and at 8 PEEP (8 PEEP-alpha). The microsphere method was used for blood flow determinations. The animals received either 5 ml.kg-1.h-1 (Group A) or 10 ml.kg-1.h-1 (Group B) of fluids. In Group A on SB-alpha, CO was maintained due to tachycardia but mean arterial pressure (MAP) decreased, renal blood flow and urine production decreased. At 8 PEEP-alpha, CO decreased despite increased heart rate (HR), MAP decreased alarmingly, renal blood flow decreased, urine production ceased and cerebral blood flow decreased, reflecting failing autoregulation. In Group B, CO increased during SB-alpha, SVR decreased, myocardial blood flow increased and organ blood flows were otherwise unchanged. At 8 PEEP-alpha, MAP, SVR, renal, pancreatic and splenic blood flows decreased in Group B. Gastric, intestinal and muscular blood flows were unchanged at 8 PEEP-alpha in Group B which is interpreted as an effect of the alpha blockade. In both groups peripheral arterio-venous shunting increased after alpha blockade.     

Development and Evaluation of a Flow-Dividing Unit for Differential Ventilation and Selective PEEP

Differential ventilation with selective positive end-expiratory pressure (PEEP) was studied in a two-compartment lung model, using one ventilator and a flow-dividing unit consisting of inspiratory flow resistors and an inspiratory threshold valve. The compliance of each lung compartment was varied between 0.15 and 0.23 1 X kPa-1 and the resistance was varied from 0 to 3.5 kPa X 1(-1) X s. The minute volume was 12 1 and the respiratory frequency 12/min, with an inspiratory:expiratory ratio of 1:2. An even distribution of ventilation to the two lung compartments was obtained with the inspiratory flow resistors or the threshold valve under all conditions studied. However, a stepwise increase in the inspiratory resistance of one lung compartment from 1.0 to 2.5 or from 2.5 to 3.5 kPa X 1(-1) X s required readjustment of the inspiratory flow resistor to achieve an even distribution of ventilation, whereas the inspiratory threshold valve needed no readjustment. Large differences in the inspiratory impedance of the two lung compartments caused asynchronous gas delivery when the ventilation distribution was adjusted by means of the flow resistors. Use of the threshold valve resulted in synchronous gas delivery. The flow-dividing unit consists of non-active elements and can thus be connected to any ventilator.     

Does a positive end‐expiratory pressure‐induced reduction in stroke volume indicate preload responsiveness? An experimental study

BACKGROUND: Increases in positive end-expiratory pressure (PEEP) are often associated with cardiovascular depression, responding to fluid loading. Therefore, we hypothesized that if stroke volume (SV) is reduced by an increase in PEEP this reduction is an indicator of hypovolemia or preload responsiveness, i.e. that SV would increase by fluid administration at zero end-expiratory pressure (ZEEP). The relationship between the cardiovascular response to different PEEP levels and fluid load as well as the relation between change in SV as a result of change in preload (Frank-Starling relationship) were evaluated in a porcine model. In addition, other measures of fluid status were assessed. METHODS: Eight, 20-22 kg, anesthetized, mechanically ventilated pigs were subjected to 0, 10, and 20 cm H(2)O PEEP at 10% (of estimated blood volume) hypovolemia, normo- and 10% hypervolemia, and to ZEEP at 20% hypervolemia. SV, cardiac output, intrathoracic blood volume and airway, esophageal, vascular pressures, stroke volume variations, left ventricular end-diastolic and end-systolic areas and respiratory variations in the diameter of the inferior vena cava were obtained. RESULTS: At hypovolemia and normovolemia, 10 cm H(2)O PEEP induced a significant decrease in SV, while no change occurred at 10% hypervolemia. SV measured at ZEEP increased from hypovolemia to normovolemia and 10% hypervolemia, while no change was found between 10% and 20% hypervolemia. The sensitivity and specificity decrease in SV by PEEP indicating an increase in SV by fluids was 60-88% and 67%, respectively, depending on the volemic (preload) levels. CONCLUSION: Although the overall results suggest that a change in SV by PEEP might predict preload responsiveness, the individual response of SV by 10 cm H(2)O PEEP and of the successive fluid administration seemed to be dependent on where on the Frank-Starling curve the heart function was located.     

间歇正压通气和呼气末正压通气对犬眼内压的影响

目的 探讨间歇正压通气(IPPV)和呼气末正压通气(PEEP)对犬眼内压(10P)的影响.方法 实验犬8只,麻醉后分别监测基础条件下和各种机械通气条件下的IOP、CVP、MAP.结果 实施20 ml/kg和30 ml/kg两种不同潮气量的IPPV时IOP差异无统计学意义.实施10、15、20cm H20三种不同压力值的PEEP时IOP均显著升高(P<0.01).结论 IPPV对IOP影响不大,PEEP可使IOP显著升高.     

