NOTA SUI LIMITI FISIOLOGICI DELLE IMMERSIONI PROFONDE IN APNEA

This article discusses the limits of deep breath-hold diving in humans. After a short historical introduction and a discussion of the evolution of depth records, the classical theories of breath-hold diving limits are presented and discussed, namely that of the ratio between total lung capacity and residual volume and that of blood shift, implying an increase in central blood volume. Then the current vision is introduced, based on the principles of the energetics of muscular exercise. The new vision has turned the classical vision upside down, moving the discussion to a different level. A direct consequence of the new theory is the importance of having large lung volumes at the start of a dive, in order to increase body oxygen stores. I finally discuss the role of anaerobic lactic metabolism as a possible mechanism of oxygen preservation, thus prolonging breath-hold duration.

Effect of early versus delayed activation of thoracic epidural anesthesia on plasma pro-atrial natriuretic peptide to indicate deviations in central blood volume during esophagectomy

Background and ObjectivesA side effect to thoracic epidural anesthesia (TEA) is hypotension induced by central hypovolemia. This study addressed whether early activation (EA) versus late activation (LA) of TEA affects plasma pro-atrial natriuretic peptide (proANP) reflecting deviations in the central blood volume (CBV). We hypothesized that EA TEA would reduce plasma proANP, thus reflecting a decrease in CBV.MethodsA randomized, controlled, single-blinded trial was conducted. Patients undergoing open esophagectomy were randomized to EA (n=25, after induction of general anesthesia) or LA TEA (n=25, after re-established gastric continuity) with the epidural catheter placed at the interspaces Th7-8 or Th8-9. Plasma proANP was determined repetitively along with hemodynamic variables and administration of fluid/vasopressors as postoperative complications were noted.ResultsWith EA TEA, plasma proANP decreased following induction of anesthesia to the end of surgery (13%; 113±68 to 99±49 pmol/L; p=0.026), but that was not the case in the LA group (3%; 97±44 to 94±49 pmol/L; p=0.565) despite equal fluid balance (+1584±582 vs +1560±563 mL; p=0.888). Accordingly, the EA group required excessive treatment with vasopressors to maintain MAP >60 mm Hg during surgery (2.7±2 vs 1.6±1.4 ephedrine boluses; p=0.033 and infusion of phenylephrine for 216±86 vs 58±91 min; p<0.001). Plasma proANP and fluid balance were correlated only for EA patients (r=0.44; 95% CI 0.04 to 0.91; p=0.033).ConclusionsEA TEA reduces plasma proANP indicating that CBV becomes affected. Based on a correlation between plasma proANP and fluid balance, a 2000 mL volume surplus of lactated Ringer’s solution is required to maintain plasma proANP stable during open esophagectomy.Trial registration number2014-002036-14 (https://www.clinicaltrialsregister.eu/ctr-search/search?query=2014-002036-14).

Muscle metaboreflex-induced central blood volume mobilization in heart failure

Underperfusion of active skeletal muscle causes metabolites to accumulate and stimulate group III and IV skeletal muscle afferents, which triggers a powerful pressor response termed the muscle metaboreflex. Muscle metaboreflex activation (MMA) during submaximal dynamic exercise in healthy individuals increases arterial pressure mainly via substantial increases in cardiac output (CO). The increases in CO occur via the combination of tachycardia and increased ventricular contractility. Importantly, MMA also elicits substantial central blood volume mobilization, which allows the ventricular responses to sustain the increases in CO. Otherwise preload would fall and the increases in CO could not be maintained. In subjects with systolic heart failure (HF), the ability to increase CO during exercise and MMA is markedly reduced, which has been attributed to impaired ventricular contractility. Whether the ability to maintain preload during MMA in HF is preserved is unknown. Using a conscious chronically instrumented canine model, we observed that MMA in HF is able to raise central blood volume similarly as in normal subjects. Therefore, the loss of the ability to raise CO during MMA in HF is not because of the loss of the ability to mobilize blood volume centrally. NEW & NOTEWORTHY In normal subjects during dynamic exercise muscle metaboreflex activation elicits large increases in cardiac output that occur via increases in heart rate, ventricular contractility, and, importantly, marked central blood volume mobilization that acts to maintain ventricular preload, thereby allowing the changes in cardiac function to maintain the increases in cardiac output. In subjects with heart failure, the ability to raise cardiac output during muscle metaboreflex activation is impaired. We investigated whether this is because of the inability to maintain ventricular preload. We found that this reflex is still able to elicit large increases in central blood volume, and therefore the limited ability to raise cardiac output likely stems from ventricular dysfunction and not the ability to maintain preload.

