Effects of sternotomy on heart-lung interaction in patients undergoing cardiac surgery receiving pressure-controlled mechanical ventilation

Donor heart ischemic time can be extended beyond 9 hours using hypothermic machine perfusion in sheep

BACKGROUND

The global shortage of donor hearts available for transplantation is a major problem for the treatment of end-stage heart failure. The ischemic time for donor hearts using traditional preservation by standard static cold storage (SCS) is limited to approximately 4 hours, beyond which the risk for primary graft dysfunction (PGD) significantly increases. Hypothermic machine perfusion (HMP) of donor hearts has been proposed to safely extend ischemic time without increasing the risk of PGD.

METHODS

Using our sheep model of 24 hours brain death (BD) followed by orthotopic heart transplantation (HTx), we examined post-transplant outcomes in recipients following donor heart preservation by HMP for 8 hours, compared to donor heart preservation for 2 hours by either SCS or HMP.

RESULTS

Following HTx, all HMP recipients (both 2 hours and 8 hours groups) survived to the end of the study (6 hours after transplantation and successful weaning from cardiopulmonary bypass), required less vasoactive support for hemodynamic stability, and exhibited superior metabolic, fluid status and inflammatory profiles compared to SCS recipients. Contractile function and cardiac damage (troponin I release and histological assessment) was comparable between groups.

CONCLUSIONS

Overall, compared to current clinical SCS, recipient outcomes following transplantation are not adversely impacted by extending HMP to 8 hours. These results have important implications for clinical transplantation where longer ischemic times may be required (e.g., complex surgical cases, transport across long distances). Additionally, HMP may allow safe preservation of "marginal" donor hearts that are more susceptible to myocardial injury and facilitate increased utilization of these hearts for transplantation.

Impact of sternotomy and pericardiotomy on cardiopulmonary haemodynamics in a large animal model

NEW FINDINGS

What is the central question of this study? Invasive cardiovascular instrumentation can occur through closed- or open-chest approaches. To what extent will sternotomy and pericardiotomy affect cardiopulmonary variables? What is the main finding and its importance? Opening of the thorax decreased mean systemic and pulmonary pressures. Left ventricular function improved, but no changes were observed in right ventricular systolic measures. No consensus or recommendation exists regarding instrumentation. Methodological differences risk compromising rigour and reproducibility in preclinical research.

ABSTRACT

Animal models of cardiovascular disease are often evaluated by invasive instrumentation for phenotyping. As no consensus exists, both open- and closed-chest approaches are used, which might compromise rigour and reproducibility in preclinical research. We aimed to quantify the cardiopulmonary changes induced by sternotomy and pericardiotomy in a large animal model. Seven pigs were anaesthetized, mechanically ventilated and evaluated by right heart catheterization and bi-ventricular pressure-volume loop recordings at baseline and after sternotomy and pericardiotomy. Data were compared by ANOVA or the Friedmann test where appropriate, with post-hoc analyses to control for multiple comparisons. Sternotomy and pericardiotomy caused reductions in mean systemic (-12 ± 11 mmHg, P = 0.027) and pulmonary pressures (-4 ± 3 mmHg, P = 0.006) and airway pressures. Cardiac output decreased non-significantly (-1329 ± 1762 ml/min, P = 0.052). Left ventricular afterload decreased, with an increase in ejection fraction (+9 ± 7%, P = 0.027) and coupling. No changes were observed in right ventricular systolic function or arterial blood gases. In conclusion, open- versus closed-chest approaches to invasive cardiovascular phenotyping cause a systematic difference in key haemodynamic variables. Researchers should adopt the most appropriate approach to ensure rigour and reproducibility in preclinical cardiovascular research.