Treatment of a flail injury of the chest

A patient with flail-chest injury, and associated abdominal and musculo-skeletal trauma, required several modes of mechanical ventilation in the Surgical Intensive Care Unit Ventilator modalities included mechanical ventilation with positive end-expiratory pressure (PEEP) and intermittent mandatory ventilation (IMV) with continuous positive airway pressure (CPAP) during 12 days of intensive respiratory care. This treatment has resulted from an evolution of ideas about pathophysiology and treatment of the flail-chest injury. Future developments portend a shorter duration of ventilatory support; alternatively, a radically new mode of therapy may simplify the care of the flail-chest injured patient.     

Bi-level positive airway pressure ventilation reduces the oxygen cost of breathing in long-standing post-polio patients on invasive home mechanical ventilation

BACKGROUND: Today, patients with chronic respiratory failure are commonly treated with non-invasive bi-level positive airway pressure ventilation, supporting spontaneous breathing. However, in conformity with previous clinical routine, many post-polio patients with chronic respiratory failure are still treated with invasive (i.e. via a tracheostomy) controlled mechanical ventilation (CMV). The aim of the study was to investigate the effect of invasive bi-level positive airway pressure ventilation on the work of breathing compared with that during the patients' ordinary CMV and spontaneous breathing without mechanical support. METHODS: Nine post-polio patients on invasive (tracheostomy) nocturnal CMV were investigated. Work of breathing was analysed by assessing differences in oxygen consumption (VO2) using indirect calorimetry. Hereby, the oxygen cost of breathing during the various ventilatory modes could be estimated and related to one another. Data on energy expenditure were also obtained. RESULTS: The oxygen cost of breathing decreased by approximately 15% during bi-level positive airway pressure ventilation compared with CMV and spontaneous breathing. There was no difference between predicted (Harris-Benedict equation) and measured energy expenditure. CONCLUSION: Invasive bi-level positive airway pressure ventilation reduces the oxygen cost of breathing in long-standing tracheostomized post-polio patients, compared with CMV. Furthermore, the Harris-Benedict equation provides a reasonable prediction of energy expenditure in this group of patients.     

The effect of positive airway pressure during pre-oxygenation and induction of anaesthesia upon duration of non-hypoxic apnoea

Positive end-expiratory pressure (PEEP) applied during induction of anaesthesia may prevent atelectasis formation in the lungs. This may increase the duration of non-hypoxic apnoea by increasing the functional residual capacity. We studied the benefit of PEEP applied during the induction of anaesthesia on the duration of apnoea until the SpO2 reached 90%. Forty ASA I-II patients were randomly allocated to one of two groups. In the PEEP group (n = 20) patients were pre-oxygenated using 100% O2 administered using a CPAP device (6 cmH2O) for 5 min. Following induction of anaesthesia, patients were mechanically ventilated (PEEP 6 cm H2O) for a further 5 min. In the ZEEP group (n = 20), no CPAP or PEEP was used. The duration of apnoea until SpO2 reached 90% was measured. Non-hypoxic apnoea duration was longer in the PEEP group compared to ZEEP group (599 +/- 135 s vs. 470 +/- 150 s, p = 0.007). We conclude that the application of positive airway pressure during induction of anaesthesia in adults prolongs the non-hypoxic apnoea duration by > 2 min.     

Use of the ProSeal laryngeal mask airway for pressure-controlled ventilation with and without positive end-expiratory pressure in paediatric patients: a randomized, controlled study

Background. Tracheal intubation and positive end-expiratorypressure (PEEP) are frequently used in children to avoid airwayclosure and atelectasis during general anaesthesia. Also, thelaryngeal mask airway (LMA     

Evaluation of a recruitment maneuver with positive inspiratory pressure and high PEEP in patients with severe ARDS

BACKGROUND: To evaluate the effect of a recruitment maneuver (RM) with constant positive inspiratory pressure and high positive end-expiratory pressure (PEEP) on oxygenation and static compliance (Cs) in patients with severe acute respiratory distress syndrome (ARDS). METHODS: Eight patients with ARDS ventilated with lung-protective strategy and an arterial partial pressure of oxygen to inspired oxygen fraction ratio (PaO2/FIO2) < or =100 mmHg regardless of PEEP were prospectively studied. The RM was performed in pressure-controlled ventilation at FIO2 of 1.0 until PaO2 reached 250 mmHg or a maximal plateau pressure/PEEP of 60/45 cmH2O was achieved. The RM was performed with stepwise increases of 5 cmH2O of PEEP every 2 min and thereafter with stepwise decreases of 2 cmH2O of PEEP every 2 min until a drop in PaO2 >10% below the recruitment PEEP level. Data was collected before (preRM), during and after 30 min (posRM). RESULTS: The PaO2/FIO2 increased from 83 +/- 22 mmHg preRM to 118 +/- 32 mmHg posRM (P = 0.001). The Cs increased from 28 +/- 10 ml cmH2O(-1) preRM to 35 +/- 12 ml cmH2O(-1) posRM (P = 0.025). The PEEP was 12 +/- 3 cmH2O preRM and was set at 15 +/- 4 cmH2O posRM (P = 0.025). The PEEP of recruitment was 36 +/- 9 cmH2O and the collapsing PEEP was 13 +/- 4 cmH2O. The PaO2 of recruitment was 225 +/- 105 mmHg, with five patients reaching a PaO2 > or = 250 mmHg. The FIO2 decreased from 0.76 +/- 0.16 preRM to 0.63 +/- 0.15 posRM (P = 0.001). No major complications were detected. CONCLUSION: Recruitment maneuver was safe and useful to improve oxygenation and Cs in patients with severe ARDS ventilated with lung-protective strategy.     

