Pulmonary shunt and cardiovascular responses to CPAP during nitroprusside-induced hypotension

Esmolol infusion during nitroprusside-induced hypotension: impact on hemodynamics, ventricular performance, and venous admixture

The impact of esmolol infusion on hemodynamics, ventricular performance, venous admixture, sympathoadrenal, and renin-angiotensin system responses during sodium nitroprusside (SNP)-induced hypotension was studied in 11 patients undergoing lymph node dissection during general anesthesia with 60% nitrous oxide and fentanyl. Radial arterial and thermistor-tipped pulmonary catheters were employed for hemodynamic monitoring. Arterial and mixed venous blood gas tensions, arterial plasma renin activity (PRA), and plasma catecholamine levels were measured. Derived hemodynamic parameters and venous admixture (Qs/Qt) data were obtained from standard equations. Transesophageal echocardiography (6 patients) was used to assess left ventricular performance using the relationship between end-systolic wall stress (ESWS) and velocity of circumferential shortening (VCFC). After surgical incision, arterial hypotension was induced with SNP alone. Esmolol was infused at each of the following rates in sequence: 200, 300, and 400 micrograms/kg/min. Each esmolol infusion lasted 20 minutes and the SNP dose was adjusted to maintain MAP at 55 to 60 mm Hg. The mean dose of SNP required to induce hypotension was 5.5 micrograms/kg/min +/- 0.5 SE. Compared to prehypotension values, SNP induced significant increases in Qs/Qt and reductions in PaO2, systemic vascular resistance (SVR), and stroke volume index (SVI). Esmolol infusion caused dose-dependent (highest with 400 micrograms/kg/min) reductions in the SNP requirement, heart rate (HR), SVI, Qs/Qt, and PRA, and also led to significant increases in SVR and left ventricular (LV) internal diameter in diastole as well as systole. Furthermore, esmolol infusion was associated with a dose-dependent downward and leftward shift of the ESWS versus VCFC relationship, implying diminished contractility.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sodium nitroprusside on ventilation-perfusion mismatching in heart failure

Sodium nitroprusside has been shown to lower arterial partial pressure of oxygen (PaO2) in patients with congestive heart failure and respiratory failure. The multiple inert gas elimination technique was used to evaluate the effect of sodium nitroprusside infusion on pulmonary gas exchange in five patients with congestive heart failure. During sodium nitroprusside infusion, mean values of cardiac output increased and mean values of arterial pressure, pulmonary artery pressure, pulmonary artery wedge pressure and pulmonary vascular resistance decreased. Cardiac output increased in each patient and PaO2 decreased in all but one patient (mean 75.6 +/- 15.1 to 68 +/- 17.5 mm Hg, p = 0.032). Distributions of ventilation and perfusion showed increased perfusion of lung units with low (less than or equal to 0.1) ventilation-perfusion ratios in all subjects during sodium nitroprusside infusion (mean 3.89 +/- 1.52 to 11.33 +/- 7.42% of cardiac output, p = 0.027, paired t test). The amount of shunt (fractional perfusion of lung units with ventilation-perfusion ratio = 0) increased in the two patients with some shunt present in the baseline measurements. The mean total low ventilation-perfusion perfusion (shunt plus ventilation-perfusion less than or equal to 0.1) was significantly increased from 4.38 +/- 1.54 to 14.7 +/- 9.37% (p = 0.023) during sodium nitroprusside infusion. Total low ventilation-perfusion perfusion was negatively correlated with mean pulmonary artery pressure and pulmonary artery wedge pressure (r = -0.949 and -0.946, respectively). Although sodium nitroprusside infusion increased cardiac output and overall oxygen transport in all patients, it worsened ventilation-perfusion mismatching. The mechanism is probably pulmonary vasodilation or increased cardiac output, or both.

Cardiovascular effects of sodium nitroprusside in cerebral aneurysm surgery

Cardiovascular effects of sodium nitroprusside (SNP) were studied in 17 patients undergoing cerebral aneurysm surgery under controlled hypotension. In 10 patients oxygen uptake was also calculated. Mean arterial blood pressure was decreased from 91 to 58 mmHg by SNP due to reductions in both total peripheral resistance (29%) and cardiac output (15%). Mean right atrial and pulmonary capillary wedge pressures were reduced during hypotension. Total oxygen uptake remained constant throughout the procedure. There was no significant change in arterial oxygen tension in the hypotensive period, but after discontinuation of SNP infusion it was significantly increased. The haemodynamic effects of SNP in patients with a recent subarachnoid haemorrhage and its effect on blood oxygenation are discussed. It is concluded that the fall in cardiac output is related to a reduction of central blood volume and might be more pronounced in these patients than in normal subjects.

Hyperkinetic shock in viral and pneumococcal pneumonias

Ten patients, suffering from severe viral or bacterial pneumonia had circulatory shock, characterised haemodynamically by normal or high cardiac output (CI = 4.1 +/- 1.2 1/min/m2) and low systemic resistance (SVR = 14 +/- 3.7 mm Hg/1/min/m2). Existence of such a hyperkinetic state greatly complicates the management of patients. Plasma volume expansion, performed in five cases of initial hypovolaemia, and Dopamine infusion (five patients) increased markedly the intra-pulmonary shunting. High level PEEP ventilation was not tolerated, despite the improvement of blood gases it produced. Extra-corporeal membrane lung oxygenation (three cases), whilst giving an initial decrease of shunting and restoring SVR, produced no long term surviors. All ten patients died from intractable shock and severe hypoxaemia. Spontaneous ventilation with positive expiratory pressure (CPAP) is believed to be an attractive alternative, due to its absence of deleterious haemodynamic effects.