The Journal of critical illness最新文献

Inverse ratio ventilation (IRV) differs from other ventilatory techniques in that it employs a prolonged inspiratory time. In theory, pressure-control IRV allows you to maintain ventilation and oxygenation with lower peak airway and end-expiratory pressures; this may reduce the potential for lung damage secondary to shearing forces. Consider pressure-control IRV for patients with acute lung disease characterized by low lung compliance, diffuse microatelectasis, and increased intrapulmonary shunting. Currently, the chief limitation of this technique is that the patient cannot breathe spontaneously during its use. The best inspiratory to expiratory ratio is the shortest inspiratory time that improves oxygenation with minimal hemodynamic compromise; depression of cardiac output will negate any potential improvement in arterial oxygenation.

In the ICU, both transthoracic and transesophageal echocardiography can assist in the acute management of a number of different disorders. In hypotensive patients, echocardiography helps distinguish between cardiogenic shock (resulting from acute myocardial infarction), septic shock, and circulatory shock (associated with a reduction in circulating blood volume); it can also help determine whether pericardial effusion or obstruction to valvular flow is producing the hypotension and suggesting pulmonary embolus. Other roles for echocardiography include differentiating left- and right-sided heart failure, assessing the extent of pericardial disease, diagnosing disorders of the thoracic aorta, and evaluating traumatic heart disease.

In patients with myocardial infarction, echocardiography helps predict outcome, as well as guide the selection of interventional strategies. By assessing the extent of valvular dysfunction and abnormalities in LV and RV size and function, echocardiography can help determine whether surgery is needed in patients with valvular heart disease. Both pulmonary artery pressures and LV function can be evaluated and quantified with ultrasonically derived data. Although echocardiography cannot document active infection, vegetations may appear as areas of valvular thickening. The role of echocardiography will continue to evolve as new techniques are developed.

Most modern ventilators have several key features in common: microprocessor control of operational and monitoring functions; electromechanical valves to control and adjust gas flow patterns; and extensive monitoring systems. In addition, these machines can provide a number of different modes of ventilation (including pressure support). Though not microprocessor-controlled, the Siemens Servo 900 series ventilators use feedback electronics to adjust inspiratory flow based on expiratory flow to meet preset volumes. In contrast, the Bennett 7200 units use microprocessor-regulated solenoid valves to deliver preset tidal volume. High-frequency ventilators deliver smaller tidal volumes at rates greater than 60 bpm.

By restoring the balance between myocardial oxygen supply and demand, intra-aortic balloon counterpulsation can aid the ischemic or failing left ventricle. Rapid inflation and deflation of the balloon reduces afterload and improves coronary perfusion. Timing of inflation and deflation is adjusted according to the arterial pressure waveform. The balloon should inflate just after the aortic valve closes and deflate just before the aortic valve opens; early or late balloon inflation or deflation blunts the therapeutic effects. Discontinue the IABP support gradually; monitor the patient frequently for any signs of hemodynamic compromise, redevelopment of ischemic chest pain, or electrocardiographic changes.

The Bennett MA-1 ventilator is a volume-cycled, constant flow generator that can act as an assistor, controller, or assist-controller. It is one of the most commonly used ventilators in clinical practice. With this unit, inspiration continues until a preset tidal volume is delivered to the patient--unless impedances to gas flow increase system pressures to a preselected limit. Thus, setting the maximum inspiratory pressure too low limits the ability of the ventilator to deliver the tidal volume, causing it to function as a pressure-cycled device. Other basic controls allow you to establish the sensitivity of the ventilator to spontaneous breathing attempts, the maximum flow rate, the frequency of respirations, and the oxygen percentage. Special controls permit delivery of a sigh breath and slowing of exhalation.

The intra-aortic balloon pump (IABP) uses the timed inflation and deflation of a balloon placed in the descending aorta to augment coronary perfusion and reduce myocardial work. The IABP is useful in a number of settings, including acute myocardial ischemia, cardiogenic shock, and mechanical complications of acute myocardial infarction; it also provides support to perioperative patients. Because the device can be inserted percutaneously, mechanical circulatory support can be instituted rapidly Complications, most of which are reversible, occur in approximately 30% of patients.

Currently available ventilators offer a number of special options to meet the needs of critically ill patients. Intermittent mandatory ventilation allows a patient to breathe spontaneously without assistance. CPAP and PEEP ensure that the patient breathes at an elevated pressure either constantly or during expiration. Pressure support ventilation allows patients to participate in breathing but provides inspiratory assistance and is most useful during weaning. Airway pressure release ventilation facilitates venous return and decreases airway pressure. Sophisticated monitors provide detailed information on the patient's status, but alarm features are somewhat unreliable. Thorough knowledge of the controls on modern ventilators can help you provide the optimum form of respiratory support.

To select a ventilator (or a ventilatory mode), consider the most basic characteristics: How is tidal volume generated (with a constant or nonconstant flow or pressure generator)? How does the ventilator trigger a changeover from exhalation to inhalation and cycle back to exhalation? How is tidal volume delivered to the patient (either directly from a power source or indirectly from an intermediate chamber)? What special functions are available? The answers to these questions will not only let you make the best selection but will also help you troubleshoot when a ventilator fails to function properly.

Percutaneous renal biopsy is useful in diagnosing a variety of kidney disorders, as well as certain systemic diseases with renal and/or nonrenal manifestations. The procedure is performed while the kidney is viewed by continuous ultrasonographic imaging. The kidney is first located with a spinal needle and the area is anesthetized. The biopsy needle is then inserted and advanced toward the capsule. When the capsule is pierced, the trochar is advanced into the renal cortex. The sheath is lowered, encompassing the cutting edge and the core biopsy sample. Complications, although rare, may include the creation of an arteriovenous malformation and laceration of the kidney or other intra-abdominal organ; close monitoring after the procedure is therefore required.