Section 5: Critical Care Medicine
Chapter 72: Respiratory Care

Full and Partial Ventilator Support

As listed in Table 72–9, full ventilator support infers that all the work of breathing necessary to maintain eucapnia is provided by the ventilator; partial ventilator support infers that both the patient and the ventilator provide essential portions of the required work of breathing to maintain eucapnia. 158  The concept of partial ventilator support is predicated on the finding that spontaneous breathing is normally beneficial for cardiopulmonary homeostasis, whereas energy expenditure that exceeds cardiopulmonary reserves is detrimental work of breathing and should be avoided. 38 

TABLE 72–9. Definitions of the Three Levels of Ventilator Support

Ventilator modes designed to provide partial support permit the patient to breathe spontaneously to whatever extent desirable without physiologic detriment, whereas the ventilator supplies the remaining work of breathing. 56  The first mode capable of providing partial support was IMV, which provided a continuous-flow system to allow spontaneous breathing between the volume-preset cycles. 159  Because of difficulties in manufacturing continuous-flow systems, SIMV was introduced. Essentially, SIMV is an assist/control volume-preset ventilator in conjunction with a demand-flow system for spontaneous breathing. The major disadvantage of demand-flow systems was a delay in providing adequate inspiratory gas flow to the patient, resulting in an imposed work of breathing that was significantly greater than that associated with continuous-flow systems. However, SIMV became almost universally utilized because demand-flow systems were preferable to continuous-flow systems from a manufacturing viewpoint.

In comparison with full ventilator support, techniques of partial ventilator support have been demonstrated to enhance cardiac output in patients with normal left ventricular function, 160  to cause less hemodynamic compromise in conjunction with CPAP/PEEP therapy, 161  and to allow for significantly greater urine output and renal blood flow. 162  However, not all patients do better with partial ventilator support. For example, patients with severe ventilatory muscle fatigue, usually those with COPD, may be treated more successfully with full ventilator support. 163  Another example is a patient with cardiogenic shock and poor left ventricular reserve, who maintains better left ventricular function and has improved peripheral perfusion when all the work of breathing is supplied by the ventilator. 164