Endotracheal tube versus face mask with and without continuous positive airway pressure (CPAP)

Various ways of delivering continuous positive airway pressure (CPAP) have been extensively studied, with little attention, however, being paid to the effects of an intubation tube compared with breathing through a face mask, with or without CPAP. Pulmonary and cardiovascular variables were measured while 12 patients recovering from coronary artery bypass grafting were spontaneously breathing at ambient airway pressure, then at 7.4 mmHg (1 kPa) CPAP, and again at ambient pressure just before extubation. The same stages were repeated immediately after extubation, with patients breathing through a tight-fitting face mask. Arterial oxygen tension (Pao₂, mean ± s.d.) was better when the patients were breathing at ambient pressure through a face mask (11.7±2.8 kPa) than when they were intubated (10.6±2.4 kPa, P < 0.05). Compared with ambient pressure, CPAP (7.4 mmHg) (1 kPa) increased Pao₂ in both modes (13.4 ± 3.5 kPa with mask, and 12.6 ± 3.5 kPa when intubated, n.s.). The best arterial oxygen saturation was measured during CPAP with a face mask (96± 1%). Cardiac output remained unchanged in all the breathing modes. After coronary artery bypass grafting, spontaneous breathing with a face mask resulted in better Pao₂ than breathing through an endotracheal tube, both with and without 7.4 mmHg (1 kPa) CPAP. This study indicates that unnecessary delay in extubation should be avoided.     

Validation of pulse contour derived stroke volume variation during modifications of cardiac afterload

Background: Left ventricular stroke volume variation (SVV) or its surrogatesare useful tools to assess fluid responsiveness in mechanicallyventilated patients. So far it is unknown, how changes in cardiacafterload affect SVV. Therefore, this study compared left ventricularSVV derived by pulse contour analysis with SVV measured usingan ultrasonic flow probe and investigated the influence of cardiacafterload on left ventricular SVV. Methods: In 13 anaesthetized, mechanically ventilated pigs [31(SD 6)kg], we compared cardiac output (CO), stroke volume (SV), andSVV determined by pulse contour analysis and by an ultrasonicaortic flow signal (Bland–Altman analysis). After obtainingbaseline measurements, cardiac afterload was increased usingphenylephrine and decreased using adenosine (both continuouslyadministered). Measurements were performed with a constant tidalvolume (12 ml kg^–1) without PEEP. Results: Neither increasing mean arterial pressure (MAP) [from 59 (7)to 116 (19)] nor decreasing MAP [from 63 (7) to 39 (4)] affectedCO, SV, and SVV (both methods). Method comparison revealed abias for SVV of 0.1% [standard error of the mean (SE) 0.8] atbaseline, –1.2% (SE 0.8) during decreased and 4.0% (SE0.7) during increased afterload, the latter being significantlydifferent from the others (P < 0.05). Thereby, pulse contouranalysis tended to underestimate SVV during decreased afterloadand to overestimate SVV during increased afterload. Limits ofagreement were approximately 6% for all points of measurement. Conclusions: Left ventricular SVV is not affected by changes in cardiac afterload.There is a good agreement of pulse contour with flow derivedSVV. The agreement decreases, if afterload is extensively augmented.     

Nasopharyngeal airway continuous positive airway pressure: a method to wean from or avoid mechanical ventilation in adults

A simple technique using a nasopharyngeal airway and a continuous positive airway pressure circuit has proved an effective alternative to a tight fitting nasal or face mask for delivery of continuous positive airway pressure. Nasal trauma, discomfort and mouth breathing are the main disadvantages.     