Pulse-wave transit time with ventilator-induced variation for the prediction of fluid responsiveness

Aim

Although pulse pressure variation is a good predictor of fluid responsiveness, its measurement is invasive. Therefore, a technically simple, non‐invasive method is needed for evaluating circulatory status to prevent fluid loading and optimize hemodynamic status. We focused in the pulse‐wave transit time (PWTT) defined as the time interval between electrocardiogram R wave to plethysmograph upstroke, which has been recently introduced to non‐invasively assess cardiovascular response. In the present study, we evaluated the efficacy of pulse‐wave transit time (PWTT) with ventilator‐induced variation (PWTTV) in predicting fluid responsiveness.

Methods

We evaluated six domestic pigs weighing 46.0 ± 3.5 kg. After anesthesia induction, electrocardiogram, femoral arterial blood pressure, plethysmograph on the tail, and carotid artery blood flow were monitored and hemorrhage was induced by withdrawing 20 mL/kg blood over 20 min; 5 mL/kg blood volume was then autotransfused over 10 min. Then PWTTV and pulse pressure variation were measured at tidal volumes of 6 and 12 mL/kg.

Results

Area under the receiver operating curve values for the prediction of a >10% change in carotid artery blood flow were 0.979 for pulse pressure variation and 0.993 for PWTTV at a tidal volume of 6 mL/kg and 0.979 and 0.979, respectively, at a tidal volume of 12 mL/kg (all P < 0.0001).

Conclusions

Measured non‐invasively, PWTTV showed similar utility to pulse pressure variation in predicting >10% changes in carotid artery blood flow induced by autotransfusion.

Temporal Changes in Ventilator Settings in Patients With Uninjured Lungs: A Systematic Review

In patients with uninjured lungs, increasing evidence indicates that tidal volume (VT) reduction improves outcomes in the intensive care unit (ICU) and in the operating room (OR). However, the degree to which this evidence has translated to clinical changes in ventilator settings for patients with uninjured lungs is unknown. To clarify whether ventilator settings have changed, we searched MEDLINE, Cochrane Central Register of Controlled Trials, and Web of Science for publications on invasive ventilation in ICUs or ORs, excluding those on patients <18 years of age or those with >25% of patients with acute respiratory distress syndrome (ARDS). Our primary end point was temporal change in VT over time. Secondary end points were changes in maximum airway pressure, mean airway pressure, positive end-expiratory pressure, inspiratory oxygen fraction, development of ARDS (ICU studies only), and postoperative pulmonary complications (OR studies only) determined using correlation analysis and linear regression. We identified 96 ICU and 96 OR studies comprising 130,316 patients from 1975 to 2014 and observed that in the ICU, VT size decreased annually by 0.16 mL/kg (-0.19 to -0.12 mL/kg) (P < .001), while positive end-expiratory pressure increased by an average of 0.1 mbar/y (0.02-0.17 mbar/y) (P = .017). In the OR, VT size decreased by 0.09 mL/kg per year (-0.14 to -0.04 mL/kg per year) (P < .001). The change in VTs leveled off in 1995. Other intraoperative ventilator settings did not change in the study period. Incidences of ARDS (ICU studies) and postoperative pulmonary complications (OR studies) also did not change over time. We found that, during a 39-year period, from 1975 to 2014, VTs in clinical studies on mechanical ventilation have decreased significantly in the ICU and in the OR.

Evaluation of stroke volume variation and pulse pressure variation as predictors of fluid responsiveness in patients undergoing protective one-lung ventilation

