Pulmonary O2 transfer during pulsatile and non-pulsatile perfusion

Autonomic control of the pulmonary circulation: Implications for pulmonary hypertension

The autonomic regulation of the pulmonary vasculature has been under-appreciated despite the presence of sympathetic and parasympathetic neural innervation and adrenergic and cholinergic receptors on pulmonary vessels. Recent clinical trials targeting this innervation have demonstrated promising effects in pulmonary hypertension, and in this context of reignited interest, we review autonomic pulmonary vascular regulation, its integration with other pulmonary vascular regulatory mechanisms, systemic homeostatic reflexes and their clinical relevance in pulmonary hypertension. The sympathetic and parasympathetic nervous systems can affect pulmonary vascular tone and pulmonary vascular stiffness. Local afferents in the pulmonary vasculature are activated by elevations in pressure and distension and lead to distinct pulmonary baroreflex responses, including pulmonary vasoconstriction, increased sympathetic outflow, systemic vasoconstriction and increased respiratory drive. Autonomic pulmonary vascular control interacts with, and potentially makes a functional contribution to, systemic homeostatic reflexes, such as the arterial baroreflex. New experimental therapeutic applications, including pulmonary artery denervation, pharmacological cholinergic potentiation, vagal nerve stimulation and carotid baroreflex stimulation, have shown some promise in the treatment of pulmonary hypertension.

Pulmonary function assessment post-left ventricular assist device implantation

Aim

The lungs—and particularly the alveolar‐capillary membrane—may be sensitive to continuous flow (CF) and pulmonary pressure alterations in heart failure (HF). We aimed to investigate long‐term effects of CF pumps on respiratory function.

Methods and results

We conducted a retrospective study of patients with end‐stage HF at our institution. We analysed pulmonary function tests [e.g. forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV₁)] and diffusing capacity of the lung for carbon monoxide (D_LCO) from before and after left ventricular assist device (LVAD) implantation and compared them with invasive haemodynamic studies. Of the 274 patients screened, final study analysis involved 44 patients with end‐stage HF who had CF LVAD implantation between 1 February 2007 and 31 December 2015 at our institution. These patients [mean (standard deviation, SD) age, 50 (9) years; male sex, n = 33, 75%] received either the HeartMate II (Thoratec Corp.) pump (77%) or the HeartWare (HeartWare International Inc.) pump. The mean (SD) left ventricular ejection fraction was 21% (13%). At a median of 237 days post‐LVAD implantation, we observed significant D_LCO decrease (−23%) since pre‐implantation (P < 0.001). ΔD_LCO had an inverse relationship with changes in pulmonary capillary wedge pressure (PCWP) and right atrial pressure (RAP) from pre‐LVAD to post‐LVAD implantation: ΔD_LCO to ΔPCWP (r = 0.50, P < 0.01) and ΔD_LCO to ΔRAP (r = 0.39, P < 0.05). We observed other reductions in FEV₁, FVC, and FEV₁/FVC between pre‐LVAD and post‐LVAD implantation. In mean (SD) values, FEV₁ changed from 2.3 (0.7) to 2.1 (0.7) (P = 0.005); FVC decreased from 3.2 (0.8) to 2.9 (0.9) (P = 0.01); and FEV₁/FVC went from 0.72 (0.1) to 0.72 (0.1) (P = 0.50). Landmark survival analysis revealed that ΔD_LCO from 6 months after LVAD implantation was predictive of death for HF patients [hazard ratio (95% confidence interval), 0.60 (0.28–0.98); P = 0.03].

Conclusions

Pulmonary function did not improve after LVAD implantation. The degree of D_LCO deterioration is related to haemodynamic status post‐LVAD implantation. The ΔD_LCO within 6 months post‐operative was associated with survival.

Change in pulmonary blood volume changes pulmonary artery systolic storage

BACKGROUND

A fraction of right ventricular stroke volume (pulmonary artery systolic storage, [PASS]), which is stored in pulmonary arteries during systole and then discharged to the capillaries, determines the diastolic pulmonary capillary blood flow and hence the capillary blood volume participating in gas diffusion. Possibility that increases in pulmonary blood volume (PBV) increase PASS, leading to an improved distribution of ventilation-to-perfusion ratios (V/Q), was examined.

