The resulting high density of the anesthetic block and the reproducibility of the technique have made of spinal anesthesia the most widely diffused regional anesthesia technique. Nevertheless the procedure still remains the regional anesthesia technique burdened by the potentially most severe side effects, the most frequent of which being systemic arterial hypotension.
Subarachnoidal administration of local anesthetics in fact is associated not only with both a motor and sensory block, but also with a blockade of the autonomic nervous system, which controls peripheral vascular tone. Sympathetic block leads to a sudden decrease in peripheral resistance, with consequent significant increase in vascular bed and relative hypovolemia.
Transient hypotensive episodes can be generally well tolerated by healthy patients, however they may lead to major complications in patients with increased cardiovascular risk. In the common clinical practice, it is usual to administer fluids empirically. An empirical preventive fluid repletion however can represent per se a risk in patients with impaired cardiac and renal functions, since a volume overload of heart chambers with reduced compliance can cause pulmonary edema or congestive heart failure.
Since about ten years both non-invasive and invasive techniques have been developed, with the specific purpose of optimizing fluid status on a rational basis. This methods are based on the correlation between the levels of mean arterial pressure (MAP), systemic vascular resistance (SVR) and cardiac output (CO), according to the equation: MAP = CO \* SVR. The latter explains why, in case of vasodilation, unless CO is increased, a lowering of SVR will inevitably translate into a decrease of MAP.
Determinants of blood pressure are:
1. volemic status;
2. vascular resistance (determined by the control of the sympathetic system);
3. cardiac output.
In case of spinal anesthesia the reduction in SVR determines relative hypovolemia (due to an increase in total vascular bed). For this reason after spinal anesthesia appears fundamental an optimization of patients' volemic status through administration of fluids (eventually associated with the administration of amines, in order to contrast massive vasodilation).
The use of methods to assess fluid responsiveness has been extensively studied in critical patients populations. However these techniques have not been studied yet in non critical patients. Spinal anesthesia is a model of pharmacological vasodilation which often generates significant hypotension, for which nowadays an evidence-based rational treatment based on a guided volemic repletion is still lacking.
WORKING HYPOTHESIS:
The use of non-invasive methods that have proven to be accurate for the identification of patient fluid-responsiveness is currently not validated in patients undergoing spinal anesthesia.
Ultrasound of inferior vena cava by is an effective method to determine the responsiveness to fluids in a patient on mechanical ventilation, however it has been proven to be inaccurate in spontaneous ventilating patients in critically ill patients, few data being available in non critical patients. The study of the inferior vena cava is based on the size of the inferior vena cava in its intra-abdominal portion (approximately 2 cm from emergency right atrium) and the pattern of its diameter changes with breathing. This measurement is obtained with M-mode ultrasound through subcostal view..
Passive leg raising test (PLRT) has been proven to be a highly accurate method in predicting fluid responsiveness in spontaneously breathing patients, in a population of critically ill patients. The method is based on the following assumption: active legs elevation, in addition to its effect of venous pool shifting from the lower limbs to the thorax, exerts a contemporary effect of stimulation on the peri-arterial sympathetic system at iliac-femoral level, this causing an orthosympathetic reflex which can increase cardiac output, possibly masking hypovolemia. Passive lower limbs raising instead has the advantage of mobilizing lower limbs venous blood (estimated 300-500 ml), without activating the orthosympathetic reflex. This allows for a quantification of the clinical response to a bolus of fluids, in terms of decrease of blood pressure variations following leg raising.
The hypothesis that investigators want to test is that the use of inferior vena cava ultrasound before spinal anesthesia, compared to the standard method (empirical fluid administration) and a non-invasive method (PLRT) can reduce the impact of systemic hypotension through an adequate titrated volemic repletion, avoiding both hypotension and fluid overload. The final purpose is to ensure spinal anesthesia in the safest possible way.
OBJECTIVES OF THE STUDY:
Aim of the study is to determine whether vena cava ultrasound, a bedside, unexpensive, non invasive methods, which have been tested as predictors of fluid responsiveness in critical patients, is effective in guiding titrated fluid repletion in a non critical population, in order both to decrease post procedural significant hypotension rate and unnecessary fluid overload in patients undergoing spinal anesthesia for elective surgical procedures.