In order to investigate whether the hemodynamic indices, including stroke volume variation (SVV) and pulse pressure variation (PPV) could predict fluid responsiveness in patients undergoing protective one-lung ventilation. 60 patients scheduled for a combined thoracoscopic and laparoscopic esophagectomy were enrolled and randomized into two groups. The patients in the protective group (Group P) were ventilated with a tidal volume of 6 mL/kg, an inspired oxygen fraction (FiO2) of 80%, and a positive end expiratory pressure (PEEP) of 5 cm H2O. Patients in the conventional group (Group C) were ventilated with a tidal volume of 8 mL/kg and a FiO2 of 100%. Dynamic variables were collected before and after fluid loading (7 mL/kg hydroxyethyl starch 6%, 0.4 mL/kg/min). Patients whose stroke volume index (SVI) increased by more than 15% were defined as responders. Data collected from 45 patients were finally analyzed. Twelve of 24 patients in Group P and 10 of 21 patients in Group C were responders. SVV and PPV significantly changed after the fluid loading. The receive operating characteristic (ROC) analysis showed that the thresholds for SVV and PPV to discriminate responders were 8.5% for each, with a sensitivity of 66.7% (SVV) and 75% (PPV) and a specificity of 50% (SVV) and 83.3% (PPV) in Group P. However, the thresholds for SVV and PPV were 8.5% and 7.5% with a sensitivity of 80% (SVV) and 90% (PPV) and a specificity of 70% (SVV) and 80% (PPV) in Group C. We found SVV and PPV could predict fluid responsiveness in protective one-lung ventilation, but the accuracy and ability of SVV and PPV were weak compared with the role they played in a conventional ventilation strategy.

Inflatable external leg compression prevents orthostatic hypotension in a patient with a traumatic cervical spinal cord injury

High thoracic spine or cervical injury may cause long-term orthostatic hypotension (OH). To stabilize hemodynamics and prevent presyncope symptoms in these patients, noninvasive management is preferable. We describe a case of a 61-year-old man who experienced presyncope symptoms as a result of severe OH due to spinal cord injury, after 60° head-up tilt position. The patient was referred to the intensive care unit where he was successfully managed with an inflatable external leg compression (ELC). Accordingly, inflatable ELC succeeded not only in improving presyncope symptoms, but also in preventing orthostatic hypotension for several hours. ELC may be an alternative way to stabilize hemodynamics and prevent presyncope symptoms in patients with OH following spinal cord injury.

[Influence of open chest conditions on pulse pressure variations]

OBJECTIVES

Pulse pressure variations are used to assess fluid responsiveness in mechanically ventilated patients. The accuracy of this index in open chest conditions remained unclear. The aim of the study was to evaluate the effect of open chest conditions on pulse pressure variations.

STUDY DESIGN

Non-interventional prospective study.

METHODS AND PATIENTS

Twenty-eight mechanically ventilated patients scheduled for open-heart surgery were included. Pulse pressure variations, peak aortic velocity, and stroke volume were measured before and after thoracotomy with pericardotomy. Measurements were made at each step and compared.

RESULTS

Neither pulse pressure variation nor peak aortic velocity and nor stroke volume variation were modified by open chest conditions (median=5% [interquartile range=6] vs 4% [6], p=NS), (20% [11] vs 17% [12], p=NS and 11% [7] vs 10% [3], p=NS) respectively. Pulse pressure variations were correlated to stroke volume before thoracotomy (r'=-0.432; p=0.02) and after thorocatomy (r'=-0.433, p=0.02).

CONCLUSION

In these studied patients, preload dependancy indices were not modified by open chest conditions. Pulse pressure variations remained correlated to stroke volume even after thoracotomy.

Less invasive indicators of changes in thermodilution cardiac output by ventilatory changes after cardiac surgery

BACKGROUND AND OBJECTIVE

We studied whether changes in less invasive, noncalibrated pulse-contour cardiac output (by modified ModelFlow, COmf) and derived stroke volume variations (SVV), as well as systolic and pulse pressure variations, predict changes in bolus thermodilution cardiac output (COtd), evoked by continuous and cyclic increases in intrathoracic pressure by increases in positive end-expiratory pressure (PEEP) and tidal volume (Vt), respectively.

METHODS

Prospective study on 17 critically ill postcardiac surgery patients on full mechanical ventilatory support, in the intensive care unit.