METHODS AND RESULTS

Included were 34 obese patients undergoing bariatric surgery. We used a nitrous oxide-airway-pneumotachographic method to measure PASS. The measurements were repeated before and after increasing PBV. In 20 patients, PBV was increased with infusion of crystalloids, which was guided by pulmonary capillary wedge pressure (PCWP). There was a good correlation between change in PASS and change in PBV (r(2) = 0.741, P < 0.0001). However, when the baseline PASS was high, changes in PASS were much less. In patients with a pulmonary artery diastolic-pulmonary capillary wedge pressure gradient ≥ 6 mmHg, the baseline PASS was correlated with pulmonary venous resistance (r(2) = 0.644, P = 0.017). In 14 patients, in whom PBV was increased with both changes in position and infusion of crystalloids, the physiologic dead space-to-tidal volume ratio (VD/VT) was measured as an index of the distribution of V/Q. There was a good negative correlation between PASS and VD/VT (r(2) = 0.697, P < 0.0001). However, at a high baseline PASS, increases in PBV decreased PASS (P = 0.0006) and increased VD/VT (P = 0.0018).

CONCLUSIONS

Changes in PBV change PASS and thereby the distribution of V/Q, depending on pulmonary venous resistance, which determines the baseline PASS.

Increased pulmonary artery systolic storage associated with improved ventilation-to-perfusion ratios in acute respiratory distress syndrome

PURPOSE

The possibility that the increased pulmonary artery systolic storage (PASS) correlates with an improved distribution of ventilation/perfusion (V(A)/Q) and hence benefits gas exchange in acute respiratory distress syndrome (ARDS) was examined. Pulmonary artery systolic storage is the fraction of stroke volume stored in PA during systole and then discharged to the capillaries. The increased PASS can augment the diastolic pulmonary capillary blood flow (PCBF), which can then increase capillary blood volume participating in gas diffusion. We examined this by assessing the correlation between PASS and physiologic dead space to tidal volume (VD/VT) ratio.

MATERIALS AND METHODS

Included were 17 patients with ARDS. By using nitrous oxide-airway-pneumotachographic method, we measured the instantaneous PCBF, from which PASS was determined. Because PASS is the same as the flow volume of PCBF during diastole, PASS was determined from the flow volume of PCBF during diastole divided by the flow volume of PCBF during a whole cardiac cycle. The VD/VT ratio, used as an index of V(A)/Q, was measured by using the Bohr equation.

RESULTS

There was a good inverse correlation between PASS and VD/VT (r(2) = 0.785, P < .0001).

CONCLUSIONS

Our data indicate that the increased PASS correlates with an improved distribution of V(A)/Q and hence benefits gas exchange in ARDS.

Right ventricular failure after left ventricular assist device implantation: the need for an implantable right ventricular assist device

Right ventricular failure after implantation of a left ventricular assist device is an unremitting problem. Consideration of portal circulation is important for reversing liver dysfunction and preventing multiple organ failure after left ventricular assist device implantation. To achieve these objectives, it is imperative to maintain the central venous pressure as low as possible. A more positive application of right ventricular assistance is recommended. Implantable pulsatile left ventricular assist devices cannot be used as a right ventricular assist device because of their structure and device size. To improve future prospects, it is necessary to develop an implantable right ventricular assist device based on a rotary blood pump.

Hemodynamics of chronic nonpulsatile flow: implications for LVAD development

Both experimental and clinical evidence suggest that pulse pressure is not required from a blood pump. End-organ function is well maintained with nonpulsatile systems, though pulse pressure may accelerate recovery from cardiogenic shock. Form follows function, so the effects of reduced pulse pressure on the arterial wall are not surprising. The ability to alter aortic wall morphology by reducing pulse pressure may have important implications for the future treatment of arterial pathology. Both centrifugal and axial-flow pumps can be miniaturized and are silent. Their reliability and user-friendly status may soon allow implantation at an earlier stage of cardiac deterioration. Doubts about the feasibility of long-term pulseless circulation are disappearing.

