Tidal Volumes in Patients with Normal Lungs: One for All or the Less, the Better?

MECHANICAL ventilation (MV) using tidal volumes (V^T) of not more than 6 ml/kg predicted body weight (PBW) has been shown to result in reduction of systemic inflammatory markers, increased ventilator-free days, and reduction in mortality when compared with V^Tof 12 ml/kg PBW in patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) (table 1).1,2In the low V^Tgroup, V^Twas reduced further to 5 or 4 ml/kg PBW if necessary to maintain plateau pressure (P^plat) at less than 30 cm H²O.1However, decreasing V^Tdid not improve outcome in three other controlled trials investing V^Tin ALI and ARDS patients, which was explained by differences in study design (table 1).3–5Using V^Tof not more than 6 ml/kg PBW comparing a high positive end-expiratory pressure (PEEP)–low inspiratory oxygen fraction (Fio²) with a low PEEP–high Fio²strategy to prevent hypoxemia did not demonstrate advantageous of higher PEEP levels in ALI and ARDS patients.6The lack of effect of higher PEEP levels was partially explained by the resulting higher P^plat. A secondary analysis of the ARDS Network database showed a beneficial effect of V^Treduction from 12 ml/kg to 6 ml/kg PBW even in patients with low P^platranging between 16 and 26 cm H²O before V^Treduction.7In this issue of Anesthesiology, Schultz et al.  8suggest the use of low V^Tventilation with PEEP levels above 5 cm H²O in patients without ALI or ARDS in absence of large-scale prospective randomized trials.

Schultz et al.  argue that in critically ill patients requiring MV for pulmonary edema, chronic obstructive pulmonary disease, congestive heart failure, aspiration, pneumonia, and trauma and after surgery not fulfilling ARDS criteria, mortality is associated with application of high V^Tand P^plat.8,9Two retrospective analyses identified high airway pressures and V^Tas independent risk factors for development of ALI and ARDS in patients requiring MV for acute respiratory failure.10,11It is of importance that these analyses included patients who were critically ill and had obviously either cardiopulmonary disease or ventilatory dysfunction and had thus per se  a certain risk to develop ALI or ARDS. In an international cohort of unselected ARDS patients, neither P^platnor V^Tbut use of low or no PEEP was associated with adjusted mortality.12Recent surveys demonstrated that V^Tin critically ill patients is on average approximately 7–8 ml/kg BW but that still V^Tbetween 12 and 18 ml/kg BW are used with low or nil PEEP.13Based on these data, it seems justified to request protective ventilator strategies in risk patients routinely and not to wait until the ALI or ARDS criteria are fulfilled. Although we do not have evidence that the ventilator settings suggested by Schultz et al. , which are essentially based on the ARDS Network protocol, are the best way to ventilate patients at risk for ALI or ARDS, they may prevent harm from the use of too-high V^Tand low or nil PEEP levels.

Potential adverse effects of protective MV should be considered in all critically ill patients. Hypercapnia may cause increased intracranial pressure, pulmonary hypertension, decreased myocardial contractility, decreased renal blood flow, and release of endogenous catecholamines. Moreover, MV with low V^Tand P^platmay promote atelectasis formation and increase requirements for higher Fio²and PEEP. To counteract cardiovascular depression caused by higher PEEP levels, fluid loading frequently associated with a positive fluid balance and/or catecholamines may be required. Therefore, all of these variables must be carefully considered and balanced when reducing V^Tin individual patients.

Another question is whether protective ventilation is beneficial in patients with healthy lungs requiring short-term MV during anesthesia. Besides airway closure and reduced lung volumes in the supine position, distortion of rib cage (and lung), cephalad shift of the diaphragm, surfactant alteration, blood shift from abdomen to thorax, or a combination of these contribute to atelectasis formation in 90% of the patients during anesthesia.14In the 1960s, use of large V^Tof approximately 15 ml/kg BW was advocated to reopen collapsed lung tissue and prevent impaired oxygenation during anesthesia.15Cyclic opening and closing caused by recruitment and derecruitment of small airways or lung units may lead to increased local shear stress (atelectrauma), which has been suggested to contribute to lung damage even in the absence of high P^plat.16However, for identical V^Tand PEEP, reducing respiratory frequency attenuates or delays damage, provided that tidal ventilatory stress is sufficiently high.17This indicates that the doses of stress will matter. Whereas a ventilator cycle is repeated 20,000–40,000 times per day for a longer period in critically ill patients, probably not more than 900 cycles are commonly applied per 1 h of anesthesia. PEEP levels up to 10 cm H²O are necessary in healthy patients during anesthesia to keep open those units that are most likely to close. However, any lung-protective benefit of PEEP is expected to be unimpressive when P^platis modest or when the lung contains few recruitable units. Atelectatic area on computed tomography slice near the diaphragm is generally approximately 5–6% of the total lung area but can exceed 15–20% during uneventful anesthesia.14This may explain why in patients with healthy lungs undergoing elective major thoracic or abdominal surgery, MV with V^Tof 12–15 ml/kg PBW and nil PEEP did not result in different pulmonary or systemic levels of inflammatory markers when compared with V^Tof 6 ml/kg PBW and PEEP of 10 cm H²O.18 

Individual factors such as obesity, pneumoperitoneum, preexisting disease, and some surgical interventions may aggravate atelectasis formation. In addition, a variety of cofactors apart from ventilator settings such as positioning; systemic inflammatory response depending, for example, on the amount of surgical trauma; and higher precapillary19and lower postcapillary20pulmonary vascular pressures are important for generation or prevention of ventilator-induced lung injury. As highlighted by Schultz et al. , smaller randomized controlled trials of perioperative ventilatory strategies during major surgery revealed nonuniform results.8The impression is that ventilatory strategy is more relevant during surgery that triggers a higher inflammatory response, such as esophagectomy or cardiac surgery. However, these studies where not designed or powered to draw clinically relevant conclusions on clinical outcome measures, but studied inflammatory markers that are likely to but not proven to be surrogate markers of clinical outcome. To avoid high plateau pressures during one-lung ventilation, it has been suggested to use V^Tof 5–6 ml/kg BW with PEEP in the absence of auto PEEP and to limit P^platto less than 25 cm H²O during one-lung ventilation.21However, application of PEEP in the dependent ventilated lung may increase pulmonary vascular resistance in this lung, diverting blood flow to the nonventilated lung, and thereby increasing intrapulmonary shunt and hypoxemia.

Although V^Tof more that 10 ml/kg PBW are probably seldom used during anesthesia, there is no sound scientific basis to consider further V^Treduction necessary when P^platis not higher than 16 cm H²O to prevent lung injury.8Hypercapnia and its side effects can be generally prevented by moderate increased respiratory rates due to reduced carbon dioxide production during anesthesia. To counteract atelectasis formation during MV with low V^Tand P^plat, higher Fio²and PEEP may be required. Especially in the presence of hypovolemia or shock, already moderate PEEP levels require fluid loading resulting in a positive fluid balance, which is a significant risk factor for major and minor morbidity and gastrointestinal paralysis after colorectal and major surgery.22To what extent postoperative complications are caused by respiratory dysfunction and ventilator settings during anesthesia is not yet clear.

Therefore, it is essential to tailor ventilator settings during anesthesia to the specific physiologic changes caused by surgery and preexisting disease of the patient, while treating the lungs gently. It may be concluded so far that the more ill the patient is, the more relevant the ventilatory strategy may be.

Department of Anesthesiology and Intensive Care Medicine, University of Bonn, Bonn, Germany. putensen@uni-bonn.de