Head-down bed rest impairs vagal baroreflex responses and provokes orthostatic hypotension

Abstract

We studied vagally mediated carotid baroreceptor-cardiac reflexes in 11 healthy men before, during, and after 30 days of 6 degrees head-down bed rest to test the hypothesis that baroreflex malfunction contributes to orthostatic hypotension in this model of simulated microgravity. Sigmoidal baroreflex response relationships were provoked with ramped neck pressure-suction sequences comprising pressure elevations to 40 mmHg followed by serial R-wave-triggered 15-mmHg reductions to -65 mmHg. Each R-R interval was plotted as a function of systolic pressure minus the neck chamber pressure applied during the interval. Compared with control measurements, base-line R-R intervals and the minimum, maximum, range, and maximum slope of the R-R interval-carotid pressure relationships were reduced (P less than 0.05) from bed rest day 12 through recovery day 5. Baroreflex slopes were reduced more in four subjects who fainted during standing after bed rest than in six subjects who did not faint (-1.8 +/- 0.7 vs. -0.3 +/- 0.3 ms/mmHg, P less than 0.05). There was a significant linear correlation (r = 0.70, P less than 0.05) between changes of baroreflex slopes from before bed rest to bed rest day 25 and changes of systolic blood pressure during standing after bed rest. Although plasma volume declined by approximately 15% (P less than 0.05), there was no significant correlation between reductions of plasma volume and changes of baroreflex responses. There were no significant changes of before and after plasma norepinephrine or epinephrine levels before and after bed rest during supine rest or sitting.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration of Early Mobilization in Intensive Care Unit Patients and Effects of Early Mobilization on Patient Hemodynamics

“…Karşılaşılan advers olay veya planlanmamış ekstübasyon oranlarının çok düşük olduğu belirtilmektedir. Mobilizasyonun entübasyon sonrası ilk gün, olabildiğince en erken zamanda başlanması önerilmektedir (7)(8)(9). Bizim çalışmamızda cerrahi geçiren hastalar ekstübasyon sonrası herhangi kontrendikasyon oluşturacak bir durum yoksa ortalama 16 saat sonra, girişimsel cerrahi işlemi uygulanan hastalar kasık kanama kontrolü ve nörolojik değerlendirilmelerine göre ve diğer hastalar ilk 24 saat içerisinde mobilize edilmiştir.…”

“…Mobilize edilen hastalarda ekstübasyon, hipertansiyon veya hipotansiyon, hastanın dizüstü düşmesi gibi istenmeyen durumlar gösterilmiştir (8). Yoğun bakım hastalarında zayıf vasküler tonus, işlevsiz bir otonomik geri besleme döngüsü ve/ veya düşük kardiyovasküler rezerv sonucu mobilizasyona adaptasyon gecikebilir (9,10).…”

Introduction to Progressive Mobility

“…Critically ill patients commonly have poor vascular tone, a dysfunctional autonomic feedback loop, and/or low cardiovascular reserve. 31,32 Often, they are left in a stationary position for a prolonged period and establish a "gravitational equilibrium" over time, making it more difficult to adapt to a position change. For patients in whom hemodynamic instability develops with a manual turn, the solution might be to train the patients to tolerate a position change rather than leaving them in a stationary supine position.…”

Efeitos cardiovasculares agudos da exposição ao ambiente microgravitacional

“…Uma vez que o intervalo R-R se correlaciona linearmente com a descarga vagal para o coração 74 , postulou-se que o distúrbio autonômico seria causado por alteração do componente parassimpático do reflexo 72,73 . Interessantemente, a mesma avaliação do reflexo barorreceptor carotí-deo já havia sido realizada no modelo de simulação HDT a 6º 75 , revelando praticamente os mesmos resultados.…”