RESULTS

In contrast to systolic pressure variation and pulse pressure variation, SVV increased from (mean +/- SD) 6.2 +/- 4.4 to 8.1 +/- 5.6 at PEEP 10 cmH2O (P = 0.064) and to 7.8 +/- 3.5% at PEEP 15 cmH2O (P = 0.031), concomitantly with a 12 +/- 7 and 11 +/- 8% decrease in COmf and COtd (P < 0.001), respectively. For pooled data, changes in COmf correlated with those in COtd (r = 0.55, P = 0.002), but changes in SVV did not. Variables did not change when Vt was increased up to 50%.

CONCLUSION

A fall in COmf is more sensitive than a rise in SVV, which is more sensitive than systolic pressure variation and pulse pressure variation, in tracking a fall in COtd during continuous (and not cyclic) increases in intrathoracic pressure, in mechanically ventilated patients after cardiac surgery. This suggests a reduction in biventricular preload as the main factor in decreasing cardiac output and increasing SVV with PEEP.

Heart-lung interactions

PURPOSE OF REVIEW

Assessment of cardiovascular stability using ventilation-induced changes in measured physiological variables, referred to as functional hemodynamic monitoring, usually requires measurement of ventilation-induced changes in venous return. Thus, it is important to understand the determinants of these complex heart-lung interactions.

RECENT FINDINGS

Several animal and human studies have recently documented that ventricular interdependence plays an important role during positive-pressure breathing, causing acute cor pulmonale. With the use of lower tidal volume ventilation in patients with acute respiratory failure, the incidence of acute cor pulmonale is decreasing proportionally. When present, however, it induces a stroke volume variation that is 180 degrees out of phase with that seen in hypovolemic states, such that left ventricular stroke volume increases during inspiration rather than decreasing as seen in hypovolemia. Further, when either tidal volume or positive end-expiratory pressure levels are varied, both stroke volume variation and pulse pressure variation are affected in a predictable manner. The greater the swing in intrathoracic pressure, the greater the change in venous return.

SUMMARY

Functional hemodynamic monitoring is becoming more prevalent. For it to be used effectively, the operator needs to have a solid understanding of how ventilation induces both pulse pressure variation and stroke volume variation in that specific patient.

Limitations of arterial pulse pressure variation and left ventricular stroke volume variation in estimating cardiac pre-load during open heart surgery

BACKGROUND

In addition to their well-known ability to predict fluid responsiveness, functional pre-load parameters, such as the left ventricular stroke volume variation (SVV) and pulse pressure variation (PPV), have been proposed to allow real-time monitoring of cardiac pre-load. SVV and PPV result from complex heart-lung interactions during mechanical ventilation. It was hypothesized that, under open-chest conditions, when cyclic changes in pleural pressures during positive-pressure ventilation are less pronounced, functional pre-load indicators may be deceptive in the estimation of ventricular pre-load.

METHODS

Forty-five patients undergoing coronary artery bypass grafting participated in this prospective, observational study. PPV and SVV were assessed by pulse contour analysis. The thermodilution technique was used to measure the stroke volume index and global and right ventricular end-diastolic volume index. Trans-oesophageal echocardiography was used to determine the left ventricular end-diastolic area index. All parameters were assessed before and after sternotomy, and, in addition, after weaning from cardiopulmonary bypass before and after chest closure (pericardium left open). Patients were ventilated with constant tidal volumes (8 +/- 2 ml/kg) throughout the study period using pressure control.

RESULTS

SVV and PPV decreased after sternotomy and increased after chest closure. However, these changes could not be related to concomitant changes in the ventricular pre-load. The stroke volume index was correlated with SVV and PPV in closed-chest conditions only, whereas volumetric indices reflected cardiac pre-load in both closed- and open-chest conditions. SVV and PPV were correlated with left and right ventricular pre-load in closed-chest-closed-pericardium conditions only (with the best correlation found for the right ventricular end-diastolic volume index).

CONCLUSIONS

SVV and PPV may be misleading when estimating cardiac pre-load during open heart surgery.