Pulmonary capillaries are recruited during pulsatile flow

Capillaries recruit when pulmonary arterial pressure rises. The duration of increased pressure imposed in such experiments is usually on the order of minutes, although recent work shows that the recruitment response can occur in <4 s. In the present study, we investigate whether the brief pressure rise during cardiac systole can also cause recruitment and whether the recruitment is maintained during diastole. To study these basic aspects of pulmonary capillary hemodynamics, isolated dog lungs were pump perfused alternately by steady flow and pulsatile flow with the mean arterial and left atrial pressures held constant. Several direct measurements of capillary recruitment were made with videomicroscopy. The total number and total length of perfused capillaries increased significantly during pulsatile flow by 94 and 105%, respectively. Of the newly recruited capillaries, 92% were perfused by red blood cells throughout the pulsatile cycle. These data provide the first direct account of how the pulmonary capillaries respond to pulsatile flow by showing that capillaries are recruited during the systolic pulse and that, once open, the capillaries remain open throughout the pulsatile cycle.

Chronic nonpulsatile blood flow. III. Effects of pump flow rate on oxygen transport and utilization in chronic nonpulsatile biventricular bypass

The relationship between blood flow and oxygen transport was studied in five calves with chronic nonpulsatile biventricular bypass. Seven days was allowed for recovery from the effects of anesthesia and operation; the natural heart was then fibrillated. Pump flows were maintained at nominal rates of 90, 100, or 120 ml.kg-1.min for 1 week each, with the sequence varied from experiment to experiment. Venous and arterial blood samples were taken at rest for blood gas analysis. Serum lactate analysis was done twice a week, on the third and seventh days after each pump flow change. Serum catecholamine levels were assayed on the seventh day of each flow rate. Progressive exercise tests were also conducted during each test segment. Basal oxygen consumption of a 4-month-old calf was 6.3 +/- 0.3 ml.kg-1.min-1. The mixed venous oxygen tension decreased when pump flow rate was reduced (29.6 +/- 1.0, 28.3 +/- 1.2, and 23.8 +/- 0.9 mm Hg at 120, 100, and 90 ml.kg-1.min-1 of pump flow, respectively), and oxygen extraction increased linearly when pump flow rate was reduced. Hemoglobin concentration significantly affected oxygen extraction rate. Serum lactate concentration increased significantly at a 90 ml.kg-1.min-1 perfusion compared with concentrations at other pump flow rates (7.81 +/- 2.42 mEq/L at 90 ml.kg-1.min-1 vs 0.71 +/- 0.19 and 0.73 +/- 0.81 mEq/L at 100 and 120 ml.kg-1.min-1, respectively; p < 0.01, analysis of variance, Scheffe F test). Maximum oxygen extraction during exercise was 78%. These results suggest that a critical flow level between 90 and 100 ml.kg-1.min-1 maintains oxidative metabolism in the calf with chronic nonpulsatile flow. The resulting oxygen delivery was slightly higher than that indicated in the literature. Maximal oxygen extraction was normal.

Pulsatile and nonpulsatile cardiopulmonary bypass: review of a counterproductive controversy

In the controversy over pulsatile and nonpulsatile perfusion, most authors have failed to recognize the fundamental physical differences between the two methods. Pulsatile perfusion is polymorphic and its form varies with both the pulsatile source and the vascular system being perfused; nonpulsatile perfusion is by definition unvarying and uniform. While many studies of hemodynamics, metabolism, organ function, microcirculation, and histology show benefits derived from pulsatile perfusion, others do not. The simplest explanation for these conflicts is that different investigators employ different forms of pulsatile perfusion, only some of which are effective. Failure to quantitate adequately the pulsatile components of flow in these studies prevents differentiation between effective and ineffective forms of pulsatile flow and makes comparison of studies difficult. Future research in this area should be directed toward definition of effective pulsatile perfusion by adequate measurement of the pulsatile components of perfusion.