Effect of Selective PEEP on Pulmonary Blood Flow following Unilateral Hydrochloric Acid Aspiration in the Dog

We investigated the effect of unilateral or bilateral positive end-expiratory pressure (PEEP) on pulmonary perfusion in 12 dogs with a hydrochloric acid aspiration injury of the left lung. The lungs were ventilated separately and PEEP was applied to the left lung at 10 cmH2O (1.0 kPa) in six and at 15 cmH2O (1.5 kPa) in six others. Measurements of the right and left pulmonary arterial blood flows (QR and QL) and venous admixture were made before, during and after PEEP. After this study, 5 and 10 cmH2O (0.5 and 1.0 kPa) PEEP were applied to both lungs in six dogs and measurements were repeated. Following the application of PEEP to the left lung, a significant decrease in QL and increase in QR were observed. However, the application of PEEP to both lungs was followed by significant decreases in both QL and QR. The cardiac output decreased slightly during unilateral PEEP and markedly during bilateral PEEP. The venous admixture decreased significantly during PEEP in all the groups. These findings indicate that selective PEEP causes a transfer of pulmonary blood flow from the injured lung to the normal lung, improving ventilation-perfusion inequality, and improves gas exchange without impeding oxygen delivery.     

Analysis of lung density by computed tomography before and during general anaesthesia

Pulmonary structure was analysed by means of computed tomography (CT) in 20 lung-healthy patients, relating tissue density to the attenuation value (AV) of a picture element. Regional density of pulmonary tissue (r_lung) was determined using mean lung density in five regions of interest (ROI_1–5) (sector method). Vertical and horizontal distributions of x-ray attenuation were analysed by density profiles, relating AV values to evenly distributed and normalised length scales. In group I (n= 12), CT-densitometry was obtained in awake, supine patients and after induction of general anaesthesia. In group II (n = 8), the effect of mechanical ventilation with positive end-expiratory pressure (PEEP, 1.0 kPa [10 cmH₂O]) was studied. In the awake state, a vertical tissue density difference between the top and the bottom of the lung was found in all patients, accounting for a mean of 0.235 g'cm^-3 (right lung) and 0.199 g'cm^-3 (left lung). Only minor changes were seen in the horizontal lung density profiles. After induction of anaesthesia, x-ray attenuation of ROI_1–4 showed no significant differences when compared with the awake state. The basal lung areas (ROI₅) revealed a significantly increased tissue density (P ≤ 0.01), reaching mean values of 0.94 g cm^-3 (right lung) and 0.814 g-cm^-3 (left lung). Similarly, vertical density profiles showed a markedly enhanced r_lung of the bottom of the lung in all patients, interpreted as atelectasis. The amount of atelectasis accounted for 4.8 ± 2.6% (right lung) and 4.7 ± 2.1% (left lung) of the intrapulmonary area. There was no evidence of “non-gravitational” inhomogeneity of density distribution seen in the horizontal density profiles. After application of PEEP, basal lung densities decreased significantly, although small basal densities remained in most patients (2.27 ± 2.57% of right intrapulmonary area [P ≤ 0.01], 2.2 ± 2.37% left intrapulmonary area [P ≤ 0.01]). Calculated alveolar recruitment was 7.7 cm² and 8.4 cm², whereas expansion of both lungs was smaller (4.3 cm² and 4.4 cm² [right and left lung]). Mean density of aerated tissue had decreased by 25%, and both horizontal and vertical attenuation profiles revealed an even distribution of r_lung. Analysis of r_lung provides useful information about regional pulmonary morphology during anaesthesia and may be related to lung function.     

Effect of continuous positive airway pressure (CPAP) in patients with chronic obstructive pulmonary chsease (COPD) depending on intrinsic PEEP levels

The application of continuous positive airway pressure (CPAP) is known to reduce inspiratory work of breathing in intubated patients with chronic obstructive pulmonary disease (COPD). This effect is caused by a decrease in elastic work related to a reduction in intrinsic PEEP. The aim of this study was to relate the decrease in inspiratory work due to CPAP to the intrinsic PEEP levels obtained during spontaneous breathing without positive pressure. Ten intubated patients with COPD who had been ventilated for acute respiratory failure were studied. Intrinsic PEEP was determined during tracheal occlusions performed at end-expiration when the patient was breathing without positive airway pressure. Inspiratory work was computed during breathing through a circuit with a CPAP of 0.5 kPa and the same circuit without positive pressure. Intrinsic PEEP-levels ranged from 0.26 to 1.31 kPa. Compared to spontaneous breathing without positive pressure, CPAP reduced the total inspiratory work per liter of ventilation (Wltot) from 1.42±0.48 to 1.24±0.50 J·1^-1 (means±SD P <0.01). This decrease was found to be related to the intrinsic PEEP-levels: the largest reductions were found in the patients with an intrinsic PEEP-level close to the CPAP-level applied. In intubated patients with COPD, the decrease in Wltot due to a CPAP of 0.5 kPa was found to be related to the intrinsic PEEP-levels present when no positive airway pressure was applied. The intrinsic PEEP measured during tracheal occlusions could be used to estimate the effect of CPAP in these patients.