Interaction between graviception and carotid baroreflex function in humans during parabolic flight-induced microgravity

The aim of the present study was to assess carotid baroreflex (CBR) function during acute changes in otolithic activity in humans. To address this question, we designed a set of experiments to identify the modulatory effects of microgravity on CBR function at a tilt angle of −2°, which was identified to minimize changes in central blood volume during parabolic flight. During parabolic flight at 0 and 1 g, CBR function curves were modeled from the heart rate (HR) and mean arterial pressure (MAP) responses to rapid pulse trains of neck pressure and neck suction ranging from +40 to −80 Torr; CBR control of HR (carotid-HR) and MAP (carotid-MAP) function curves, respectively. The maximal gain of both carotid-HR and carotid-MAP baroreflex function curves were augmented during microgravity compared with 1 g (carotid-HR, −0.53 to −0.80 beats·min−1·mmHg−1, P < 0.05; carotid-MAP, −0.24 to −0.30 mmHg/mmHg, P < 0.05). These findings suggest that parabolic flight-induced acute change of otolithic activity may modify CBR function and identifies that the vestibular system contributes to blood pressure regulation under fluctuations in gravitational forces. NEW & NOTEWORTHY The effect of acute changes in vestibular activity on arterial baroreflex function remains unclear. In the present study, we assessed carotid baroreflex function without changes in central blood volume during parabolic flight, which causes acute changes in otolithic activity. The sensitivity of both carotid heart rate and carotid mean arterial pressure baroreflex function was augmented in microgravity compared with 1 g, suggesting that the vestibular system contributes to blood pressure regulation in humans on Earth.

Transjugular intrahepatic portosystemic shunt: impact on systemic hemodynamics and renal and cardiac function in patients with cirrhosis

Transjugular intrahepatic portosystemic shunt (TIPS) alleviates portal hypertension and possibly increases central blood volume (CBV). Moreover, renal function often improves; however, its effects on cardiac function are unclear. The aims of our study were to examine the effects of TIPS on hemodynamics and renal and cardiac function in patients with cirrhosis. In 25 cirrhotic patients, we analyzed systemic, cardiac, and splanchnic hemodynamics by catheterization of the liver veins and right heart chambers before and 1 wk after TIPS. Additionally, we measured renal and cardiac markers and performed advanced echocardiography before, 1 wk after, and 4 mo after TIPS. CBV increased significantly after TIPS (+4.6%, P < 0.05). Cardiac output (CO) increased (+15.3%, P < 0.005) due to an increase in stroke volume (SV) (+11.1%, P < 0.005), whereas heart rate (HR) was initially unchanged. Cardiopulmonary pressures increased after TIPS, whereas copeptin, a marker of vasopressin, decreased (−18%, P < 0.005) and proatrial natriuretic peptide increased (+52%, P < 0.0005) 1 wk after TIPS and returned to baseline 4 mo after TIPS. Plasma neutrophil gelatinase-associated lipocalin, renin, aldosterone, and serum creatinine decreased after TIPS (−36%, P < 0.005; −65%, P < 0.05; −90%, P < 0.005; and −13%, P < 0.005, respectively). Echocardiography revealed subtle changes in cardiac function after TIPS, although these were within the normal range. TIPS increases CBV by increasing CO and SV, whereas HR is initially unaltered. These results indicate an inability to increase the heart rate in response to a hemodynamic challenge that only partially increases CBV after TIPS. These changes, however, are sufficient for improving renal function. NEW & NOTEWORTHY For the first time, we have combined advanced techniques to study the integrated effects of transjugular intrahepatic portosystemic shunt (TIPS) in cirrhosis. We showed that TIPS increases central blood volume (CBV) through improved cardiac inotropy. Advanced echocardiography demonstrated that myocardial function was unaffected by the dramatic increase in preload after TIPS. Finally, renal function improved due to the increase in CBV. Recognition of these physiological changes significantly contributes to our clinical understanding of TIPS.